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E L S E V I E R Tectonophysics 264 (1996) 175-189

TECTONOPHYSICS

Crustal image of the Ionian basin and its Calabrian margins

L. Cernobori a,b,*, A. Him c, J.H. McBride d, R. Nicolich a, L. Petronio a, M. Romanelli a, STREAMERS/PROFILES Working Groups

a DINMA-Univ. of Trieste, Via Valerio 10, Trieste, Italy b CNR-lst. Talassografico, Via le R. Gessi 2, Trieste, Italy

c LPG. de Paris, pl. Jussieu 4, Paris, France a BIRPS, Bullard Laboratories, Madingley Road, University of Cambridge, Cambridge, CB30EZ, UK

Received 1 March 1995; accepted 30 November 1995

Abstract

Previous seismic investigation of the crustal structure in the Ionian basin has been limited to shallow penetration seismics of the 1970's, characterized by inadequate source power and low fold. Earlier OBS and ESP seismic refraction experiments have not been able to firmly resolve one of the principal scientific problems for this region which is whether the Ionian basin is floored by oceanic crust or by highly attenuated continental crust. A second elusive problem is the nature of the transition of the boundaries between the Ionian basin and its margins.

In this paper we describe and interpret new deep seismic reflection and wide-angle data collected in the western Ionian Sea and the Calabria region of Italy. One of the principal features of our multichannel reflection data beneath the Ionian basin is a band of 'layered' high-amplitude reflections near the base of the crust. This band shows a quasi-monochromatic (ca. 8 to 10 Hz) frequency and a traveltime thickness of 1 to 1.5 s. These images contrast with the well known reflection patterns of Mesozoic oceanic crust investigated in the Atlantic Ocean. There is evidence that the low-frequency band dips down towards the edge of the Malta Escarpment (ME), where landward-dipping reflectors separate continental and intermediate type crust in the central tract of the ME. The increased traveltimes of the lower-crustal reflectors and Moho, from the basin towards the southern and eastern margins of southern Calabria, could be partially due to the velocity pull-down effect of the sedimentary pile of the arc, although a true dip of 15 to 18%, over 60 km distance, can be substantiated. Moreover, the reflecting band maintains its reflectivity and thickness until its abrupt termination beneath the Ionian continuation of the Calabrian compressional crustal structures. The coincident acquisition of wide-angle seismic data and marine reflection seismic data provided a landward extension of the survey which will complement existing geologic information on the deep framework of the Ionian basin and its Calabrian margin.

Keywords: Ionian Basin; multi-channel seismic reflection; Calabrian margin; lower crust seismic stucture; crustal thinning

1. Introduction

The western Ionian Sea (Fig. 1), which has been surveyed by the S T R E A M E R S seismic acquisit ion

* Corresponding author. Tel. + 3 9 40 676-3478; Fax: + 3 9 40 676-3497.

project in early 1992, is underlain by a deep basin where the sedimentary cover can be more than 8 km thick, as known from both reflection and refraction data (Finetti, 1982; Makris et al., 1986; Ferrucci et al., 1991; de Voogd et al., 1992; Truffert et al., 1993), and includes Mesozoic to Quaternary sequences (Dercourt et al., 1986). The basin is lo-

0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PI1 S 0 0 4 0 - 1 9 5 1 ( 9 6 ) 0 0 1 2 5 - 4

176 L. Cernobori et a l . /Tectonophysics 264 (1996) 175-189

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Fig. 1. Location map of the STREAMERS profiles (ION-l, -2, -3, -4, -5, -6) in the western Ionian (thick continuous lines). Additional hatched boxes mark the portion of the profiles shown in Figs. 3 to 7. The land-stations location (TS-1) and the Serre profile are indicated onshore Calabria. Dashed lines indicate DSS/79 (Ferrucci et al., 1991), DSS/81 (Makris et al., 1986) and ESP locations (de Voogd et al., 1992). A = Calabrian-Peloritan nappes; B = Maghrebian arc; C = Hyblean plateau; 1 = main vertical extensional faults; 2 = thrust faults on land.

cated within a complex geological area and played a crucial role in the Tertiary and Quaternary tectonic interaction between the African and European continental margins. Extensional and compressive processes have developed over a short time span in the central Mediterranean since the Tortonian: ar-

eas of stretched crust, crustal underplating, and a thrust belt in which arc-shaped structures have developed comprise the interesting tectonic features of this region. Nevertheless, geophysical and structural data distinguish distinct crustal domains and de- limit the kinematic evolution of their margins which

L. Cernobori et aL / Tectonophysics 264 (1996) 175-189 177

was strictly controlled by N-S collision between the African and European plates:

- the Pelagian block that exhibits a normal to thin continental crust affected by rifting processes;

- the thinned and stretched area of the southern Tyrrhenian and Ionian domains, characterised by a transitional or oceanic crust;

- the orogenic belt represented by the Apennine and Maghrebian thrust system and by the Calabrian arc (detailed structurations in C.N.R., 1983).

The timing of the deformation suggests that the Tortonian to Quatemary tectonics of the central Mediterranean is dominated by the extrusion of the Calabrian arc towards the east, laterally restrained by the N-S impingement of the continental indenter of the Pelagic domain (Reuther et al., 1993). The mech- anism of the interaction between the Calabrian block and the Ionian domain and the nature of the Ionian crust and its active or passive role in the collisional process, are crucial points for the understanding of the recent evolution of this complex area. Accord- ing to a model of Mesozoic-Cenozoic plate-tectonic evolution of the Eastern Mediterranean area (Der- court et al., 1986), the extension within the Ionian basin began in the Late Jurassic (150-130 Ma). In the Aptian (about 110 Ma) a rotation of Apulia with respect to Africa initiated the rapid subsidence of the Ionian basin. The basin was expanding in the Late Cretaceous and ceased its spreading in Cretaceous- Paleocene time, which generally corresponds to the collision north of Apulia. Gealy (1988) assigns the spreading event, which led to the development of the Eastern Mediterranean, to the Middle Cretaceous.

Geophysical data (seismic, gravimetric and heat flow) are consistent with the suggested models of thinning and basin formation. In particular, in the western Ionian Sea, the depth of the Moho rises to 19-20 km and finally reaches about 16-18 km under the Ionian Abyssal Plain. Makris et al. (1986), Ferrucci et al. (1991), de Voogd et al. (1992) and Trnffert et al. (1993) have identified the Moho depth from high, at least 8.1-8.2 km/s, refraction velocities on OBS and ESP profiles. This P-wave velocity corresponds to a density of 3350 to 3450 kg/m 3 (Truffert et al., 1993) which, in turn, correlates to either a depleted mantle or with the presence of eclogite (transformation of magmatic basalt lenses into dense eclogite rocks after the cooling of the

basin). Furthermore, the positive Bouguer anomalies, ranging up to about 310 mGal at the centre of the Ionian abyssal plain (Morelli et al., 1975) in spite of the rather deep Moho, provide convincing evidence of a dense lens in the uppermost mantle. Moreover, in the western Ionian basin the heat-flow values do not exceed 40 mW/m 2 (Della Vedova and Pellis, 1992). This provides evidence of general cooling related to the tectonic subsidence of the Ionian basin.

The STREAMERS survey provides new additional constraints on the tectonic development of this region by imaging the reflectivity structure at higher resolution than previously available. The most unexpected and impressive image obtained with the seismic data described in this paper is that of a layered band at the base of the crust, 1 to 1.5 s thick, with monochromatic (8-10 Hz) reflectors and strong amplitudes. According to the above mentioned DSS and ESP results, this band corresponds to a wide, thick (3.5 to 4.0 km), laminated lower crust or crust- mantle transition, to which an interval velocity of 6.9 to 7.1 km/s was assigned. It appears to be a distinctive signature for the Ionian basin and, although this area has been thought to be floored by oceanic crust, differs from true oceanic reflection images as observed in the eastem Atlantic Ocean (McBride et al., 1994). Furthermore, this distinctive band can be traced from the basin centre to its margins and then used as a marker of the Moho topography and of deformations in the Ionian region since its formation. This layered lower crust has also been clearly seen in a profile shot in 1993 with a single-bubble airgun array on board M/V Nadir (Ifremer) in the frame of a study of Mt. Etna, Sicily (Cernobori et al., 1994; Avedik et al., 1995).

2 . T h e g e o l o g i c a l f r a m e w o r k

2.1. The abyssal plain in front of the central sector of the Malta Escarpment (ME)

Geophysical data indicate that the ME separates two distinct domains corresponding to the Pelagian and Ionian Seas. Casero and Roure (1994) interpret the evolution of the ME as an essentially Torto- nian feature, subdivided, along a N-S direction, into tracts (Fig. 1), each of which had a distinct geodynamic evolution due to important SW-NE-trending

178 L. Cernobori et al./Tectonophysics 264 (1996) 175-189

dislocations. Each tract shows structurally different characteristics: the middle and southernmost ones have developed parallel to zones characterised by equal facies depositions during the Mesozoic, but the northern tract crosscuts them (Casero et al., 1988). The central sector corresponds to a continental margin facing an ancient sea. However, in analogy to other Mesozoic margins, the central ME could have been affected by listric faulting and subdivided into tilted blocks of various sizes. The transition from a thinned continental to a true oceanic crust towards the centre of the basin, as interpreted by de Voogd et al. (1992), is also a possibly important part of this story.

2.2. The Ionian margin of southern Calabria

The Calabrian arc is divided into two sectors. The northern one includes the Sila crystalline units and a stack of Apennine carbonate formations partly overthrust by the Sila granite nappes. Since the Creta- ceous, the northern sector, which was closely linked to the evolution of the Apennine chain, has undergone nearly uninterrupted deformation along an area perpendicular to the major Apennine orogenic move- ments. The southern sector is linked to the evolution of the Sicilian-Maghrebides chain (Boccaletti et al., 1984). A dramatic uplift of this region (1-2 mm/yr.) occurred in the Middle-Late Pleistocene with the development of normal faulting tectonics. Detailed onshore and some offshore geological information are given in the Structural Model of Italy (C.N.R., 1983).

From a tectonic point of view, the southern sector is the most active area of the central Mediterranean. Cassinis and Ranzoni (1987) have recognised, underneath Calabria, a belt with a dense concentration of earthquakes, subhorizontally distributed and confined close to the crust-mantle boundary at 20 to 25 km depth. This information is supported by the records obtained by a local seismological net (Cara- belli et al., 1988) that reveals a layer possessing a strong seismicity at a depth range of 15 to 25 km. A second seismically active layer has been found at 50 km depth. These seismically active structures abut on an area in the Tyrrhenian Sea, with deep-seated seismicity (hypocentres located at depths reaching more than 400 km).

From the above observations, active subduction of the Ionian crust beneath the Calabrian arc has been hypothesised. Other authors (Patacca and Scan- done, 1989) maintain that the subduction system is by now composed only of a narrow relic slab which is now mechanically weak. Moreover, they forecast the collapse of lithospheric blocks by gravity into the mantle, which may be related to modern seismic activity. However, the opening of the Tyrrhenian Sea can be interpreted as an asymmetric passive rifting developed as a consequence of the N-S collision between the African and European plates or may be the consequence of low-angle shear displacement (i.e., a shear plane displaces the entire crust and it is followed by a uniform stretching of the lower and more ductile parts of the lithosphere). Such a delamination model could explain the deformation style of the continental lithosphere and the uplift of the mantle due to tectonic denudation. Another model could be proposed which involves the active role of the mantle with asthenospheric intrusions as prod- ucts of a transformation of the lithospheric mantle (and crust), enhanced by a thermal anomaly and fluid supply from deep mantle sources. The southeastward motion of a less viscous and mobilised asthenospheric mass would then be the kinematic source of the obduction of the Calabrian crustal units onto the quasi-oceanic Ionian crust (Locardi and Nicolich, 1988).

Ferrucci et al. (1991, and references therein) have recognised the base of the crust (Calabrian Moho?) at depths of nearly 30 km, offshore the southern tip of Calabria, and at depths of about 18 km in the Serre region. More recently, a reflection seismic profile has been acquired in the Serre mountain area (LUschen et al., 1992). The interpretation of these data confirms the relatively thin crust of the Calabria block, characterised by reflective internal structures and with the base at a depth of approximately 18 to 20 km, dipping southwards. The distribution of interval velocities within the block, as revealed by refraction seismic and modelling techniques applied to the reflection section, never exceeds 6.0-6.4 km/s until the base of the block is reached at 6.5 to 7.5 s (about 20 km depth). At the base, the velocity is poorly constrained but is not greater than 7.5 km/s. Therefore, we may consider the crustal structures imaged by the Serre profile as representing a stack

L. Cernobori et al. / Tectonophysics 264 (1996) 175-189 179

of strongly deformed and sheared metamorphic units related to the Apennine collisional orogeny.

3. STREAMERS data acquisition and processing

Previous discussion of crustal structure beneath the Ionian basin was limited by shallow penetration seismic of the 1970's which was shot with low source power and low fold coverage (Finetti, 1982). More vigorous programs, employing the full power of multi-channel reflection seismic, have recently been effective in imaging the crust underneath the oceans (Rosendhal et al., 1992). These images of oceanic crust have highlighted sharp lateral variations in the form of dipping events which cut through the crust implying a non-uniform structure for oceanic crust (McBride et al., 1994).

The acquisition of the STREAMERS data was carried out with the Geko-Prakla M/V Bin Hai 511 using a 180-channel 4.5-km streamer and a 7118- inch 3 tuned airgun source with shot spacing of 50 or 75 m providing high-fold coverage (3000% or 4500%). The six lines (ION-l, -2, -3, -4, -5, -6, in Fig. 1) show good signal quality down to the lower crust, due to the very large volume and areally extensive airgun array. BIRPS was the technical operator.

The major results obtained include: penetration through the whole crust of the Ionian Sea, resolution of the deep framework of the basin margins, hints regarding the sediment/lower-crust relations, and the successful acquisition of coincident near-normal in- cidence and wide-angle data that allowed landward extension of the marine coverage.

The processing sequence included conventional techniques with minor modifications designed to en- hance deep reflections. One important step was the array simulation applied as a time variant weighted trace mix, applied in the shot domain to simulate weighted receivers arrays up to 200 m overall length. This mixing suppressed steeply dipping back-scattered noise and helped solve spatial alias- ing problems when frequency-wavenumber filtering was required. The post-stack processing was aimed at the enhancement of the low-frequency lower-crust reflecting band through resampling at 16 ms and the application of robust band-pass filters. A further application of an adaptive AGC (AAGC) enhanced the

main reflectors and provided the basis for the line drawing interpretation.

Advanced processing was applied only to the ION-1 line. Here a deconvolution before stack (D.B.S.: operator length of 300 ms, lag of 32 ms with two windows) was tested. This deconvolution operator has improved the image of the upper sedimentary crust but did not affect the lower-crustal reflectivity. A decisive factor in the processing sequence was the cancellation of the first coherent sea-bottom multiple which has been obtained mak- ing use of the median filtering technique normally utilised in VSP data processing. The data were first aligned, according to the moveout of the first multiple, and then the linearised coherent events were rejected according to specified moveouts. An inverse NMO correction restored the original position of the signals. Removing the high-energy first multiples from the sections usually generates a 'shadow zone' in the amplitudes which cannot be recovered by standard scalers. The adaptive AGC, utilising two windows of different length (the shorter one being adopted for the shadow zones), has counterbalanced this effect.

The processing has been done in Athens by the Petroleum Corporation of Greece (DEP-EKY) and at the DINMA-University of Trieste.

4. Profile examples and line drawing interpretation

The line drawing of the STREAMERS Ionian profiles is given in Fig. 2. The line-drawing represents a first interpretation of the seismic lines, mark- ing only the main reflections that have good spatial correlation. Only on line ION-l, where the reflection layering is sub-horizontal and without complex tectonic disruptions, it was possible to recognise stratigraphic units (Fig. 3). The stratigraphic correlation was made following the interpretation given by Casero et al. (1984, 1988). The topmost horizon (Fig. 3) corresponds to the base of the Late Pliocene and Quaternary clays. Beneath this we have identified the Early Pliocene Trubi unit, and the Messinian salt. This last unit cannot be very thick (only a few hundred metres) because there is no evidence of salt domes and, based on outcrops in Sicily and Calabria, we prefer the interpretation of a quasi-transparent

180 L. Cernobori et al. / Tectonophysics 264 (1996) 175-189

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layer of pre-evaporitic marls and sands, more than 1 km thick, to be the principal stratigraphic component of the Messinian. The pinching-out of the Messinian and of the well-layered Tortonian shales and marls towards the ME (Casero et al., 1988) attests to the importance of the post-Tortonian tectonic evolution of the area. In fact, the Tortonian is locally absent in the wells drilled in the Malta-Ragusa platform (e.g. the well Pilade est 1, Fig, 1). The correlation of the seismic lines to drill holes confirms the continuity through the Escarpment of the Serravallian to Oligocene formations. The Cretaceous carbonates are underlain by Jurassic limestones, but we cannot make inferences about the Triassic or deeper crystalline units. However, Triassic limestone has been collected at the base of the wall of the Escarpment (Bizon et al., 1985).

As the lower-crust reflective band dips towards the ME, it appears to form an arch, which becomes more dramatic towards the northern tract of the ME (Cernobori et al., 1994). In the middle of the ION-1 profile, the base of the crust can be identified at 16- 17 km depth. An important tectonic lineament (L in the section of Fig. 3) separates this part of the Ionian

abyssal plain from the Calabrian arc structures and the central Ionian Sea. A strike-slip interpretation has been made for this feature (see also C.N.R., 1983) and its recent activity is suggested by the presence of syn-rift deposits apparently of Pleistocene age (P in Fig. 3). The lineament sharply cuts the stack of the reflectors below the Messinian pre-evaporitic deposits down to the base of the crust. To the east of it, we enter a severely mobilised domain, where traces of reflective patterns of the carbonate formations of the Ragusa-Malta area are still recognisable and where the topography of the base of the crust appears variable because of deformation and/or velocity pull-up or pull-down in the reflection times by lateral velocity variations.

Profile ION-2 (Fig. 4) moves northward into the Calabrian arc domain. Evidence of north-verging thrust planes appears on the seismic line. The thick sediments consist of a series of thrusts that affect the sedimentary cover. The sediments are piled onto the front of the basin and are severely deformed as suggested by the very low P-velocities (Makris et al., 1986). We have not recognised any outstanding feature beneath the deformed sediments, other than

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the lower-crustal reflecting band which is still quite visible and confirms the depth of the Moho, situated at the base of reflectivity, at 18 to 20 km along the line. Therefore, we have a slight crustal thickening towards Calabria.

Profile ION-3 (Fig. 5) furnishes an image of the interaction of the Ionian with the Catabrian block. The superficial, unconsolidated sediments overlying this crustal block are affected by major thrusting and a wide piggy-back basin (the Spartivento basin). This basin is related to accommodation processes occurring at the rear of the accretionary wedge which maintain the basin's stable geometry in response to

underplating. The line shows only a few indications of the seaward extension of the Calabrian block with southward-dipping reflectors recognised at 7 to 10 s. An important flexure of the Ionian lower crust can be documented from the topography of the band of lower-crustal reflectors. The time-deepening can be partially due to the velocity pull-down of the sedimentary pile of the arc, but a true dip of approximately 18% over 60 km distance appears to be genuine, i.e., Moho deepening from about 20 km to 30 km. The average velocity used for the depth conversion (approximately 5 km/s from the sea bottom) has been obtained from the stack ve-

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184 L. Cernobori e t aL /Tectonophysics 264 (1996) 175-189

locity analyses integrated with the DSS data from Makris et al. (1986). The abrupt termination of the deep-crustal signature, typical of the Ionian domain, below the southern tip of the Spartivento basin, can be related to a poor signal-to-noise ratio at 15 s. It could also be an effect of a sharp velocity increase in the strata corresponding to the overthrust Calabrian crystalline units, or possibly to extreme dips, or even

to tectonic discontinuities at depth in the collision zone.

Similar images, from the eastward extension of the Calabria block, appear on line ION-4 (Fig. 6). Offshore well data (Luciana 1: AGIP, 1977) validate the presence of crystalline units, which can be easily followed along the line (dipping eastwards from 1 to 5 s). The sediments accumulated over the top of the

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L. Cernobori et aL /Tectonophysics 264 (1996) 175-189 185

block represent Plio-Quaternary clays, Messinian sands, shales and limestone, Tortonian marls and shales and a stack of other units, probably comprising Mesozoic carbonates, cropping out in Calabria (C.N.R., 1983) and transported downslope by gravity sliding. The image of internal structures of the crystalline crust down to the base of the crust at 21-22 km depth (about 10 s) are similar to those recorded on land in the Serre (Ltischen et al., 1992). The dip of these structures (approximately 10 ° towards the southeast) is also confirmed. At the eastern termination of the ION-4 profile we observe the collision structures within the Ionian crust. Here, we invoke the aid of the refraction data (Ferrucci et al., 1991), as the poor signal-to-noise ratio of the reflection data

provides only sparse information. According to the wide-angle data, the Ionian Moho is again at 30 km depth with a termination just beneath the seaward extension of the Calabrian crystalline crust (thick black dot in Fig. 6).

The signature of the Ionian lower crust is again well-imaged on profile ION-5. The flexure of the Ionian Moho is of the order of 15 ° over a distance of about 60 km. The Calabrian arc structures are also well described, with large thrusts transporting consolidated sediments, as implied by the higher velocities, 2.7 to 4.0 km/s, derived from the stacking velocities and from Makris et al. (1986).

On the profile ION-6 (Fig. 7) we return to a very thin crust (Moho at a depth of about 16 km,

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sea bottom at 3 kin, as on ION-l). On this line we do not observe any important influence from the Calabrian arc, but the seismic response does not reveal significant layering or bedding of the sedimentary crust. The lower-crust reflective band is clearly arched with indication of dips both towards the Calabrian arc (west) and the Hellenic arc (east), a pattern comparable to that described by Ferrucci et al. (1991, Fig. 3).

5. Sea-land connections

Land stations in Calabria, emplaced on outcrops of the granitic basement, recorded the sea airgun shots when the ship was approaching the coast from the south (line ION-3) and when it was steam- ing away towards the east (lines ION-4 and -5). A single station (Reftek jr.) with Mark-L4-3D/2 Hz geophones and one DFS-V recording system with 48 channels, two tape units and sixteen Mark L4-3D/2 Hz geophones distributed in a large array of 540 x 300 m, were installed. The recording conditions were extremely severe with repeated interruptions of the operations at sea, an unfavourable environment and bad weather conditions with a very strong wind.

An example of the data is shown in Fig. 8. The records were stacked for all the sixteen stations of the array and summed (weighted mix) for five con- secutive shots in order to increase the S/N ratio. The time section was corrected for the water depth with the replacement of the water by sediments having a velocity of 3.0 km/s. The maximum source-receiver offset, for which a coherent signal was recorded, was about 82 kin, not enough to obtain a refracted first arrival from the base of the crust. Bright wide-angle multi-cyclic reflections from the base of the crust were obtained in the LMO sections (Fig. 8a, with a reduction velocity of 6 km/s), from the shots along the line ION-4 and the beginning of the line ION- 5. In the NMO section (Fig. 8b, obtained after the application of a velocity function with 4350 m/s at twt = 0 s, 5700 m/s at twt = 12 s) the base of the crust corresponds to a reflective marker at about 10

s, which correlates to the reflectors seen in the ION-4 multichannel line (about 22 km depth).

6. Concluding remarks

Earlier problems of signal-to-noise ratio and penetration in the crust of the Ionian Sea and its margins could be overcome by using up-to-date oil exploration reflection seismic techniques. A well tuned source, with a large airgun array supplying ca. 120 1 per shot and coverage as great as 30- or 45- fold with a 4.5-km streamer were crucial. Advanced data processing improved the definition of the seismic stratigraphy within the basin and also of the structures at the collision margin. A highly reflective interval within the lower crust has been recognised. This reflective interval terminates against the margins that have been deformed by collision or where the crust thickens towards a passive continental margin (i.e., central tract of ME). Its topographic variations suggest recent mobilisation of the mantle possibly associated with extensional structures of the northern tract of the ME. Alternatively, the topographic variation may be linked to the tectonically controlled subsidence of the basin such as transcur- rent motion along crust-cutting faults in response to the stresses applied at the active margins (Ceruobori et al., 1994), and to the loading of materials on a thin, cold, and brittle crust. We noticed the differences with the images of the deep-crustal data in the Mesozoic eastern Atlantic Ocean. Comparative data in the Mediterranean do not confirm the presence of a reflective band in the 'oceanic' areas (de Voogd et al., 1991), but landward-dipping reflectors and crustal thickening by a factor greater than five were observed across the passive continental margin in the Gulf of Lions. These features can be compared with the seismic data on the prolongation of the ION-1 line across the ME (Nicolich et al., 1995).

The rapid subsidence of the Ionian basin still remains unclear within the framework of the reconstruction of Dercourt et al. (1986): why did the rotation of Apulia with respect to Africa, during the

Fig. 8. Refraction and wide-angle data from land stations (array of 16 geophones, TS-1 in Fig. 1). In (a) a LMO correction with a velocity reduction of 6 km/s was applied. In (b) a NMO correction was utilised for the correlation and landward extension of the ION-4 profile.

L. Cernobori et a l . /Tectonophysics 264 (1996) 175-189 187

• 000

5.000

18 k m offset from land station 82 . . . • , . - ..:~.~.~::..~:,:'..~::.~ ~ . . - z . ' . . : ' ~ - . - . z ~ . ~ ? ~ : S ~ . : ~ . ~ . -

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km • 000

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5 . 0 0 0 0 S 1 0 1 5 2 0 KM

I

i0.000 1 0 . 0 0 0

t 5 . 0 0 0 15. 0 0 0

tec4914 Fig. 8

b )


Aptian , result in rapid subs idence in the adjacent

bas in? In the course o f ex tens ion (Late Jurassic) ac-

c o m p a n i e d by the th inning of the l i thosphere , the

roo f o f the under ly ing as thenosphere m o v e d up-

ward. The man t l e mater ia l upl i f ted at the roo f o f

the as thenosphere cou ld have par t ia l ly mel ted , ow-

ing to the reduc t ion o f pressure. Possibly, a por t ion

o f the m a g m a t i c mel t pene t ra ted through the crust

and led to the vo lcan ic act ivi ty in this area in the

Late Jurassic . This m a y have pervas ive ly affected

the lower por t ion o f the crust chang ing its petro-

logic and phys ica l character is t ics , as wel l as those

o f the lower l i t hosphe re -uppe r as thenosphere , es-

pec ia l ly when the t empe ra tu r e -p re s su re condi t ions

changed. Tec tonic subs idence o f the basin has devel -

oped subsequent ly . This final tec tonic stage o f basin

fo rmat ion does not contradic t the mode l o f Dercour t

et al. (1986) but ra ther c o m p l e m e n t s the mode l by

exp la in ing ver t ica l m o v e m e n t s wi th in the crust of the

Eastern Medi te r ranean .

Acknowledgements

We are indeb ted to M. Loukoyannak i s and Dep-

Eky for the data process ing . Thanks are due to

the G e c o - P r a k l a s taff and crew of the vesse l Bin-

Hai 511. We are grateful to H.B. Hi r sch leber and

to an a n o n y m o u s r ev i ewer for their helpful com-

ments and sugges t ions that improved the manuscr ipt .

Funds for the data acquis i t ion have been main ly pro-

v ided by EC contract S T R E A M E R S , J O U 2 90-CT- 00132 (contractors : A. H i m , IPG de Paris; E. Banda,

C.S.I .C. Barce lona ; D. Blundel l , R H B N Univ. o f

London ; L.A. M e n d e s Victor, Univ. o f Lisbon; R. Nico l ich , D I N M A Univ. o f Tr ies te ; J. Drakopou-

los, Univ. o f Athens ; N. La lechos , PPC Athens)

and for the data p rocess ing by E C contract PRO-

F I L E S , J O U 2 -CT93-0313 (Contractors: A. H im,

IPG de Paris; R. Nico l ich , D I N M A Univ. o f Trieste;

A. Lymberopou los , PPC Athens) . Addi t iona l funds were also p rov ided by the authors ' inst i tut ions and

nat ional research p rog rammes .

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J.H., Cernobori, L. and the STREAMERS/PROFILES Groups, 1995. Appraisal of a new low frequency seismic pulse generating method on a deep seismic reflection profile in the central Mediterranean Sea. First Break, 13(7): 277-290.

Bizon, G., Muller, C. and Vieban, F., 1985. Les s6diments M6sozo'iques et C6nozo'/ques de Mer Ionienne (Campagne Es- canned 3: Escarpement de MaRe, Mont Alfeo et Monts de Medine). Etude biostratigraphique: foraminifers, nannoplanc- ton, microfacies. Rev. Inst. Fr. Petrol., 38(5): 575-602 (in French, with English abstract).

Boccaletti, M., Nicolich, R. and Tortorici, L., 1984. The Cal- abrian Arc and the Ionian Sea in the dynamic evolution of the central Mediterranean. Mar. Geol., 55: 219-245.

Carabelli, F., Migani, M. and Moia, E, 1988. Rete sismica ENEL di Gioia Tauro: primi risultati del rilevamento sismico. Quaderni ISMES, 235.

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Casero, P., Cita, M.B., Croce, M. and De Micheli, A., 1984. Tentativo di interpretazione evolutiva della scarpata di Malta basata su dati geologici e geofisici. Mem. Soc. Geol. Ital., 27: 233-253 (in Italian, with English abstract).

Casero, P., Cita, M.B., Croce, M., Frisia, S., Hieke, W. and Nicolich, R., 1988. Malta Escarpement, Alfeo Sea-Mount and Victor Hensen Sea-Hilt: a key to plate tectonic evolution of the western and eastern Med. since Mesozoic. ODP proposal presented at ECOD and CIESM workshops, unpublished report.

Cassinis, R. and Ranzoui, A., 1987. Contribution of controlled source seismology to the study of seismogenesis: examples from the Italian transitional area. Tectonophysics, 140:81-91.

Cernobori, L., Nicolich, R., Romanelli, M., Him, A., Sachpazi, M., Avedik, F. and Gallart, J., 1994. The Sicilian margin of the Ionian basin and Mt. Etna. EGS/1994-Grenoble, Ann. Geophys., SE2, C36, Abstr.

C.N.R., 1983. P.F. Geodinamica, Structural Model of Italy, sheet n.6, SELCA, Firenze.

Della Vedova, B. and Pellis, G., 1992. New heat flow density measurements in the Ionian Sea. Ani VIII Convegno GNGTS, pp. 1133-1145.

Dercourt, J., Zonenshain, L.P., Ricou, L.E., Kazmin, V.G., Le Pichon, X., Knipper, A.L., Grandjacquet, C., Sborshikov, I.M., Geyssant, J., Lepvrier, C., Pechersky, D.H., Boulin, J., Sibuet, J.C., Savostin, L.A., Sorokhtin, O., Westphal, M., Bazhenov, M.L., Lauer, J.P. and Biju-Duval, B., 1986. Geological evolution of the Tethys belt from the Atlantic to the Pamirs since the Lias. Tectonophysics, 123: 241-315.

de Voogd, B., Nicolich, R., Olivet, J.L., Fannucci, F., Burrus, J. and ECORS-CROP Working Group, 1991. First deep seismic reflection transect from the Gulf of Lions to Sardinia. AGU, Continental Lithosphere: Deep Seismic Reflections. Geody- namics, 22: 265-273.

de Voogd, B., Truffert, C., Chamot-Rooke, N., Huchon, P., Lalle- mant, S. and Le Pichon, X., 1992. Two-ships deep seismic soundings in the basin of the Eastern Med. Sea (Pasiphae cruise). Geophys. J. Int., 109: 536-552.

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Ferrucci, F., Gaudiosi, G., Him, A. and Nicolich, R., 1991. Ionian Basin and Calabrian Arc: new elements by DSS data. Tectonophysics, 195: 411--419.

Finetti, I., 1982. Structure, stratigraphy and evolution of Central Mediterranean. Boll. di Geof. Teor. ed Appl., XXIV, 96: 247- 312.

Gealy, W.K., 1988. Plate tectonic evolution of the Mediterra- nean-Middle East region. Tectonophysics, 155: 285-341.

Locardi, E. and Nicolich, R., 1988. Geodinamica del Tirreno e dell'Appennino centro-meridionale: la nuova carta della Moho. Mem. Soc. Geol. Ital., 41: 121-140.

Ltischen, E., Nicolich, R., Cernobori, L., Fuchs, K., Kern, H., Kruhl, J., Persoglia, S., Romanelli, M., Schenk, V., Siegesmund, S. and Tortorici, L. (Italian~3erman lower crust group), 1992. Calibration of a seismic 3-component reflection- refraction experiment on the exposed lower crust in Calabria. Terra Nova, 4: 77-86.

Makris, J., Nicolich, R. and Weigel, W., 1986. A seismic study in the Western Ionian Sea. Ann. Geophys., 6, B: 665~578.

McBride, J.H., White, R.S., Henstock, T.J. and Hobbs, R.W., 1994. Complex structure along a Mesozoic sea-floor spreading ridge: BIRPS deep seismic reflection, Cape Verde abyssal

plain. Geophys. J. Int., 119: 453478. Morelli, C., Gantar, C. and Pisani, M., 1975. Bathymetry, gravity

and magnetism in the Strait of Sicily and in the Ionian Sea. Boll. Geofis. Teoret. Appl., 17: 39-58.

Nicolich, R., Avedik, E, Cernobori, L. and Him, A., 1995. Ionian basin deep crustal structures and its western margins. Rapport du XXXIV Congr~s de la CIESM, 34:113 (abstr.).

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Reuther, C.D., Ben-Avraham, Z. and Grasso, M., 1993. Origin and role of major strike-slip transfers during plate collision in the central Mediterranean. Terra Nova, 5: 249-257.

Rosendhal, B.R., Meyers, J., Groschel, H. and Scott, D., 1992. Nature of the transition from continental to oceanic crust and the meaning of reflection Moho. Geology, 20:721-724

Truffert, C., Chamot-Rooke, N., Lallemant, S., de Voogd, B., Huchon, P. and Le Pichon, X., 1993. The crust of the West- ern Mediterranean Ridge from deep seismic data and gravity modelling. Geophys. J. Int., 114: 360-372.

ELSEVIER Earth and Planetary Science Letters 168 (1999) 243–254

Foredeep geometries at the front of the Apennines in the Ionian Sea(central Mediterranean)

Carlo Doglioni a,Ł, Saverio Merlini b, Giuseppe Cantarella b

a Dipartimento di Scienze della Terra, Universita La Sapienza, P. le A. Moro 5,00185, Rome, Italy

b ENI–AGIP, V. Emilia 1, 20097, San Donato Milanese, Italy

Received 29 January 1999; revised version received 1 March 1999; accepted 3 March 1999

Abstract

A new regional seismic section in the Ionian Sea across the Apennines belt and related foreland shows how the presentforedeep geometry may be an example for interpreting discontinuous, tilted and deformed earlier basins now incorporatedin the internal parts of the accretionary wedge. Onlap stratal terminations of the foredeep sediments on the forelandmonocline may simulate downlap geometries once involved and tilted by back-thrusting. The geometry of the Ionianforedeep is controlled by the dip of the regional monocline, and internally by the variable dip and length of the limb of theexternal fold, which may be either foreland-verging or hinterland-verging. The generation of a new fold within the foredeepsplits the basin into a new foredeep toward the foreland and a thrust-top basin toward the hinterland. The thrust-top basindimension is primarily controlled by the distance between the two folds and related thrusts at its margins. The foredeep, inits overall history, is composed by a series of concave heterogeneous lenses, progressively displaced and piled up towardthe foreland to the east. The formation of each sedimentary lens is controlled by the development of a new fold and thecontemporaneous retreat of the regional monocline which creates new accommodation space. The complex 3D geometryof the Apennines foredeep mainly results from lateral variations of the latter parameters, as well as variations in sedimentsupply and eustasy. 1999 Elsevier Science B.V. All rights reserved.

Keywords: fore-arc basins; thrust faults; seismic profiles; Apennines; Ionian Sea

1. Introduction

Foredeeps are those basins located at the mar-gin of orogens or accretionary wedges, and they arecharacterized by lens shaped clastic sedimentary se-quences which are controlled by a number of wellknown factors, i.e., subsidence, sediment supply, eu-stasy and climate [1]. The internal geometry of these

Ł Corresponding author. Tel.: C39-6-4991-4549; Fax: C39-6-4454-729; E-mail: [email protected]

active margin basins is well differentiated from thatof passive margin basins, both in terms of lithologyand geometry of the basin. Foredeeps are locatedand propagate on top of a regional monocline [2]usually dipping toward the interior of the belt withangles ranging between 1º and more than 10º [3].The internal geometries of foredeep basins are no-toriously controlled by growth folds (e.g., [4,5]).The Apennines accretionary wedge presents a Plio–Pleistocene somewhere 8 km deep basin which un-derwent subsidence rates higher than 1 mm=yr. This

0012-821X/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 0 5 9 - X

244 C. Doglioni et al. / Earth and Planetary Science Letters 168 (1999) 243–254

Fig. 1. Location of the seismic section M5 (Fig. 2) in the frame of the Apennines Arc. The arc migrated ‘eastward’ since the earlyNeogene. In grey is the frontal compressive part of the Apennines accretionary wedge which is followed to the west by an extensionaltectonic wave.

foredeep provides excellent examples of geometries[6] which may occur in similar basins.

During the last decade, a large effort for studyingthe crust of Italy and the surrounding seas (ProjectCROP) has been performed by a pool composedby the Consiglio Nazionale delle Ricerche of Italy,CNR, the national oil company, Agip, and the na-tional electric company, Enel. The several seismiclines provided new insights on the structure of theItalian crust. Offshore sections are the most read-able seismic lines, and in this paper we present inparticular section M5 (Fig. 1) at the front of the off-shore Apennines in the Ionian Sea. This paper aimsto present in particular a few details of this sectionwhich highlights original geometries and kinematicsof the foredeep evolution (Fig. 2).

2. The Apennines accretionary wedge

The Apennines belt is an arc shaping the Ital-ian Peninsula, from Piemonte-Monferrato in north-ern Italy, down to the northern Africa-Maghrebides(Fig. 1). The arc formed on top of a west-directedsubduction zone which retreated ‘eastward’ duringthe last 30 Ma [7–12]. The convex part of the arc isthe area where the roll-back of the subduction hingehas been maximum. The most arcuate part of theApennines Arc is Calabria. The arc migrated ‘east-ward’ about 775 km during the Late Oligocene ina section crossing northern Calabria, the TyrrhenianSea, Sardinia, and the Provencal basin [12]. Thisvalue decreases moving either toward the northernApennines or to the south, toward Sicily and theMaghrebides. The southern Apennines and Calabria,

C.D

oglioniet

al./Earth

andP

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168(1999)

243–254245

Fig. 2. Regional seismic section Crop M5 of the Ionian Sea across the Apennines accretionary wedge and its Apulian foreland. See location in Fig. 1. Note that the accretionarywedge is lower than the foreland.


being located in the most arcuate part of the belt,travelled eastward at the fastest rates (3–4 cm=yr;[11,13,14]. The earlier foredeep stages were locatedmore internally and to the west they are now aban-doned due to the high speed of the roll-back. Withinthe southern Apennines and Calabrian Arc there areremnants of the earlier foredeep since the EarlyMiocene (‘pre-Irpinian’) [15], as it was observedin the central-northern Apennines [16]. The fasteastward advancement of the southern Apenninesforedeep is clearly evidenced by highly shortenedPlio–Pleistocene deposits, originally located in theinternal side of the foredeep, or discovered in wellswhere the Pliocene sediments have been overriddenby Mesozoic thrust sheets.

The Apennines foredeep has been interpreted asgenerated by the ‘eastward’ roll-back of the hingeof the subduction induced by the slab pull or by therelative eastward mantle flow [14,17]. There the sub-sidence is among the fastest on the Earth’s surfaceand it exceeds 1 mm=yr [3]. The foredeep eastwardmigration in the Apennines should reflect the ve-locity of the roll-back of the Adriatic slab, whichretreated faster in its southern part where the Ionianoceanic lithosphere was undergoing subduction, incontrast with the slower northern continental part.This appears to be true both for the early Neogenehistory of the foredeep but also for the later-to-pre-sent stages of the subduction [18].

3. Seismic section M5 in the Ionian Sea

This seismic section is one of the most completelines across the Apennines front and its foreland.The line runs from offshore eastern Calabria (south-ern Italy) to the northeast, offshore southern Puglia.In the Calabrian part the section crosses extensionalfaults which are well known inland to the northin Campania and Basilicata to form grabens (e.g.,the Vallo di Diano, Val d’Agri) and be responsiblefor the high seismicity of the southern Apennines[19]. Moving northeastward, the section exhibits anirregular seafloor, indicating active or very recenttectonic activity. Below the Messinian unconformity(Fig. 2) back-thrusts deform the Miocene sequencesand they determine the formation of triangle zonesin the central part of the section. The front of the ac-

cretionary wedge is marked by an eastward-vergingthrust overriding the narrow foredeep deposits of theTaranto trench. (Fig. 3). The foreland is character-ized by a steep westward dipping monocline made ofcontinental crust with an about 6 km thick MesozoicApulian carbonate platform and a thin sequence ofTertiary calcarenites and limestone. A few normalfaults also disrupt the seafloor of the foreland whichis even more elevated than the accretionary wedge tothe west (Fig. 2).

From the main section, two details have beenextracted in order to have clearer views of the frontof the accretionary wedge and the present foredeep(Fig. 3) and a more internal part of the wedge whichwas formerly the front of the belt, probably duringthe Late Miocene (Fig. 4). These data allow us togive a look into the complicated geometries of theaccretionary wedge. Dating of sediments is based onprojected unpublished Agip wells.

The frontal section (Fig. 3) shows a 8.5 km wideforedeep, with a seafloor about 2350 m deep, andsediments onlapping the westward-dipping mono-cline. The onlap shows a progressive eastward dis-placement. The accretionary wedge is composed ofPlio–Pleistocene sediments. The internal detail ofthe section (Fig. 4) has been migrated and depth-con-verted. It is characterized by two main back-thrustsverging toward the southwest, generating two trian-gle zones. Between the two hinterland-verging rampsthere is a basin which is made of two wedges, thelower pointing toward the foreland, the upper point-ing toward the hinterland. The lower wedge appearsas an earlier external foredeep now involved by theaccretionary wedge, and it is limited at the base byapparent downlap stratal terminations at about 5 kmdepth. The overlying upper wedge is a later thrust-top basin onlapping the lower wedge with stratalterminations shifting in the opposite direction towardthe southwest.

4. Discussion on the M5 section

Fig. 5 is an interpretation of the main featuresvisible on Fig. 4. This spectacular section indicatesthat the two-stage basin shape is controlled by thedistance between the two back-thrusts in the up-per wedge, that this upper basin formed during the

C. Doglioni et al. / Earth and Planetary Science Letters 168 (1999) 243–254 247

Fig. 3. The frontal thrust and related fold of the Apennines accretionary wedge in the Ionian Sea with the related foredeep. Deep faciessediments onlap the foreland monocline. See location in Fig. 2.

growth of the two anticlines (in particular it pinches-out on the back-limb of the internal anticline tothe southwest), and that the lower foreland pinch-ing-out wedge has an apparent downlap of sedi-ments toward the right. The Messinian unconformityeroded the two folds and post-dates the underly-ing sediments. The Miocene sequences are laterallydisplaced, with the younger one more to the right,

toward the foreland, and laterally overlying the mar-gin of the previous internal section. Therefore, therelative depocentres are progressively displaced to-ward the east. Fig. 6 proposes an interpretation of thekinematics of the geometries occurring in Fig. 4: theoriginal foredeep sedimentary wedge (first stage) isincorporated and tilted by a back-thrust with syntec-tonic sedimentation (second stage). Original onlap


Fig. 5. Interpretation of Fig. 4. The original sediments onlapping the foreland have been tilted by the ramp of the back-thrust of a trianglezone. They now simulate an apparent downlap plane (foreland pinching-out wedge), Miocene 1. While it uplifted, the growth fold to theleft determined a wedge of sediments onlapping its internal limb and pinching-out in the opposite direction toward the left (hinterlandpinching-out wedge), called Miocene 2. The section may be divided into two main sedimentary packages which are differentiated on thebasis of their tectonic control.

geometries are tilted to appear as downlap strata ter-minations. The first sedimentary wedge is thinningtoward the foreland, whereas the second overlyingbasin is thinning toward the hinterland on the back-limb of the fold. Therefore, the first stage shouldcorrespond to the external foredeep located on theforeland monocline and limited to the southwest bythe frontal fold of the accretionary wedge. The sec-ond stage of the basin formed instead on top ofactive thrusts, and the first-stage foredeep became apiggy-back basin confined by two hinterland-vergingstructures (Fig. 6). These geometries and kinemat-ics may be a key to unravel along-strike inlanddeep geometries in the southern Apennines, e.g., the

Fig. 4. Detail of internal part of the Apennines accretionary wedge in the Ionian Sea. This is characterized by triangle zones. M DMessinian unconformity. See location in Fig. 2. This section has been migrated and depth-converted.

Sant’Arcangelo basin where onshore seismic dataare of lower quality but the outcrops are spectacular[20,21].

4.1. Internal geometries of the thrust belt

Within a foreland-propagating accretionary wedgethere may form back-thrusts. When associated to abasal decollement, a back-thrust forms a triangle zone[22]. This feature is well developed within the M5seismic section (Fig. 5). The association of thrustsmay generate different combinations of vergenceswhich may be entirely or partly opposite to the accre-tionary wedge main direction of propagation (Fig. 7).


Fig. 6. Kinematic interpretation of the geometric structural and stratigraphic pattern of Fig. 4. The frontal early foredeep stage thinningtoward the right is interpreted as the present setting at the front of the Apennines accretionary wedge. The front could have later beeninvolved by a back-thrust. This generated the opposite pinching-out of the sedimentary packages associated with the two-stage evolution.Original onlap geometries then appear as a downlap plane. The intra-folds basin is contemporaneously filled by a wedge thinning towardthe west. The shape and width of the basin is controlled by ramp distances and vergences.

Basin margins are shaped by the dip and evolution ofthe fold limbs: internal limbs are longer and less steepwith respect to frontal fold limbs which are frequentlyshort and overturned in fault-propagation folds [23].

In one of his classic papers on thrust belt struc-tural analysis, Mitra [24] shows how the distancebetween the ramps and the ramp length control thefinal internal geometry of an imbricate fan or a du-

plex. When the distance between the ramps is largeand the displacement of the associated thrusts smallenough to maintain an undeformed flat sequence be-tween the back limb of the external fold and thefore-limb of the internal fold, there may form a syn-folding basin, e.g., a thrust-top basin, or also calledpiggy-back basin [25]. In this respect, when the dis-tance between the ramps varies, the basin dimension


Fig. 7. Two examples (A and B) in which M1 and M2 are sedimentary packages differentiated by the growth of the second fold whichsplits the basin into two sub-basins. The depocentre of M2 is shifted toward the foreland with respect to M1. The central piggy-backbasin forms on top of older external foredeep sediments and it is controlled by the distance of the two ramps. (A) The fold vergencescoincide with the regional vergence. (B) The fold vergences are opposed to the regional vergence. The foredeep sediments are thinningboth toward the foreland monocline and toward the back-limb of the frontal fold which is longer and less steep with respect to example(A). The scale is based on the examples included in Fig. 2.

also varies. In other words, the shape and dimensionof a piggy-back basin forming on the top of an ac-cretionary wedge may be analyzed in terms of rampdistances. In Fig. 4 it is evident that the width of thebasin (about 7 km) confined between the two anti-clines is proportional and determined by the distance

between the two folds which is in turn controlledby the distance between the ramps of the two mainthrusts (about 11 km).

The foreland migration of the southern Apenninesforedeep since the Miocene to recent times gener-ated a variety of lens-shaped basins progressively


displaced eastward. This is evident inland, for in-stance for the Tortonian Gorgoglione Flysch [26],and it occurs at the front of the northern Apenninesin the Po basin [6]. As already demonstrated by RicciLucchi [16] for the northern Apennines, the onlap onthe regional monocline contemporaneously migratedtoward the foreland to the east. The basins shapes arecontrolled by thrust-ramps distance, fold vergence,dip of the foreland monocline, and the along-strikevariation of the former parameters. Two thrusts canlaterally branch into a single thrust and the distancebetween the two ramps may decrease to zero; there-fore, also the possible associated thrust-top basinnarrows (see the example of the Gorgoglione Flysch[26]). Usually the distance among major ramps inthe Apennines ranges between 5 and 25 km at thefront of the accretionary prism. Large thrust sheetsof more that 40–50 km of displacement have alsobeen interpreted, which implies an equal amountof ramps distance. The Apennines foredeep sedi-ments are sometimes so irregular to form severalisolated sub-basins. This could partly explain thelarge number of formational names which have beenintroduced in the literature to describe the Neogeneand Quaternary foredeep stratigraphy of this belt.This large variety may be explained by the frequentundulations of the thrust belt, along transfer zonesinduced by the inherited lateral variations in therheological parameters of the pre-existing Mesozoicstratigraphy and the underlying basement.

4.2. Foredeep evolution

The foredeep basin is in general a sedimentarywedge bounded internally by the frontal thrust, whileit expands laterally toward the foreland, with a pro-gressive onlap of the foredeep sediments [27]. Theformation of a new anticline within a foredeep basinmay generate two sub-basins, a new more externalforedeep sensu stricto, and a thrust-top basin (Fig. 7)which forms on top of the earlier frontal foredeep.From that moment on, the two basins are also dif-ferentiated in terms of sediment supply from theforeland monocline, the hinterland, or other exter-nal sources along the foredeep axis. This is usuallycontextual to a progressive retreat of the forelandmonocline which determines subsidence and newaccommodation space. The stratigraphic packages

may be punctuated and differentiated in shape bythe growth of the new fold, which has a given dis-tance with respect to the pre-existing internal fold.The frontal fold may have either foreland vergence(Fig. 7A) or hinterland vergence (Fig. 7B). The maindifference is the onlap on the frontal fold which oc-curs respectively on the forelimb or in the back-limb.Back-limb is less steep and longer with respect to theforelimb, and this generates different shapes of theinternal margin of the basin. The schematic sectionsof Fig. 7 represent a model of the kinematics of fore-deep propagation based on the geometries observedin the M5 seismic section. In particular Fig. 5 showsthe effect of back-thrusting in generating an isolatedsub-basin (Fig. 7B).

The development of a fold may start at anymoment during a third-order sea-level fluctuation.Therefore, the eustasy-controlled depositional se-quences [1] may be perturbed by the isolation ofnew basins which determine variations in the sedi-ment supply and in the water depth in the area of thegrowing fold.

5. Conclusions

The seismic section M5 of the Apennines accre-tionary wedge in the Ionian Sea (Fig. 2) supportsthe interpretation that the geometry and kinematicsof foredeep and piggy-back basins are primarily con-trolled by the dip of the foreland monocline, thedistance between thrust ramps, and the vergence ofthe single folds with respect to the regional vergencewhich may be either in the same direction or op-posite. Subsidence in foredeeps controls the dip andretreat rate of the foreland monocline, parameterswhich are primarily determined by the subductiontype [3]. Different geometries of basins associatedwith active margins result by the combination andvariation of these parameters and the other sedimen-tary factors such as sediment supply and eustasy.As it is a classic rule in geology, the present ge-ometries of a foredeep may be powerful keys inilluminating and interpreting internal shortened com-plex parts of an accretionary wedge (Fig. 6). Theexample of the Ionian Sea foredeep allows us topredict other variable patterns of foredeep geometryand kinematics. A common feature in the Apennines


is the formation of concave lenses progressively dis-placed and piled up toward the foreland to the east(Fig. 7). The stratigraphic packages may be punctu-ated and differentiated in shape by the growth of anew fold, its vergence and distance with respect tothe pre-existing internal fold. In this regard, tecton-ics could deeply influence depositional sequences,independently from eustasy.

Acknowledgements

The paper benefited from critical reviews by ananonymous referee who made a thoughtful reviewand by J.P. Rehault. Many thanks to A. Bernasconi,D. Bernoulli, G. Mariotti, G. Pialli, P. Pieri, C. Sauliand M. Tropeano for helpful discussions. The CNR(grants 97.00246.CT05, 98.00228.CT05) and Murst(cofinanziamento 1997) supported this study. [RV]

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[7] P. Scandone, Origin of the Tyrrhenian sea and Calabrianarc, Boll. Soc. Geol. Ital. 98 (1979) 27–34.

[8] J.P. Rehault, J. Mascle, G. Boillot, Evolution geodynamiquede la Mediterranee depuis l’Oligocene, Mem. Soc. Geol.Ital. 27 (1984) 85–96.

[9] A. Malinverno, W.B.F. Ryan, Extension in the TyrrhenianSea and shortening in the Apennines as a result of arcmigration driven by sinking of the lithosphere, Tectonics 5(1986) 227–245.

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[12] E. Gueguen, C. Doglioni, M. Fernandez, Lithospheric boud-inage in the western Mediterranean back-arc basins, TerraNova 9 (1997) 184–187.

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[20] J.-C. Hippolyte, J. Angelier, F. Roure, P. Casero, Piggy-back basin development and thrust belt evolution: structuraland paleostress analysis of Plio-Quaternary basins in theSouthern Apennines, J. Struct. Geol. 16 (1994) 159–173.

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Tectonophysics 324 (2000) 239–265www.elsevier.com/locate/tecto

Deep structure of the Campanian–Lucanian Arc(Southern Apennine, Italy)

A. Menardi Noguera, G. Rea *ENI–AGIP Division, 20097, San Donato Milanese, Italy

Abstract

Recent exploration activities and successful discovery of large oil fields in the upper Val d’Agri area (SouthernApennine, Italy) have encouraged the collection of large quantities of geological and geophysical data, and theirintegration into comprehensive geological models. Here we present a series of structural profiles based on severalhundred kilometres of seismic lines (two- and three-dimensional ) and the results of many deep wells. The best seismiclines have been extensively reprocessed, and also tied to magnetotelluric data. Geological interpretations of those lineshave been interactively tested, and verified by gravity and magnetic modelling and structural balancing.

The wells and seismic data demonstrate that the fundamental tracts of the deep architecture of the orogen, as itappears today, were developed only at the end of the Late Pliocene. The final closure of the ensialic shear-zoneresponsible for the subduction of at least 150 km of continental lithosphere originally underlying the Lagonegro Basincan be dated as Early Pliocene. The involvement of the Inner Apulian Platform in the core of the belt and theactivation of a new and more external ( left-lateral ) oblique ensialic shear-zone can be constrained to the LatePliocene–Pleistocene. The seismic profiles clearly show the underthrusting of the Outer Apulian Platform unit beneaththe Inner Apulian Platform unit.

The thick-skinned character of the Plio-Pleistocene contractional structures is evident from the structural profiles.This geometry is constrained by gravity–magnetic data and structural considerations, according to the relatively steepangle of dip (about 10°) of the Apulian foreland beneath the chain.

The structural profiles depict the general architecture of the Campanian–Lucanian Arc: the Apennine Platformand Lagonegro Unit are largely allochthonous and a part of the Lagonegro basement could be buried below theApennine Platform sole-thrust. The Neogene plastic successions of the Apennine Platform and Lagonegro Basin weredetached and now outcrop in the Bradanic Foredeep. The Inner Apulian Platform constitutes a huge thrust-sheet,structured as a wide antiform in the core of the belt; its regional culmination roughly corresponds to the Val d’Agriarea. Extensional and transtensional structures cross cut the pre-existing compressional structures as shown by theQuaternary valleys of Vallo di Diano and Val d’Agri.

The profiles shown also highlight the existing contrast between the deep structures in the core of the belt and thetectonic style of the Lagonegro, Liguride and Sicilide Units. These units were in fact subjected to brittle–ductile deformationunder metamorphic (part of Liguridi Units) and deep diagenetic to anchizonal conditions (Lagonegro Units) and wereaffected by a general overthrust-shear directed towards the NE. The Inner Apulian Platform was, on the contrary, involvedin the thrust-belt together with its basement, whereas the Outer Apulian Platform, affected by A-subduction, underwent aregional bending with associated normal faulting. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Neogene tectonics; Southern Apennine; structural evolution; thick-skinned tectonics

* Corresponding author. Present Address: Nigerian Agip Oil Co. NAOC, Mile 4, Ikwerre Road, P.O. Box 923, Port Harcourt,Nigeria, Tel. : 084-236400-9/3226; Fax 084-236400-9/3526.

E-mail address: [email protected] (G. Rea)

0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0040-1951 ( 00 ) 00137-2

240 A.M. Noguera, G. Rea / Tectonophysics 324 (2000) 239–265

1. Introduction B–B∞ (Golfo di Policastro, S.Fele, Venosa). Thesesections were chosen since they cut through theentire thrust-belt–foredeep–foreland system start-The intense exploration work in the Southern

Apennine over the last decade has produced a ing from the innermost areas and traverse the mainoil fields discovered to date. They are thus emi-sharp increase in data and knowledge that has led

to the discovery of the Val d’Agri oil fields. The nently suitable to exemplify the model that hasbeen developed, and which is described below.geological model and structural evolution pro-

posed for the Campanian–Lucanian Apennine isbased on the interpretation of a grid (30×20 km2grid) of regional seismic profiles tied to a large 2. Structure and regional geology of the

Campanian–Lucanian arc (CLA)number of wells. The best seismic lines from thisgrid were reprocessed and migrated with identicalschemes. The interpreted profiles were then depth- The CLA lies between the WNW–ESE-trending

Ofanto River syncline to the north and the E–W-converted and tested through gravity and magneticmodelling. The sections were integrated with data trending structures of the Calabrian Arc to the

south (Figs. 1 and 2). The structural axes of theacquired from stratigraphic revisions carried outin all the available wells and the up-holes drilled CLA trend WNW–ESE and NW–SE in the north-

ern sector and NW–SE and N–S in the southernduring seismic investigations. This integration ofsurface and subsurface geological data allowed part. The Ofanto syncline represents the zone of

separation between the Campanian–Lucanian andconfident interpretation of the seismic horizons ofthe allochthonous cover of the Internal Apulian the Molise Southern Apennine sector. The CLA is

overprinted at the southernmost boundary by thePlatform, the target of hydrocarbon explorationin the Val d’Agri. E–W-trending structures of the Mt. Bulgheria–Mt.

Coccovello–Mt. La Spina alignment. These struc-Selected profiles were subjected to structuralbalancing tests. The interpretation of regional pro- tures are the expression of a thrust sheet trending

NW–SE to the south (Figs. 1 and 2) and deformedfiles were checked at several stages against inter-pretations of magnetotelluric data and finally by a left-lateral strike-slip fault zone known as the

Linea del Pollino (D’Argenio, 1966; Ghisetti andmodified with the aid of detailed seismic gridsacquired, reinterpreted or reprocessed during the Vezzani, 1982; Moussat, 1983).

The Southern Apennine thrust-belt is character-exploration cycle. It was thus possible to givegeological significance to some reflectors and signal ised by several geophysical and geological peculiar-

ities compared with the Alps or the Andean thrust-patterns that would otherwise have been givenlittle consideration. This method was used to over- belts. Among these the Southern Apennine thrust-

belt is characterised by the absence of thick crustcome resolution problems that often make of littlevalue the regional interpretations developed in a along the axis of the belt, the coexistence of

shallow and deep seismicity, low morphologicalmainly two-dimensional way. Detailed geologicalmapping carried out in areas where the connection and structural elevation and the association with

a back arc basin (Doglioni et al., 1996).between surface geology and subsurface geologyand geophysics is particularly critical supported The Southern Tyrrhenian Basin is characterised

by marked crustal thinning and by an oceanicthis activity. The research was performed withparticular care to reduce to a minimum the implicit crust in correspondence with the Marsili Basin and

Mt. Vavilov where MORB composition basaltsdifferences between the different scales of observa-tion among field, well, detailed seismic and regional outcrop. Southern Tyrrhenian extensional activity

is coeval with compression at the Southernseismic studies. The results of these studies, whichpaid special attention to the three-dimensional Apennine front based on ODP data (Malinverno

and Ryan, 1986; Kastens et al., 1987; Pataccadevelopment of the structure, were made to beincorporated into the geological sections A–A∞ et al., 1992a,b).

The inner and the axial zones of the Southern(Capo Palinuro, Val d’Agri, Fiume Basento) and

241A.M. Noguera, G. Rea / Tectonophysics 324 (2000) 239–265

Fig. 1. Main structural elements of the Southern Apennine.

Apennine have no deep crustal roots as shown by Amato et al., 1993; Alessio et al., 1995). The deepseismicity occurs with hypocentral depths of 300–the seismic refraction data (Nicolich, 1981;

Scarascia et al., 1994); however, these data are 500 km, which have been related to the west-dipping subduction of the Neotethyan lithosphereinsufficient in resolution and density. According

to the literature, the Moho runs parallel to the and Apulian plate lithosphere (Giardini andVelona, 1991; Amato et al., 1991; Patacca et al.,bottom of the Apulian Platform and reaches a

depth of about 35 km close to the Cilento 1991; Doglioni et al., 1994). The subduction of alithospheric slab to a depth of 500 km is alsoPeninsula (Adriatic Moho). At the immediate

Tyrrhenian offshore its depth is recognised as being documented by tomographic images (Spakman,1988).about 20–25 km (Tyrrhenian Moho; Figs. 3 and

4; Giese and Reutter, 1978; Nicolich, 1981; The Bradanic foredeep and the Murge peri-pheral bulge (Royden and Karner, 1984; Royden,Scarascia et al., 1994).

The distribution of regional seismicity shows 1993) are the expression of the Apulian lithospheresubduction. This gives rise to the clear positivethe existence of both shallow and deep earth-

quakes; the shallow earthquakes have hypocentral gravity anomaly of the Murge bulge and thenegative anomaly of the Bradano foredeep (Fig. 5;depths of less than 30 km with extensional or

transtensive focal mechanisms (Pingue et al., 1988; Amato and Selvaggi, 1991; Royden, 1993), which


Fig

.2.

Stru

ctur

alm

apof

the

CL

A.


Fig. 3. Moho depth contour map and location of crustal cross-section, modified after Scarascia et al. (1994).

Fig. 4. Crustal cross-section through the Southern Apennine, after Scarascia et al. (1994); see Fig. 3 for location.


Fig. 5. Bouguer gravity anomaly, density 2.6 g/cm3. Note the positive anomaly of the foreland (Murge) and the large NW–SE negativeanomaly of the foredeep.

reaches its minimum value just outside the the central Apennine thrust-belt; see Bally et al.(1986) and Lavecchia et al. (1987)]. The alterna-Apennine Arc. This cannot be justified only by the

low-density foredeep sediments (Royden et al., tion in space and time of carbonate platforms andbasins has given rise to considerable diversity in1987).

Magnetic data (Arisi Rota and Fichera, 1987) the structural style observable in the variousSouthern Apennine tectono-stratigraphic units.show the magnetic basement to be relatively high

in the axial part of the belt and to sink in a These characteristics, together with the discontinu-ous nature of the paleogeographic domains, makewesterly direction starting from the foreland.

The present configuration of the Southern the balancing and modelling of geological profilesa difficult task. For example, field, wells and seismicApennine is the result of progressive polyphase

non-coaxial deformation characterised by a succes- image data prove that the basinal facies terrainsof the Lagonegro Units (see further on) are highlysion in time of different stress regimes consisting

of a series of compression, extension and strike- allochthonous. These units have undergone large-scale internal brittle–ductile deformation. In con-slip phases. This deformation history has involved

different paleogeographic domains in a pro- trast, the footwall of the nappes, consisting of athick succession of carbonate platform faciesgressively eastern position (Sgrosso, 1986; Casero

et al., 1988). The structures derived from these [Inner Apulian Platform (IAP)], was subjectedmainly to brittle deformation and limited shorten-deformations are, in general, markedly non-cylin-

drical and originated in non-plane strain ing (see below). As a consequence, disharmonicdeformation between the IAP end the allochtho-conditions.

As regards mechanical stratigraphy, the terrains nous units is very common.The present foreland (Fig. 2) is represented byforming the Southern Apennine belt cannot be

considered as a single multi-layer sedimentary unit the Outer Apulian Platform (OAP) consisting of7000–8000 m thick carbonate sequence. The base[e.g. the Umbro-Marchigiano Basin involved in


of this sequence (penetrated by the Puglia 1 well ) assume that deformation propagated from theWest (within the belt) towards the East (outsideconsists of dolomites and clastic sediments of

transitional environment (Lower Triassic). The the belt). In palinspastic reconstruction the struc-turally highest units should therefore be placed inreflecting horizons corresponding to this strati-

graphic level, easily recognisable on regional the most internal paleogeographic position. Thepaleogeography, which may be deduced fromseismic sections, show that the Apulian carbonate

sequences gradually thicken from NW to SE along direct and elementary application of this principle(Ogniben, 1969), leads, therefore, to the followingthe Salento Peninsula. The thickening is particu-

larly evident for the Cretaceous interval. west to east arrangement of basinal and carbonateplatform domains (Fig. 6):In the axial zone of the belt, the OAP

underthrusts the IAP carbonates and is tectonically $ Liguride–Sicilide basinal domain$ APPcovered by the stack of thrust sheets deriving from

the deformation of the Lagonegro Basin (LB) $ LB$ Apulian Platform.described by Scandone (1967, 1972) and

D’Argenio et al. (1974). The Mesozoic thrust The Liguride basin was formed on oceanic crust(Neotethyan Domain), whereas the APP, LB andsheets of the LB tectonically lie over their Tertiary

cover and these in turn lie over the Lower Pliocene Apulian Platform were formed on continental crust(Adria plate), which probably thinned out in corre-stratigraphic horizons of the IAP. These sheets

tectonically underlie the allochthonous units of the spondence with the LB [for a review see Sgrosso(1993)].Apennine Platform (APP) and the Sicilide and

Liguride Complexes, which represent the geometri- In an alternative to this paleogeographic model,other authors (D’Argenio et al., 1974, 1993;cally highest structural units. Piggyback basin

deposits of Upper Miocene (Monte Sacro and Sgrosso, 1986; Marsella et al. 1995) presentedinterpretations that differ considerably as regardsGorgoglione cycle) to Plio-Pleistocene age

(Calvello and S. Arcangelo basins) unconformably the tectonic evolution and paleogeography of theSouthern Apennine. In particular, Marsella et al.cover the entire thrust sheet stacks. The

Quaternary Bradanic cycle of the foreland covers (1995) reproposed the original model of Selli(1962), attributing a more internal location of thethe allochthonous thrust front.

Commonly accepted geological models for the LB in relation to the APP.From a morphological and structural point oftectonic evolution of the Southern Apennine

Fig. 6. (a) Sintetic lithostratigraphic columns and thrust geometry of the main tectono-stratigraphic units of the CLA.


view, the orogen may be subdivided (Fig. 2) into lateral component of motion) and the associatedback-thrust in the western limb of the structuresan internal zone comprising the area between

Cilento and Vallo di Diano, an axial zone roughly is related to the ramp geometry.According to the theoretical models ofcorresponding to the Mt. Sirino and Mt. Vulturino

mountain groups and an external zone comprising Sanderson (1982), wrench-type shear arises if shearstrain varies for differential transport of thrustthe Bradano Valley and Murge. This simple tripar-

tition reflects the structural style and evolution of sheets. The existence of the CLA implies both thepresence of differential transport of thrust sheetsthe Apennine Orogen. In the internal zone the

compressional structures that affect the Liguride and of a heterogeneous wrench shear componenton a horizontal plane. Considering that the IAPcomplex and APP units are, in fact, overlain by

the effects of the Plio-Quaternary extensional tec- was subjected to brittle deformation, the wrench-shear component should be manifest along the arctonics that originated large-scale systems of NW–

SE-trending normal faults dipping both westward through strike-slip faults or transfer zones betweendifferent bending and folded sectors of the belt.and eastward. This zone underwent an important

uplifting phase in the Lower Pleistocene Detailed seismic mapping of the top of the IAPcarbonates along the entire CLA led to the identi-(Calabrian), which ceased to be active in the Upper

Pleistocene (Amato and Cinque, 1992; Cinque fication of these discontinuities as lateral ramps ofthe major thrust-sheets. The en-echelon geometryet al., 1993; Westaway, 1993).

In the axial part of the belt, the Lagonegro of the lateral ramps (with NE–SW and NW–SEtrend) of the main thrusts mapped within the IAPUnits are exposed through some tectonic windows

in the footwall of AP carbonates and Liguride and indicates the presence of complex transfer zones,which often show significant dip of IAP carbon-Sicilide nappes. The Lagonegro Units are featured

by a series of polyphasic folds and thrust sheets ates. It is in these zones that the Calvello, Potenzaand Ofanto basins were formed.[break thrust folds, sensu Willis (1893), in Fischer

et al. (1992)]. According to Mazzoli (1992) the These transfer zones are represented in subsur-face by the lateral ramps of the Val d’Agri deepLagonegro Units underwent an E- to NE-verging

deformation and were successively refolded by structures (IAP). The huge thrust sheet penetratedby Costa Molina 2 well (Fig. 7) represents theroughly N–S-oriented shortening. These units were

finally deformed by a post-nappe folding phase, regional culmination of the compressional struc-tures in the carbonates. This configuration suggestswhich generated the wide antiforms easily recogni-

sable on large-scale geological maps. The main that the overall shortening of the IAP varies alongthe trend of the arc and that it reaches its maximumaxes of these antiforms determine the outcrops of

Lagonegro Units in tectonic windows and lie along in the central sector.The Monte Alpi tectonic window (a peak havingconvex to the east alignments. These alignments

repeat the cartographic trend common to all the the same name as the oilfield but lying muchfarther south) represents the only outcrop of themajor units of the Campanian–Lucanian segment

of the Southern Apennine representing the struc- IAP (Fig. 2). This unit, in fact, comes to thesurface due to the effect of a transpressive backture known as the ‘‘CLA’’. According to Cinque

et al. (1993), the formation of this arc dates to the thrust [see van Dijk et al. (2000)].In the axial zone of the belt the effects of strike-Upper Pliocene. The integrated interpretation of

seismic and geological data, shows that the shallow slip and extensional tectonics are documented bythe Vallo di Diano and Val d’Agri faults. The axialstructures of the CLA originated from the IAP

deep structuring, which has been stratigraphically zone was affected by strong uplifting starting fromthe end of the Lower Pleistocene, as proven bydated by hydrocarbon exploration wells as no

older than Upper Pliocene (e.g. Costa Molina 2 Sicilian deposits outcropping 1000 m above sealevel (Cinque et al., 1993).well, Fig. 7).

The IAP contractional structures are interpret- Finally, in the external zone the Cretaceous–Miocene basinal units (External Flysch Complex)able as non-cylindrical ramp anticlines (with a left-


Fig

.7.

Stru

ctur

alcr

oss-

sect

ion

acro

ssth

eC

LA

(A–A

∞pro

file)

and

grav

ity

mod

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g.T

hegr

avit

ym

odel

ling

was

mad

eus

ing

the

follo

win

gde

nsit

yva

lues

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ide

and

Sici

lide

Com

plex=

2.4

g/cm

3,P

iggy

back

basi

nsan

dA

PP

clas

tics

terr

ains=

2.4–

2.45

g/cm

3,E

xten

alF

lysc

hC

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2.4

g/cm

3,L

B=

2.55

g/cm

3,IA

Pan

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tic

succ

essi

on=

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and

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aleo

zoic

and

Bas

emen

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68g/

cm3,

Man

tle=

3.2

g/cm

3.


overthrust the Bradano foredeep. The leading edge defined and enables the setting of upper and lowerof this complex represents the buried thrust front limits to the main deformation phases connectedof the Southern Apennine. with the evolution of the orogen (Sgrosso, 1986;

More to the east the foreland constituted by Casero et al., 1988; Patacca and Scandone, 1989).the OAP is westward dipping with a relatively The order adopted in the following summarisedsteep angle of around 10°. The vast anticlinorium descriptions of the stratigraphic units composingof the Murge corresponds to the peripheral bulge the thrust-belt–foredeep–foreland system proceedsof the foreland (Royden and Karner, 1984; Patacca from top to bottom and from the internal to theet al., 1992a,b), the extreme limit of the system external part of the system.deformed during the orogeny.

Proceeding from the Murge towards the axialzone of the belt, seismic and well data (Sella et al., 2.1.1. Apennine piggyback basins1988) show a gradual dipping of the OAP and its This definition includes various sedimentaryPliocene terrigenous cover below the external cycles of Lower Pleistocene to Middle Mioceneallochthonous complex. The regional dip of the age. The fan delta deposits of the Sauro and AgriOAP below the belt progressively increases Cycles of the S. Arcangelo Basin (Pieri et al.,towards the SE. The marked bending of the 1994) constitute the most recent cycles (Lower–Apulian Platform is also shown by the present Middle Pleistocene and Lower Pleistocene). Thearrangement of the hydrographic grid influenced Middle Pliocene sedimentary cycle is well devel-by the Apulian Platform deep structures. oped throughout the CLA (S. Arcangelo, Calvello,

Ofanto, Ariano and Benevento basins) and forms2.1. Stratigraphic framework a typical transgressive–regressive sequence depos-

ited on top of the thrust sheets as they advancedThe Southern Apennine represents a small por- towards the Apulian foreland (Amore et al., 1996).

tion of a thrust and fold belts system extending The Upper Miocene deposits are represented incontinuously from the central Mediterranean the CLA by the well-known Altavilla–Anzanothrough North Africa, Sicily and the Italian penin-

cycles, consisting mainly of evaporitic sedimentssula. This system developed during the Alpine

and subordinately of terrigenous depositsCycle (Trias–Quaternary) by deformation of the(Crostella and Vezzani, 1964). The latter are dis-southern margin of the Mesozoic Tethyscordantly deposited on the S. Bartolomeo Fm.,(Laubscher and Bernoulli, 1977). The paleogeog-which represents a large-scale succession, in placesraphy of this passive margin in the Mesozoic timesover 1500 m thick, consisting of turbiditic mud-was characterised by an alternation of carbonatestones with metamorphic pebble inclusions at theplatforms and pelagic basins (Dercourt et al.,bottom and by an overlying pelitic sandstone1992). Starting from Cretaceous–Paleocene timessequence (Crostella and Vezzani, 1964). This suc-this margin began to shorten severely, originatingcession, belonging to the ‘‘Irpine Unit’’ familythe Alpine belts. The Apennine deformation took(Cocco et al., 1972) and dated to the Upperplace during the Neogene, its evolution beingLanghian, was recently reattributed to the Lowerdirectly determined by the relative motions of theMessinian (Patacca et al., 1991).European and African plates (Dewey et al., 1989).

The Gorgoglione Fm. sedimentary cycleThe age of the first terrigenous sediments depos-outcrops widely in Basilicata and unconformablyited on the carbonate platforms or in the basinsoverlies the Liguride and Silicide Units and theand the age of the piggyback basins are linked toAlbidona Fm. It consists of a thick (often overthe migration of the bending of the foreland litho-1500 m) sequence of quartzarenites with pebblessphere and to the incorporation into the thrust-passing to pelitic sandstone deposits at the top.belt of progressively more outlying domains of theRecent dating places the Gorgoglione formationApulian plate. The dating of these events allows

the progression of the deformation to be clearly from Langhian to Tortonian (Patacca et al., 1991).


2.1.2. Liguride–Sicilide Complex correlation with the Helmintoid Flysch of theNorthern Apennine. The terrains of SicilidiThis complex includes the allochthonous succes-

sions that constitute the highest units in the chain, Affinity mapped in the Cilento region by Bonardiet al. (1988) and the Sicilidi Unit outcropping inwhich were deposited in a basin lying partially on

the oceanic crust. The Liguride Units are repre- the Sele Valley could represent internal units. TheSicilidi Units mapped in the axial zone of the beltsented by Frido, Crete Nere, Saraceno and

Albidona formations (Amore et al., 1988). These (e.g. Mt. La Ricciola, Mt. Tangia) and the strati-graphic successions described by Pescatore et. al.units are made of Jurassic to Oligocene rocks

(Ogniben, 1969; Knot, 1994) incorporating ophio- (1988) could have been deposited during the UpperCretaceous–Lower Miocene on the LB (see below).litc suites including blueschist-facies, showing a

lower greenschist-facies retrograde overprint (Celloand Mazzoli, 1999). According to Knot (1994), 2.1.3. APP Unit

This unit consists of a series of stratigraphicthese units are the remnants of a subductioncomplex, whereas the Albidona Fm. represents a successions containing westward transition facies

(towards the Liguride Basin), internal platformforedeep basin situated between the Liguridi accre-tionary wedge to the west and the AP to the east. facies and eastern transition facies (towards the

LB). The internal transition succession (thicknessAccording to Bonardi et al. (1985) the AlbidonaFm. has been attributed to the Burdigalian– 1500–2000 m) outcrops in the Capri Island and at

Mt. Bulgheria. It consists of shallow waterLanghian and interpreted [according to Selli(1962)] as a piggyback basin deposited on Liguridi, Triassic–Liassic carbonates followed by resedi-

mented carbonates interbedded with marls ofAP and LB units. New stratigraphic data on theAlbidona Fm. (Baruffini et al., 1999) shows that Lower Cretaceous–Miocene age.

The Alburno–Cervati succession represents thethis formation can be interpreted as an Eoceneforedeep basin situated upon the Liguride Units open shallow platform facies. Triassic dolomites

and dolomitic limestones form the base of theto the west and the AP to the east.The formations constituting the allochthonous succession, followed by a monotonous succession

of shallow-water carbonates showing a strati-Silicide complex (Ogniben, 1969) are of Cretaceousto Lower Miocene age. They are lithologically graphic gap between the Upper Cretaceous and

the Paleocene. Miocene sediments (Cerchiara Fm.,dominated by varicoloured clays, resedimentedlimestones with intercalations of red marls (similar Selli, 1962) follow in conformable transgression as

progressively deepening carbonate slope faciesto the Helmintoid–Flysch of the NorthernApennine) and fine sandstones. The overall thick- characterised by final complete drowning, as

shown by the development of Upper Burdigalianness is over 2000 m. Detailed stratigraphic dataand a new definition of the Sicilide Unit in the terrigenous deposits (Bifurto Fm., Patacca et al.,

1992a,b). Clastic sedimentation ended with theSouthern Apennine are lacking, so two differentattributions are followed in the literature: deposition of rudites and sandstones of

Serravallian–Tortonian age (Santo, 1996). The$ internal origin (Ogniben, 1969; Amore et al.,1988; Patacca and Scandone, 1989, Monaco thickness of this succession is about 5000 m.

The Mt. Foraporta succession, structurally lyingand Tortorici, 1995).$ external origin, deposited on LB (Mostardini between the Alburno–Cervati Unit (below) and

the Triassic dolomites of the Mts Maddalena Unitand Merlini, 1986; Casero et al., 1988; Pescatoreet al., 1988). (Bonardi, 1966), outcrops to the west of the

Lagonegro town. The Mt. Foraporta successionAccording to Casero et al. (1988) andMostardini and Merlini (1986), the Sicilide Units has been described by Boni et al. (1974), and

Scandone (1972). It consists of resedimented blackon the APP are related to a large back thrustactivity during the first phase of basin inversion. dolomites and bivalves limestones (Lithiotis facies

Auct., Bonardi, 1966) dating to the UpperRegional evidence suggests an internal origin ofpart of the Sicilide Complex, as inferred by the Triassic–Lower Jurassic. The thickness is about


500 m. According to Boni et al. (1974) the Mt. $ shales and limestones (Galestri Fm., LowerCretaceous);Foraporta succession represents an euxinic intra-

platform basin originally located between the $ resedimented limestones, calcilutites and marls(‘Red Flysch’, Auctorum, Upper Cretaceous –Alburno–Cervati and Maddalena Units.

The Mt. Maddalena succession shows evidence Lower Miocene).The latter formation is in stratigraphic continuityof a transition facies between the APP and the LB

(Marsella and Pappone, 1986; Marsella, 1988). This with the ‘Numidian Flysch’ (Lower Burdigalian;Sgrosso, 1986; Patacca et al., 1992a,b).2000 m thick sequence consists of restricted plat-

form dolomites and limestones from the Triassic– According to the interpretation originally pro-posed by Ogniben (1969) and followed byLower Jurassic interval. These are followed by slope

resedimented carbonates of Middle Jurassic to Scandone (1972), D’Argenio et al. (1974) andCarbone et al. (1988, 1991), the terranes outcrop-Paleogene age. Numidian Flysch (Burdigalian–

Langhian, Patacca et al., 1992a,b) represents the ping along the external edge of the belt representthe Upper Miocene part of the LB sequence,Miocene deposits. The Maddalena Unit foredeep

phase is documented by the Castelvetere Fm. con- detached and overthrust beyond its Lower-Miocenic–Mesozoic substrate. This external com-sisting of turbidite deposits with inclusions of con-

glomerates containing crystalline rock pebbles plex (Pescatore et al., 1988) is, in fact, constitutedby the ‘Red Flysch’ Fm., passing upward to the(Pescatore et al., 1969). According to Patacca et al.

(1992) this formation was deposited at the Numidian Sandstones. These quartzarenites arelargely represented in the Mediterranean areaTortonian–Messinian boundary.(Durand Delga, 1980; Ogniben, 1963), and accord-ing to Wezel (1970) their origin is from the slopeof the African Platform. The Numidian Sandstones2.1.4. Lagonegro Unit

The LB sequences are structured into two main cover the APP and the LB, and according toPatacca et al. (1992) they were deposited in aregional tectonic units, which in turn can be subdi-

vided into numerous tectonic sub-units. The upper foreland basin not yet reached by the compres-sional front.unit is about 2000 m thick and the lower one is

1500 m thick. The LB evolution starts with the The Serrapalazzo Fm. (Upper Tortonian), con-sisting of marly limestones, sandstones and marls,deformation of a carbonate platform (Ladinic lime-

stones, organogenic member of Monte Facito Fm.) stratigraphically covers the Numidian Sandstones.The Serrapalazzo Fm. is considered as the distalunder an extensional tectonic regime associated

with crustal thinning and regional subsidence of the portion of the foredeep deposits in the UpperTortonian. According to Pescatore et al. (1988),Hercinian basement (Wood, 1981). Starting from

Upper Triassic the LB probably constituted the the terranes outcropping in the Middle BasentoValley and Stigliano ridge, attributed to the Silicidewestward extension of the southern branch of the

Neo-Tethys Ocean (Ciarapica and Passeri, 1998). Units (Argille Varicolori, Corleto Perticara andTufiti di Tusa), could represent the UpperThe representative ‘calcareous–siliceous–marly

succession’ (Scandone, 1967, 1972; Wood, 1981; Cretaceous–Neogene portion of the LB depositedin the axial part of the basin while the FlyschMiconnet, 1983, Marsella et al., 1991) consists,

from bottom to top, of: Rosso Fm. was deposited near a carbonateplatform.$ siliciclastic deposits including Ladinic lime-

stones and dolomites covered by pelagic lamelli-branch-bearing shales (Monte Facito Fm., 2.1.5. Apulian Platform Unit

Two large tectonic units, the IAP and the OAP,Lower Triassic–Upper Triassic);$ cherty limestones (Calcari con Selce Fm., Upper represent the Apulian Platform. The IAP outcrops

in the Mt. Alpi di Latronico (Sgrosso, 1988) andTriassic);$ radiolarites and siliceous siltstones (Scisti Silicei is known in the subsurface from well and seismic

data. The OAP forms the present foreland.Fm., Jurassic);


According to Mostardini and Merlini (1986), the the remains of an Upper Miocene thrust-belt thatIAP is characterised by shortening features and underwent a major translational event during thethe OAP by extensional features. Pliocene. The low morphological and structural

The stratigraphic succession of the IAP pene- elevation make the reconstruction of deep cross-trated by wells is generically constituted of a Lower sections without use of seismic lines and well dataand Upper Cretaceous carbonates sequence with an impossible task. The fundamental outlines ofshallow-water and carbonate basin transition the Southern Apennine deep structure, as seenfacies. ‘Scaglia facies’, representing parts of the from regional seismic grids and well stratigraphy,Paleocene, were penetrated by some wells in Val have only been acquired at the end of the Lowerd’Agri (e.g. Costa Molina 2 well ). Pliocene. It was in this period that the final activity

The Miocene evolution of the Apulian Platform of the ensialic shear zone took place. This eventtestifies to a progressive drowning, demonstrated led to the subduction and total disappearance ofby the deposition of increasingly deeper carbonate the basement of the LB and to the thrusting ofslope facies limestones. The Upper Miocene por- the Lagonegro Units and internal units, as welltion of this succession consists of regressive facies as the onset of deformation of the Apulian(mudstones passing to anhydrite and gypsum in Platform.the Messinian). The deposition of mudstones and A more recent event datable to the Plio-sandstones shows the existence of a foredeep sedi- Pleistocene is the structuring of the Apulianmentary cycle at the end of the Lower Pliocene. Platform and the activation of a new ensialic shear

The OAP features a thick succession (7000– zone that led the IAP to overthrust the OAP.8000 m) of shallow-water carbonates whose oldest The geological sections show the thick-skinnedportion is only known through deep subsurface nature of the Plio-Pleistocene thrust-belt (Figs. 7investigations. The base of this succession is and 8), an interpretation that differs from thoseformed of Permian slates and continental sand- proposed by Mostardini and Merlini (1986),stones unconformably covered by a transgressive

Finetti et al. (1996), Marsella et al. (1995), Pataccasuccession formed by calcareous breccias and sand-and Scandone (1987) and Picha (1996). All thesestones of Scythian age (Puglia 1 well; Ricchetti,authors consider the orogen to be thin-skinned1994). The calcareous breccias bear some fusuli-with generalised detachment of all the sedimentarynids, indicating a Paleozoic carbonate platformcovers from the basement. Casero et al. (1988)source. The Middle–Upper Triassic successionand Roure et al. (1991) envisaged a quite differentconsists of dolomites and marls followed by dolo-tectonic style; they postulated an efficient detach-mites and anhydrites, which pass upwards toment between basement and cover in the ApulianJurassic limestones and dolomites (UgentoPlatform and the thrusting of the basement alongDolomites) and to the Cretaceous limestones ofa deep shear plane, this plane defining the existencethe Calcari di Cupello Fm.of a basement indenter.Discontinuous bodies of Tertiary and

The thick-skinned interpretation of the CLA isPleistocene limestones are known in the subsur-in agreement with the magnetic basement geometryface. These limestones are conformably covereddescribed by Arisi Rota and Fichera (1987). It isby on-lapping foredeep Middle Pliocene terrige-also backed up by the seismic recognition andnous deposits. These are, in turn, conformablyinterpretation extended to a regional level of deepoverlain by sediments of the Bradanic Cycle, repre-reflecting horizons relating to the bottom of thesenting a Pleistocene regressive megasequenceApulian carbonates, which is attributed to the(Patacca et al., 1992a,b).Lower Triassic (Figs. 7 and 8).

The geological sections (Figs. 7 and 8) clearlyshow the notable contrast between the tectonic3. Regional structural profilesstyle of the deep structures at the core of the chain,which form the hydrocarbon targets, and the tec-Oil exploration in the Southern Apennine has

proven that the exposed part of the orogen is just tonic style of the Lagonegro, Sicilide and Liguride


Fig

.8.

Stru

ctur

alcr

oss-

sect

ion

acro

ssth

eC

LA

(B–B

∞pro

file)

and

grav

ity

mod

ellin

g.F

orth

ede

nsit

yva

lues

see

Fig

.9.


nappes (detached thrust sheets). The LagonegroUnits on the top of the IAP consist exclusively ofsedimentary cover characterised by brittle–ductileinternal deformation in a metamorphic environ-ment that does not exceed the anchizone (Pozzuoliet al., 1977) and represents the effect of a north-eastward verging overthrust shear. The UpperCretaceous–Neogene covers of the LB weredetached and transported towards the forelandwith a duplex geometry (see below).

On the other hand, the deformation undergoneby the IAP also involved the basement, while theOAP was affected by extensional tectonics due tothe progressive bending of the foreland whoselithosphere was undergoing ensialic subduction.The structures of the Lagonegro Units and theallochthonous units above them, which can bereferred to the detachment and translation phase,were thus transported onto the sole-thrust of theseunits and thus are completely unrelated to the

Fig. 9. Vitrinite and bitumen reflectance from the Val d’Agrideep structures of the IAP. This is also indepen-wells are characterised by a major discontinuity along the thrustdently demonstrated by geochemical data: all thesurface at the Lagonegro–Undifferentiated Miocene unit con-

vitrinite reflectance diagrams from the wells show tact, where the maturity level sharply decreases.a saw-tooth profile with a sharp downturn of theindexes in correspondence with the sole-thrust ofthe nappes (Fig. 9). The sole-thrust of the nappes 7, 8 and 10). The response is excellent up to the

maximum recording time (6 s, TWT ), which forconsists of a thick shear zone with high fluidpressure and is folded at regional level by the the velocities involved corresponds to depths of

over 12 km below sea level. The outcrops mainlyreverse fault system affecting the IAP.The structural arrangement and evolution consist of APP terrains with their terrigenous cover

overthrusted by the Liguride and Sicilide Units.described here also provide an explanation for thesuccessful oil exploration efforts in the Val d’Agri At the western side of the geological section, the

Mesozoic and Miocene successions appear thick-area, which is located in the internal part of thethrust-belt, and thus in a fairly unfavourable posi- ened due to the effect of the frontal ramp of Mt.

Bulgheria (Fig. 7). The APP consists of varioustion for hydrocarbon exploration.The three-part morphological–structural subdi- tectonic units, from top to bottom: the Alburno–

Cervati Unit, Foraporta Unit, Monte Marzanovision used here to explain the CLA on the basisof surface data shows a natural correspondence Unit and Mts Maddalena Unit. The common

detachment level of these units is probably locatedwith the deep-lying structural styles illustrated bythe geological sections (Figs. 7 and 8). The descrip- at the bottom of the Triassic (Figs. 10 and 11).

In the Cilento Peninsula subsurface, the seismiction of the sections is thus presented proceedingfrom the internal zone towards the external zone. lines utilised for the A–A∞ and B–B∞ geological

sections show that the horizons attributed to theTriassic–Lower Paleozoic and the basement are3.1. Internal zonedeformed into a thick, complex system of duplexes.The latter overthrusts the IAP, which in the CilentoSeismic data are particularly good for the

Internal Zone ( lying between the coastline and the Peninsula subsurface appears to be characterisedby a particular seismic facies (Fig. 10). We believewestern side of the Mts Maddalena Unit, Figs. 2,


Fig

.10.

Mig

rate

dse

ism

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eac

ross

the

Cile

nto

Reg

ion

(a)

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Fig. 11. Geometrical relationship between Alburno–Cervati–Maddalena and Marzano Units before strike-slip tectonics.

this facies may refer to the buried margin of the section A–A∞ along an NE–SW transect (Fig. 2),the Alburno–Cervati Unit appears to be limitedApulian Platform (transition facies), probably

towards the LB. The nappes of the LB could by a WNW–ESE-trending system of left-lateraltranstensive faults dipping to the north-east. Thistherefore have overthrusted the IAP along its

carbonate platform slope during the LB tectonic system also controls the morphology of theTanagro Valley (Ascione et al., 1992a). The Monteinversion phase (Upper Miocene times).

Surface mapping in the Alburno–Cervati region Marzano Unit that overlies the Mts MaddalenaUnits (see Fig. 11) constitutes the eastern end ofpoints to the existence of two antiforms with

WNW–ESE-trending axial planes, characterised this valley.In the internal zone the normal faults that affectby a slight NW plunge. These features are cut by

a Plio-Pleistocene left-lateral transtensive system the IAP and the Liguride Complex are featured atdepth by listric geometry. On seismic lines theof NNW–SSE- to WNW–ESE-trending faults dip-

ping about 60° to the SW and NE (Ascione et al. fault block rotations are evident. The main faultsdip towards the Tyrrhenian Sea and flatten at the1992a,b). The NE–SW-trending fault system dis-

places the Alburno–Cervati structures towards the bottom of the AP sole-thrust, which is probablyreactivated as a low-angle normal fault (Figs. 7, 8NW in the Sele Plain.

The AP sole-thrust is clearly visible on the and 10).seismic images that show its progressive shallowingeastward up-dip (Figs. 7, 8 and 10). This disconti- 3.2. Axial zonenuity lies at 7000 m below sea level and reachesthe surface along the western flank of the Val The axial zone extends from the western side of

the Diano Valley to the S. Fele and Stiglianod’Agri. Proceeding northwards with respect to the


Ridge. This area corresponds to the culmination the shortening of the IAP is maximum along theB–B∞ regional section (Fig. 7) and decreasesof the IAP. The difference in tectonic style of the

frontal portion of the AP and Lagonegro Units towards NW (Fig. 8). The comparison of the twosections presented demonstrates the NW axialwith respect to the IAP structures is very clear.

The LB terranes are structured into two large, plunge of the IAP and the tectonic thickening ofthe Lagonegro terranes in this direction.regionally overthrusted tectonic units (upper and

lower units, see Fig. 2), in turn subdivided into In the Val d’Agri (Figs. 7 and 12) a transtensive( left-lateral ) fault affects the pre-existing compres-numerous tectonic elements. Surface mapping and

subsurface data (Figs. 2 and 7) show that the sive structures and also the basement. The NW–SE-trending and westward dipping ‘Vallo di Dianolower unit outcrops extensively in the axial culmi-

nation zone of the CLA corresponding to the Mt. fault’ represents one of the main extensional linea-ments, which can be observed from both seismicVolturino, Mt. Enoc and Mt. Caldarosa ridge and

continues across the Val d’Agri until Mt. Sirino data and field geology. The east-dipping normalfault to the east of the Vallo di Diano (Fig. 8)(Fig. 2). Towards north and east (Figs. 2 and 8)

the tectonic thickness of the upper unit exceeds could be inferred as the extension of the Irpiniaearthquake seismogenetic fault [23/11/80; see5300 m from the ground level in the area of S.

Fele town, while to the south the upper unit is Funicello et al. (1988) and Pingue et al. (1988)]. Inthis case, this fault should also cut the IAP andpartially eroded and outcrops only at Mt. Torrette

and Timpa di Roccarossa (Fig. 2). The Lagonegro would transfer the displacement to a thrust fault ofthe IAP (listric fault) or the displacement could beUnits overthrust formations of the External Flysch

Complex and the Lower Pliocene of the IAP. accommodated in the ductile crust (planar fault).The leading edge thrust of the IAP is character-The surface units show high shortening, whereas

the deep carbonates are affected by overthrusting ised by a ( left-lateral transpressive) oblique ramp(NW–SE trend) that involves the basement andwith slight displacement that involves the Lower

Triassic, and probably the Paleozoic and the crys- may separate the Adriatic Moho from theTyrrhenan Moho.talline basement. The effect of the oblique ramp

of the IAP leads to a variable displacement of the Although the IAP units are also detached fromthe underlying basement (Figs. 7 and 8), the lead-crystalline basement on cross-sections. A regional

interpretation of the seismic profiles shows that ing edge thrust of the IAP could be detached on

Fig. 12. The Southern Apennine thrust tectonics are clearly exposed along the slopes of the Val d’Agri, a Quaternary graben originatedby transtensive faults, where the APP limestones spectacularly overthrust the Lagonegro Unit (LB). Lagonegro and other underlyingMiocene Flysch are the allochthonous cover of the IAP reached by wells at an average depth of 3000 m below sea level.


the Adriatic Moho (crustal delamination) and the cross-sections (Hossack, 1979), it was possible tosatisfactorily balance and restore the APP (Fig. 13,shortening transferred to the ductile crust.

Left-lateral transpressive structures are also pre- section A–A∞). The restoring of Lagonegro thrust-nappes was carried out in a simplified way owingsent within the OAP in the area explored by the

wells Tursi 1 and Rotondella 1, as seen from the to the polyphase deformation affecting these units.The APP was balanced and restored exclusivelyregional seismic grid sections and the field geology

(Fig. 2). along the A–A∞ section (Fig. 8), since this liesperpendicular to the main structures. Consideringthe regional scale of the section, the effects due to3.3. External zonethe transtensional tectonics identified in theAlburni Mountain region were negligible.The external zone represents the portion of the

orogenic wedge not yet affected by the extensional The seismic lines across the APP allow recogni-tion of the platform bottom and top only. Thetectonics, which involve the whole of the internal

and axial parts of the thrust-belt. Here the alloch- internal geometry was therefore reconstructed byintegrating the depth-converted seismic interpreta-thonous terranes are represented by the Neogene

portion of the LB detached from their substratum tion with the surface structural data and strati-graphic thickness investigated in accordance with(External Flysch Complex). The buried thrust

front of the external zone was active in the north- the classic principles of Woodward et al. (1989)and Mitra (1992). The first step was the recon-ern part of CLA at least until the end of the Plio-

Pleistocene (G. inflata Zone). According to evi- struction of the shortening structures beforenormal faulting. Then the geometrical hypothesesdence from the Calvino 1 well (Fig. 8), it also

involves the Upper Pliocene sediments in an inter- were finally checked through structural balancingand restoring.cutaneous back-thrust, whereas the buried thrust

front in the Southern part of the CLA was active According to field and well data (Contursi 1well ), the APP foredeep terrigenous sequences(Fig. 7) until the Pleistocene (Emiliano).

The OAP is affected by disjunctive tectonics were assumed to be detached from their substra-tum and only recognisable at the footwall of Mt.(Sella et al., 1988) originated by the regional

bending of the Apulian lithosphere (Doglioni Puglie anticline.The eastward thinning of the APP is linked toet al., 1994).

The most evident structural elements in the the facies variations which lead to the transitionfrom the shallow platform (Mt. Cervati and Mt.sections shown in Fig. 7, Figs. 7 and 8 are NW–

SE-trending normal faults. The main faults dis- Alburno) to the carbonate slope deposits (Mt.Maddalena), and probably to the intra-platformplace the top of the OAP downthrown to the west

that gradually dips below the buried thrust front basin of the Foraporta Unit.The shortening1 of the AP in this sector, asof the belt. In some cases these faults clearly

displace the bottom of the OAP carbonates. calculated between the considered pin lines, is 23%.Since the LB nappes are affected by polyphaseThe seismic images of the external zone also

demonstrate the existence of growth structures deformation (Torrente, 1990; Mazzoli, 1992) gen-erated in a deep diagenetic to anchizonal metamor-within the carbonate sequence, indicating synsedi-

mentary extensional tectonics of probable Middle phic environment (Mazzoli, 1993; Pozzuoli et al.,1977) they are difficult to balance and restore. ItCretaceous age.is not always possible to describe the structureswith the necessary precision. Therefore, they have3.4. Geometrical modelling and structural evolutionbeen restored in a simplified form (Fig. 14). The

The selected sections are roughly oriented paral-lel to the regional compression direction (NE– 1 The shortening R was calculated using the expression R=SW ). Notwithstanding the presence of strike-slip (Li−Ld/Li)×100, where Li is the initial length, and Ld the

deformed length.structures that reduce the accuracy in balancing


Fig. 13. Balanced and restored structural cross-section through Alburno–Cervati and Maddalena Mts (APP). The eastward thinningof the AAP is related to basinal and transitional facies between the Alburno–Cervati platform and the Foraporta–Maddalena facies.The calculated shortening is 23%.

original width of the BL along the examined The shortening1 of the Apulian Platform (IAPand OAP) along section A–A∞ can be hypothesisedsections was estimated to be 130 km at least,

implying a minimum shortening1 of 51% (Fig. 14). as being not less than 15%, while along section B–B∞ shortening is estimated as being not less than 22%.The seismic lines across the OAP clearly show

a westward down-dip of the horizons representing The sum of the lengths of the various paleogeo-graphic domains (APP, LB, IAP and OAP)the bottom and top of the thick Apulian carbonate

sequence. included between the considered pin lines can beestimated to be about 330 km. The calculated totalTo the west of the IAP thrust front, the bottom

of the carbonate succession is only discontinuously shortening1 is therefore over 62%. This value is aminimum estimate, but nonetheless compatiblerecognisable. In particular, the geometry of the

oblique ramp and the degree of overthrusting of with the dimensions of the segment of lithospherethat was subducted overall. The deep seismicitythe IAP onto the OAP cannot be deduced directly

from the seismic images. In order to quantify this data (Giardini and Velona, 1991) and tomographicimages (Spakman, 1988) for the Southernshortening, and above all to ‘geometrically close’

the Southern Apennine thrust system, gravity mod- Tyrrhenian Sea indicate a lithospheric slab downto a depth of 500 km. Therefore, the involvedelling, using the Moho geometry and refraction

seismic data published by Scarascia et al. (1994), sedimentary cover does not exceed the dimensionsof the slab that underwent subduction.was performed.


Fig. 14. Palinspastic reconstruction for the A–A∞and B–B∞ profiles. Stage 1: until Burdigalian–Langhian times, the APP, LB andApulian Platform formed the foreland of the developing orogen; only the innermost sectors of the APP were already overthrust bythe Liguride and Sicilide nappes. The original length between the pin lines can be estimated as about 330 km. Stage 2: during theSerravallian, the APP began to shorten and be thrust over the LB, which in turn started to be deformed under overburden conditionsand its continental substratum was subducted. The already-deformed Lagonegro Units and the APP overthrust the Apulian Platformduring the Early Pliocene.

The set of available geological and geophysical gates regularly from SW to NE during theNeogene. The envisaged structural evolution isdata allows us to propose a structural evolution

model (Fig. 15) in which the deformation propa- described below.


Fig. 15. Structural evolution of the Southern Apennine (see text for explanation).

In a time T1 (pre-Langhian) the sedimentation strate and translated, with the aid of gravity slid-ing, towards the Apulian foredeep (Fig. 15). Wedomain of the APP, LB and Apulian Platform

constituted the foreland and the Apennine defor- consider that after this event the External FlyschComplex sole-thrust was breached (sensu Butler,mation front only affected the innermost domains

of the APP, which was overthrusted by the 1987) by a subsequent thrusting that put theMesozoic LB sequence on top of the MioceneLiguride and Sicilide nappes. Subsequently, during

the Serravallian–Lower Messinian (T2) the APP terranes. Deposits of the Gorgoglione, MonteSacro and S. Bartolomeo piggyback basins thenstarted to become shortened and overthrusted the

LB, which in turn deformed under overburden overlaid the Liguridi, Sicilidi, APP and LB nappes.In a successive phase ( Upper Messinian–Lowerconditions. We hypothesise that during this phase

the Meso-Cenozoic terranes forming the External Pliocene, phase T3) the deformed LB and APPoverthrusted the Inner Apulian Platform. ThisFlysch Complex were detached from their sub-


compressional deformation phase lasted until the master faults from which the normal (westward-dipping) faults observed in the Cilento PeninsulaLower Pliocene (G. margaritae–G. punticulataare branching. The effects of extensional tectonicsZone) leading to the complete subduction of thelinked with the rifting and sea floor spreading ofthinned (?) lithosphere of the LB. The crystallinethe Tyrrhenian Basin are particularly evident in thebasement in the subsurface of the Cilentoinner and axial zones of the belt, whereas they seemPeninsula probably sutures up the ancient ensialicto be more limited proceeding towards the easternshear zone along which subduction of the thinnedfront.continental crust of the LB took place.

From the Early Pleistocene onward, extensionalFinally, in the Upper Pliocene (T4), the IAPsystems characterised by E–W-trending normaland OAP were deformed. Overthrusting of the IAPfaults were generated close to the Tyrrhenian coaston top of OAP involved the Paleozoic strata and(e.g. Sele Plain). These still-active faults are relatedthe basement. The most important part of theto the Thyrrhenian opening (Patacca et al., 1992b)deformation of the IAP seems to have been origi-and are compatible with the opening direction ofnated from the left-lateral transpressive activity ofthe Marsili Basin (Sartori, 1989). As an alternateits oblique ramp, which cut the basement andhypothesis, these faults have been related to thedivided a Tyrrhenian Moho to the west from anlongitudinal extensional stress component due toAdriatic Moho to the east. This oblique thrustingthe oroclinal bending of the Apennine–Calabrian(with a left-lateral component of motion) originatedArc system (Oldow et al., 1993) or to the finalthe huge antiform of the CLA. It refolded at thelateral stretching induced by the subducted slabregional scale the previously formed stack of nappes(Doglioni, 1991).controlling the geometry of the intra-arc basin.

Extensive uplifting has been documented in theIn the Middle Pleistocene the effects of the left-internal zone of the chain during the Calabrian,lateral strike-slip tectonics induced by the south-whereas these phenomena occur in the axial zoneeastward migration of the Calabrian Arc becomeduring the Sicilian (Cinque et al., 1993). This uplift

predominant. This strike-slip system (WNW–ESE and the recent extension of the belt have beentrending) is well known in the Calabrian–Lucanian related to the isostatic relaxation after theborderland. It corresponds to the area lying detachment of the lithospheric slab undergoingbetween the Pollino Massif and the ridge explored subduction (Cinque et al., 1993; Westaway, 1993;by the wells Rotondella 1, Tursi 1 and Letizia 1. Hyppolyte et al., 1994).

We consider the Mt. Bulgheria–Pollino thrustsheet (Fig. 1, Fig. 2) to be an outcropping expres-sion of a buried ramp that allowed the Calabrian– 4. ConclusionsLucanian border structures to overthrust the CLA.This deformation zone could lie along the south- The structural interpretation of new geologicalwestern part of the buried margin between the and geophysical data acquired for hydrocarbonApulian Platform and the LB. exploration suggest a geological model of the

The extensional events, which affected the chain Campanian–Lucanian sector of the Southernfrom the Tortonian onwards with the formation of Apennine thrust-belt that is outlined as follows.oceanic lithosphere in the Tyrrhenian Basin, fol- The thin-skinned tectonic models derived fromlowed a migration pattern that progressively investigation of the non-involved basement zone ofaffected the external sectors of the orogen. They the Rocky Mountains, utilised by Bally et al. (1986)occurred parallel to the migration of compressional for the interpretation of the Umbro-Marchigianofronts. Some seismic sections close to the Apennine and by Marsella et al. (1995), MostardiniTyrrhenian Sea (Cilento Region), provide good and Merlini (1986) and Patacca and Scandonedocumentation of important eastward-dipping dis- (1987) for the Southern Apennine, can explaincontinuities which can be interpreted as deep (about shallower structures, but are not as robust as models10–15 km) normal faults that delaminate the that include the deep involvement of basement in

terms of regional features.Tyrrhenian crust (Fig. 10). These appear to be


The Southern Apennine deep structures have to be addressed also to the colleagues of the(Apulian Platform deformation) are essentially Southern Apennine Exploration Team andPlio-Pleistocene in age and related to a basement- Structural Geology Department for helpful discus-involved thrust tectonics. The IAP constitutes a sions on the Southern Apennine Geology and thehuge thrust sheet with oblique thrust ramps ( left field mapping. Thanks to P. Bernardelli, D. Casinilateral ), structured as a wide antiform in the core Ropa and I. Giori for having provided the gravitybelt. The deformation of the IAP led to refolding modelling. Special mention is accorded to A.and breaching (sensu Butler, 1987) of the overlay- Argnani and C. Cooper for the critical review anding allochthonous nappes and generated large tec- helpful suggestions. Thanks to S. Mazzoli and thetonic windows along the axial zone of the thrust- anonymous reviewer for the constructive criticismbelt. and G. Rossi and A. Pizzochero for the computer

The overthrusts propagated in a piggyback graphics.sequence (sensu Butler, 1987) towards the fore-land, regularly involving progressively more exter-nal units. Only the minor thrust sheets within the Referencesnappes (APP, LB, etc.) and the IAP could havedeveloped out of sequence (sensu McClay, 1992). Alessio, G., Esposito, E., Gorini, A., Porfido, S., 1995. Detailed

The basement in the Cilento Peninsula subsur- study of the Potentino seismic zone in the Southern Apen-nine, Italy. Tectonophysics 250, 113–134.face probably represents the hanging-wall of an

Amato, A., Cinque, A., 1992. Il bacino Plio-Pleistocenico diancient ensialic shear zone, where the LBCalvello (Potenza): evoluzione geologica e geomorfologica(thinned?) crust was subducted. di un piggyback basin. Studi Geol. Camerti Volume Speciale

The well-known left-lateral strike-slip tectonic 1992/1, 181–189.activity in the Calabrian–Lucanian borderland can Amato, A., Selvaggi, G., 1991. Terremoti crostali e sub-crostali

nell’Appennino Settentrionale. Studi Geol. Camerti Volumebe interpreted as the surface effect of a regionalSpeciale, 1991/1, 75–82.buried lateral ramp. This allowed overthrusting of

Amato, A., Cimini, B., Alessandrini, B., 1991. Struttura delthe structures of the Calabrian–Lucanian border-sistema litosfera–astenosfera nell’Appennino Settentrionale

land on the CLA and the south-eastward migration dai dati di tomografia sismica. Studi Geol. Camerti Volumeof the Calabrian Arc. Speciale, 1991/1, 83–90.

Amato, A., Alessndrini, B., Cimini, B., Frepoli, A., Selvaggi,Extensional features induced by the TortonianG., 1993. Active and remnant subducted slabs beneath Italy:opening and oceanisation of the Tyrrhenian Basinevidence from seismic tomography and seismicity. Ann.are superimposed onto the pre-existing compres-Geofis. 36 (2), 201–214.

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On the Mesozoic Ionian Basin

R. Catalano,1 C. Doglioni2 and S. Merlini31 Dipartimento di Geologia, UniversitaÁ di Palermo, Italy2 Dipartimento di Scienze della Terra, UniversitaÁ La Sapienza, Roma, Italy3 Eni-Agip SpA, S. Donato Milanese, Italy

Accepted 2000 July 14. Received 2000 February 28; in original form 1999 July 14

SUMMARY

New seismic re¯ection pro®les of the Italian deep crust project CROP provide newinsights on the structure of the Ionian sea. In spite of the Apennines and HellenidesNeogene subduction zones, two conjugate passive continental margins are preservedat the margins of the Ionian sea, along the Malta escarpment to the southwest andthe Apulian escarpment to the northeast. The Ionian sea is likely to be a remnant of theMesozoic Tethys Ocean, con®ned by these two conjugate passive continental margins.The transition from continental to oceanic crust appears sharper to the northeast than tothe southwest. The basin between southeast Sicily and southwest Puglia was about330 km wide and suggests a low spreading rate. The inferred oceanic ridge should havebeen ¯attened by thermal cooling and buried by later sediments.

Based on stratigraphic and structural constraints to the north in the Apenninesbelt, the ocean continued to the northwest. This palaeogeography is supported by theseismicity of the Apennines slab underneath the southern Tyrrhenian sea, which impliesdowngoing oceanic lithosphere. The adjacent absence or paucity of deep seismicity doesnot imply absence of subduction, but rather it can be interpreted as due to the moreductile behaviour of the subducted continental lithosphere. Surprisingly, we note thatwhere the oceanic inherited basin is subducting underneath the Apennines, in thehangingwall of the subduction hinge there are outcropping slices of continental crystallinebasement previously deformed by the Alpine orogen.

Key words: Ionian sea, Mediterranean, Mesozoic, oceanic crust, passive margin.

I N T R O D U C T I O N

The Ionian sea is located in the central-eastern part of the

Mediterranean Basin, bordered by southern Italy to the west

and north, Greece to the east, and offshore Libya to the

south (Fig. 1). It is a deep sea (in many places below 3000 m).

Important geophysical studies on the Ionian sea have been

performed during the last three decades (Hinz 1973; Panza &

Mueller 1979; Farrugia & Panza 1981; Makris 1981; Calcagnile

et al. 1982; Finetti 1982, 1985; Makris et al. 1983, 1986;

Morelli 1985; Leister et al. 1986; Della Vedova & Pellis 1989;

Ferrucci et al. 1991; Mongelli et al. 1991; de Voogd et al. 1992;

Cernobori et al. 1996; Piromallo & Morelli 1997). In contrast,

few papers have described and interpreted the geological and

geodynamic evolution of the Ionian sea, owing to the deep-water

conditions (Selli 1962). The ODP sites located in this basin

were very few and too super®cial (Ryan et al. 1973). Based

on gravity, seismic refraction and re¯ection data, the crust is

considered to be between 15 and 20 km thick (Locardi &

Nicolich 1988; Nicolich 1989; De Voogd et al. 1992; Scarascia

et al. 1994).

The Ionian Sea represents a key area for the understanding

of the evolution of the Mediterranean geodynamics, both for

the Apennines and Hellenic subduction zones (Scandone 1980;

Angelier et al. 1982; Royden et al. 1987), and for the Mesozoic

Tethyan palaeogeography (Bernoulli et al. 1979; Bernoulli &

Lemoine 1980; Dercourt et al. 1986; Lemoine et al. 1986). This

basin has been considered by Le Pichon (1982a) as a land-

locked basin or a trapped crust (Letouzey 1986). In spite of this

crucial location and of the several studies, we still have several

doubts about the nature and evolution of the Ionian sea. There

are papers that have described its oceanic nature (Finetti 1982;

De Voogd et al. 1992; Finetti et al. 1996; Stamp¯i et al. 1998),

but there are also articles which alternatively propose that the

Ionian Basin is of denser continental crust (Farrugia & Panza

1981; Calcagnile et al. 1982) or a highly re¯ective interval within

the lower crust (Cernobori et al. 1996).

The Ionian sea and its relationships with the surrounding

areas is crucial in understanding the processes that controlled

the Apennines subduction zones which consumed different litho-

spheres of the foreland. It is evident that the largest south-

eastward advancement of the Apennines accretionary wedge

Geophys. J. Int. (2001) 144, 49±64

# 2001 RAS 49

occurs in the Ionian sea, offshore Calabria, and the largest

expansion of the Tyrrhenian sea is located right behind to

the west of the Calabrian arc (Malinverno & Ryan 1986;

Doglioni 1991). In contrast, in the adjacent parts to the north

of the Ionian sea in the southern Apennines, and to the south-

west, in Sicily, the Apennines are less advanced towards the

foreland and the Tyrrhenian backarc extension is less pro-

nounced. Additionally, the front of the Hellenic arc (that is, the

Mediterranean ridge) is more advanced towards the southwest,

corresponding to the deep Ionian Basin.

Several authors described the passive continental margin of

the Malta escarpment (Cita et al. 1981; Scandone et al. 1981;

Casero et al. 1984; Charier et al. 1987; Cinque et al. 1993;

Hippolyte et al. 1994). However, it was not comprehensively

described as the conjugate margin of the Apulian swell on

the other side of the Ionian Sea. In this research we want to test

whether the Malta escarpment offshore east Sicily and the

Salento±Apulian offshore southwest Puglia (Rossi & Borsetti

1974; Sorel 1976; Auroux et al. 1985; Charier et al. 1988;

Ciaran® et al. 1988; Ricchetti et al. 1988; Favali et al. 1990;

Gambini & Tozzi 1996) are two conjugate passive continental

margins of Triassic±Cretaceous(?) age, separated by a basin,

i.e. the Ionian Ocean.

The Malta escarpment (Fig. 1) is a physiographic feature

which has been tectonically controlled since Triassic times. Rocks

dredged on the Malta escarpment span from the Mesozoic

to the Tertiary (Cita et al. 1981; Scandone et al. 1981). The

pre-Cretaceous lower fault systems (Casero et al. 1984) can be

correlated with the evolution of a Mesozoic continental margin

(Charier et al. 1987). Post-Tortonian and Late Pliocene±

Pleistocene extensional tectonic reactivation yields high angle

and listric characteristics of the normal faults in the eastward-

tilted blocks (Torelli et al. 1998). Depositional geometries of

pre-Messinian rocks prograde towards the Ionian Sea to the east.

Our main goal is to show a few new details and a new

interpretation to be added to the debate on the origin of the

Ionian Sea, based on new seismic lines acquired by CROP

(CROsta Profonda, the Italian deep crustal project founded by

CNR, ENI-Agip and Enel), tied with geophysical industrial

data and inland ®eld observations.

G E O D Y N A M I C S E T T I N G

The Ionian abyssal plain (Bigi et al. 1989) is surrounded

by a variety of geodynamic settings (Boccaletti et al. 1984).

The Ionian lithosphere is subducting underneath Calabria

to the northwest (Caputo et al. 1970, 1972; Gasparini et al.

1982; Cristofolini et al. 1985; Selvaggi & Chiarabba 1995;

Mele 1998). The associated accretionary wedge advanced

in the Ionian Sea (Tramutoli et al. 1984; Pescatore & Senatore

1986; Senatore et al. 1988; Doglioni et al. 1999), mainly

involving the sedimentary cover on top of it (Finetti 1982).

Very shallow deÂcollements deform the Messinian evaporitic

sequences, leaving the underlying crust mainly undisturbed. To

the east, the Ionian lithosphere is subducting underneath Greece

(Le Pichon & Angelier 1979; Le Pichon 1982b; Christova &

Nikolova 1993). The southern and southwestern margins of the

Ionian Sea are the areas which have not yet been involved

in Tertiary and Quaternary shortening of the Apennines and

Hellenic subduction zones (Fig. 1). There, the original shape of

the Ionian margins may be more easily studied and they show

morphology, water depth and geophysical signatures of passive

margin style. The Bouguer gravity map (Fig. 2) shows values

mainly between 130 and 250 mGal in the Ionian abyssal plain;

Figure 1. Locations of the seismic sections used in the paper.

50 R. Catalano, C. Doglioni and S. Merlini

# 2001 RAS, GJI 144, 49±64

the values decrease moving underneath Calabria to between 20

and 30 mGal due to the subduction of the Ionian lithosphere.

Della Vedova & Pellis (1989) described low heat ¯ow values

(30±40 mW mx2) in the Ionian Basin (Fig. 3) and proposed an

early Mesozoic age for the oceanic crust.

Ciminale & Wasowski (1989) described the Hyblean plateau

(or Ragusa plateau) magnetic anomaly, and compared it with

the East Coast magnetic anomaly of the eastern US Atlantic

passive continental margin, where this anomaly is interpreted

as related to magmatic intrusions. Along the Malta escarpment

in other CROP seismic re¯ection pro®les (C9421, M23) a huge

intrusion may be inferred (Catalano et al. 2000).

In the section between Sicily and Puglia, the Ionian Sea

should be a complete oceanic section containing an aborted

oceanic ridge of Mesozoic age. Its relief is lost by thermal

cooling and hidden by thick pelagic deposits, ranging in age

from Jurassic to Tertiary, and by the overlapping Apenninic

thrust sheets. If this is the case, the Ionian sea would be one of

the oldest oceanic crusts in the world.

T H E C R O P S E I S M I C D A T A

New seismic pro®les have been acquired in the Ionian sea and

other Italian seas and inland by the CROP project. In this

paper we present segments of lines M3 and C9434 and the line

C9422. These seismic pro®les are particularly important for

understanding the early history of the Ionian Sea (Fig. 1). The

acquisition parameters and the entire set of seismic lines will

soon be published as an atlas by the partners of the CROP

project.

The ®rst segment is along the Malta escarpment (Fig. 4), at

the transition between the Sicilian mainland and the deep Ionian

Basin. This topographic margin represents one of the most

important features of Mediterranean geology and physio-

graphy (Scandone et al. 1981; Scarascia et al. 1994). The high

topographic gradient corresponds to a major change in crustal

thickness and composition (Finetti 1982). For a geological inter-

pretation of this margin see Scandone et al. (1981) and Casero

et al. (1984). The other line we show is located on the other side

of the Ionian Sea, along the Apulian±Salento margin (Fig. 5),

where there is a sudden submarine topographic step, separating

the stratigraphic successions of the Apulian swell from the deep

Ionian Basin; Fig. 6 is located halfway between the former

sections (Figs 4 and 5). Fig. 7 is a further complete section of

the southern part of the Ionian Sea.

A stratigraphic reconstruction of the three different sections

is presented in Fig. 8, and Fig. 9 is a synopsis of the Ionian

Sea based on the seismic and stratigraphic data. Seismic facies

along M3 show high-frequency and good continuity patterns,

and locally transparent or chaotic characters. In the central

part of Fig. 4 (M3), the strong ¯at-lying re¯ectors are about

1 s two-way-time thick, which could correspond to the pre-

Miocene deep-water succession on a stretched continental crust

(Casero et al. 1984).

Upwards, the acoustic body is re¯ection-free or it shows a

chaotic pattern, and it is topped by high-amplitude horizons

with frequent diffraction. Its thickness ranges between 0.4 s in

the abyssal plain and about 1 s two-way-time westwards, near

to the continental rise. The body has been interpreted as con-

sisting of Miocene clastics and Messinian evaporites (Cernobori

et al. 1996). Thin Plio-Pleistocene cover is widespread all over

the section.

The deep crustal levels eastwards of the Malta escarpment

(Fig. 4) show homogeneous acoustic characteristics and con-

stant thickness (about 1.5±2 s two-way time, i.e. 6±7 km). The

Figure 2. Gravimetric map of the Ionian Sea, courtesy of ENI-Agip.

The Mesozoic Ionian Basin 51

# 2001 RAS, GJI 144, 49±64

re¯ecting band is formed by layered high-amplitude re¯ectors

at the base of the crust (located at 8±9 s two-way time, i.e.

15±17 km, Fig. 7). The layered crust has a traveltime thickness

of about 1 s. It is overlain by a seismically transparent band

topped by a high-amplitude and discontinuous re¯ector, charac-

terized by diffraction, located at about 7±6.5 s two-way time,

shallowing towards the abyssal plain (Catalano et al. 2000).

Similar images of the crust and depth of the Moho are

described by De Chassy et al. (1990) in the North Atlantic

margin (see their section AGC 85±1/2). However, Cernobori

et al. (1996) disagreed with the idea that the Ionian crust has an

oceanic nature, interpreting the layered re¯ections as the lower

crust of an extremely thinned continental crust, intruded by

upper mantle rocks (Makris et al. 1986). In Fig. 7 the layered

crust shows offsets which we interpret as possible faults with

extensional components.

The seismic image of the conjugate margin is seen at the

Salento±Apulian escarpment (Fig. 5, line C9434) that separates

the Apulia swell, composed of carbonate platform seismic facies

from the Ionian abyssal plain. The escarpment view is disturbed

by diffuse diffractions. However, on the right-hand side, both

the top and the bottom of the Apulian Triassic±Jurassic±

Cretaceous carbonate platform are easily recognizable. This is

well known inland and from industrial seismic lines as a body

approximately 2 s two-way-time thick (about 6 km thickness).

The transition to deep-water crust is sharper than along the

Malta escarpment opposite to the southwest. The crust to

the southwest of the Apulian margin is poorly re¯ective, and the

sea¯oor is in many places more than 3000 m deep, even up to

more than 4000 m, depths which are typically oceanic.

Fig. 6 is another segment of seismic line C9434, halfway

between the two former sections (Figs 4 and 5). The sea¯oor

shows an anomalous depression which could be associated either

with a con®ned thermal subsidence or more probably with the

lateral ramps of thrust planes of the Apennines accretionary

wedge. Surprisingly, however, the underlying 4 s shows crust

with re¯ectors dipping from both sides of the section towards

the central depression, but they could be multiples. The Moho

depth is unclear; there are signi®cant re¯ectors at about 9 s

depth.

The abrupt morphologies of the Malta and Apulia escarp-

ments indicate that there have been recent reactivations of

these margins. In particular, the Malta escarpment is active in

terms of seismicity and magmatism. These tectonics were inter-

preted by Doglioni et al. (1998) as being due to the right-lateral

transtension generated by the differential roll-back between the

Ionian sea and the eastern Sicily lithospheres.

S T R A T I G R A P H I C C O N S T R A I N T S

The area can schematically be divided into three main sectors:

the Hyblean Plateau to the southwest (Sicily), the Ionian deep

basin, and the Apulian swell to the northeast (Puglia) (Fig. 8).

We omit, for the sake of simplicity, the allochthonous thrust

sheets in the Apennines accretionary wedge.

The Hyblean plateau (Bianchi et al. 1987) has a Triassic±

Neogene sequence more than 5 km thick, lying above a 20±25 km

thick continental crust with African af®nity. The main base of

the sedimentary cover consists of a thick (more than 2.5 km)

Upper Triassic carbonate platform (tidal ¯at dolomites facies),

Figure 3. Heat ¯ow values of the Ionian Sea (after Della Vedova & Pellis 1989).


# 2001 RAS, GJI 144, 49±64

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# 2001 RAS, GJI 144, 49±64

passing to Uppermost Triassic±Lower Liassic Streppenosa

basinal carbonates. The carbonate platform later drowned to

pelagic facies. The eastern margin of the Hyblean plateau is the

Malta escarpment (Fig. 1), where the sea¯oor rapidly becomes

deeper towards the Ionian Sea. Clear cross-sections of the

Malta escarpment were provided by Casero et al. (1984).

The Ionian Sea (Biju-Duval et al. 1982) exhibits about

4.5±5 km of sedimentary cover of seismically interpreted

pelagic facies from Triassic (?) to present, apart from the

Messinian evaporites, resting on top of a `basaltic' layer of

oceanic nature. Fig. 8 shows the interpreted stratigraphic

column with the relative seismic velocities.

In contrast, the Apulian swell (Auroux et al. 1985) to

the northeast (Fig. 5) lies on a crystalline continental crust.

The sedimentary cover starts with a siliciclastic sequence of

Late Permian±Early Triassic age, covered by an approximately

Figure 5. The Salento escarpment separates the Apulian swell from the Ionian abyssal plain. It is considered to be a stretched passive continental

margin. It is sharper than the Malta escarpment and it appears as its conjugate margin.

Figure 6. Halfway between the Malta and Salento conjugate passive continental margins, at about 165 km from each margin, there should be an

aborted oceanic ridge. The relief of the oceanic ridge should have been lost by thermal cooling and the later burial by sediments. However, in the

shallow crust (sedimentary cover), this area is affected by the front of the Apennines accretionary wedge, which may have generated rough

topography.


# 2001 RAS, GJI 144, 49±64

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# 2001 RAS, GJI 144, 49±64

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# 2001 RAS, GJI 144, 49±64

5 km thick carbonate platform sequence with interbedded

evaporites and a lateral pelagic transition at the top in the Late

Cretaceous±Early Tertiary.

The Hyblean Plateau (Fig. 4) and the Apulian swell (Fig. 5)

show typical passive margin sedimentary successions, well

known inland in Sicily and the Apennines, whereas the Ionian

Basin indicates a persistent deep-water environment (Fig. 7).

Section C9422 (Fig. 7) exhibits a half-lens-shaped body in its

southwestern segment. This body is about 100 km wide and it

is marked by irregular internal re¯ections. We do not have any

constraints on its nature; possible interpretations are con¯ict-

ing, for example, either a basaltic ¯ow or a deep-water clastic

fan sourced from North Africa. The last interpretation brings

to mind the Early Miocene Numidian sands of the southern

Apennines (Patacca et al. 1992). The overlying sediments onlap

the northeastern margin of the lens.

We may be tempted to link or to compare the Ionian

Sea stratigraphy to the Lagonegro sequences of the southern

Apennines to the north; however, several differences probably

occur between the two realms. First, the Lagonegro sequence

starts with relatively shallow-water facies (Monte Facito

of Middle Triassic age, also containing olistholiths of Late

Permian shallow-water facies). The shallow-water environment

is probably associated with a continental crust of 20±30 km

thickness. The overlying Calcari con Selce and the cherty

Liassic Scisti Silicei more probably resemble a deep-water

pelagic setting. Similar deep-water facies occur in the Imerese

and Sicanian basins in Sicily. However, their shallow-water

substratum constrains their position on a Permian±Triassic

stretched continental crust. Therefore, the deep-water facies

of the Ionian Sea cannot simply be considered as the southern

equivalent of the Lagonegro sequences of the southern

Apennines due to their different crustal substrate. Moreover,

these Lagonegro tectonic units have overriden the Apulian

platform (Mostardini & Merlini 1986) and the restoration of

the thrust sheets indicates a very different palaeogeography of

the Apulian, Lagonegro and Ionian Sea sequences (Zappaterra

1994) with respect to their present location in the Apennines

accretionary wedge (Casero et al. 1988; Sella et al. 1988;

Boccaletti et al. 1990; Marsella et al. 1995). A palaeogeographic

reconstruction is proposed as shown in Fig. 10, where the

Lagonegro sequences were located a few hundred kilometres

westwards of their present position, before their involvement

in the Apennines accretionary prism. However, the fast Liassic

subsidence which generated the upward deepening of the

Lagonegro and Sicilian sequences could well be associated with

the opening of the Ionian Ocean.

W H A T I S T H E A G E O F T H E I O N I A N S E A ?

New lines of evidences are in favour of the old idea that the

Ionian Sea is ¯oored by oceanic crust. The Ionian Sea has a

thin 8±9 km oceanic crust and 6±8 km of sedimentary cover of

Mesozoic and Tertiary age (de Voogd et al. 1992). In their

interpretation, the entire crust has a maximum thickness of

17±19 km. The interpreted oceanic crust of the Ionian Sea has

been interpreted as being Early Jurassic by Finetti (1982). We

interpret the top of the oceanic crust in our sections at about

6.5±8.0 s two-way time. Based on the low heat ¯ow values in

the Ionian abyssal plain at about 4000 m depth (34 mW mx2),

Della Vedova & Pellis (1989) proposed an age of 180±200 Myr for

the oceanic embayment; the 90 km thick lithosphere (Calcagnile

& Panza 1981) also supports an old age for this crust. In Sicily,

deep-water pelagic Permian fauna are known throughout the

Permian. Catalano et al. (1991) speculated that Sicily belonged

either to the Permian Tethyan Ocean or to a Permian rift with

thinned continental crust, a continuation of Palaeotethys further

to the east. Magmatism along the Malta escarpment and on the

Hyblean plateau started during Triassic times and lasted into

the Tertiary. Later Pliocene and Pleistocene magmatism was

emplaced along the same trend (e.g. Mount Etna).

Figure 9. Reconstruction of the Ionian Ocean showing the assumed Mesozoic age of sea¯oor spreading. The rates of opening appear to be very low.

In this interpretation, the Apulian and Hyblean plateaux were originally connected. Continental rifting might have started in Late Permian±Triassic.

# 2001 RAS, GJI 144, 49±64


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# 2001 RAS, GJI 144, 49±64

Therefore, due to the lack of direct information (magnetic

anomalies, well data, etc.) on the age of the Ionian sea, we

support the idea that continental rifting started in the Late

Permian and Triassic, later evolving to oceanic spreading.

However, we do not have reliable constraints on the end of this

rifting; based only on the heat ¯ow data and the assumed age

of the sediments overlying the crust, we interpret a possible

Late Cretaceous±Early Tertiary age for the abortion of oceanic

spreading. Based on these assumptions and the width of

the basin, we hypothesize low values of oceanic spreading

(<1 cm yrx1?).

The two conjugate Mesozoic passive margins (i.e. Malta and

Salento) have been slightly deformed by later, mainly Neogene

and Quaternary tectonics. However, we interpret the present

topographic steps along the margins, between shallow water

and deep water, as being mainly inherited from the Ionian

Ocean rifting. The two margins have been used or reactivated

as transfer zones during the larger rollback of the Ionian

lithosphere subduction with respect to the adjacent continental

margins. Moreover, at shallower crustal depths, these margins

have also controlled the lateral advancement of the Apennines

accretionary prism, where right-lateral and left-lateral trans-

pression tectonics occurred, respectively, along the Malta and

Salento margins.

I S T H E R E A M E S O Z O I C O C E A N I C R I D G EI N T H E I O N I A N S E A ?

Since the Ionian Basin is a Mesozoic Ocean with its preserved

passive continental margins, we expect to recognize an oceanic

ridge halfway between the two margins. The assumed ocean is

about 330 km wide and therefore we checked on seismic line

C9434 at about 165 km whether there is any evidence of an

oceanic ridge (Fig. 6). At that point the section shows an

anomalous depression of the sea¯oor at about 4 s two-way

time and the dips of the re¯ectors converge below this small

3 km wide basin. However, we suspect that this anomaly could

be related to the presence of folds associated with the Apennines

accretionary prism, since the seismic pro®le is located far inside

the front of the accretionary wedge (Fig. 1). Moreover, some of

the signals at depth could be multiples. We also expect thermal

subsidence along an aborted oceanic ridge, which would have

hidden the original morphological relief of the mid-ocean ridge.

The 3±6 km thick sedimentary cover above the oceanic crust

generated further lithostatic subsidence, depressing and hiding

the assumed oceanic ridge.

C A L A B R I A B A S E M E N T A N D T H EI O N I A N O C E A N

It is noteworthy that the reconstruction of the northwestern

prolongation of the Ionian oceanic crust based on the seismic

re¯ection data of this study matches the occurrences of the

basement rocks of Calabria and northeast Sicily (Fig. 10).

Apparently, where there is downgoing oceanic lithosphere in the

subduction, Alpine±Variscan metamorphic and intrusive rocks

crop out in the hangingwall. The basement rocks of Calabria

and Sicily (Platt & Compagnoni 1990; Bonardi et al. 1994)

occur to the south of the Sangineto Line in northern Calabria

and to the northeast of the Taormina Line in northeast Sicily

(Bigi et al. 1989). This unexpected relationship could probably

be related to the different depths of the deÂcollement planes as

a function of whether there is oceanic or thinned continental

crust in the footwall of the subduction zone. In fact, the

crystalline basement continues both northwest of Calabria and

west-northwest of the Peloritani±Sicily outcrops, but it is buried

or below sea level, as indicated by dredging in the Tyrrhenian sea

(Kastens et al. 1988). The occurrence of basement continental

rocks in the hangingwall of the subduction of oceanic litho-

sphere is the best evidence for an almost complete sinking of

the slab, without signi®cant accretion from the footwall to

the hanging wall of the subduction of crustal material. The

sedimentary cover overlying the Ionian oceanic crust is largely

excluded from the subduction, being involved in the offshore

accretionary wedge.

T Y R R H E N I A N S E I S M I C I T Y A N D T H EI O N I A N O C E A N

There is another relevant relationship between the Ionian north-

western palaeogeographic prolongation and the seismicity. In

fact, the so-called seismically active Tyrrhenian slab (Amato

et al. 1993; Selvaggi & Chiarabba 1995) mainly follows the

natural continuation of the Ionian Ocean towards the northwest

(Fig. 10). This observation supports the oceanic nature of the

downgoing lithosphere. However, we know that the shortening

and sedimentary facies in the Apennines accretionary wedge

in Sicily and the southern Apennines (respectively to the west

and north of the Ionian Basin and Calabria) imply subduction

of continental lithosphere stretched during the Permian and

Mesozoic. These lithospheric segments adjacent to the Ionian Sea

are seismically mute, or with much lower seismicity. However,

the kinematics predicts several hundred kilometres (400±500 km)

of shortening in the accretionary wedge of those areas. These

values imply at least an equivalent amount of subduction.

Therefore, the shortening which is visible in the accretionary

prism where the deep-slab seismicity is lacking or attenuated

suggests that subduction has occurred all along the Apenninic arc.

Marson et al. (1995) described negative gravimetric anomalies

below the southern Apennines, interpreting these data as an

indication of the absence of the slab. However, these mass

balances with respect to the Ionian area could be attributed

to the lighter continental origin of the slab underneath the

southern Apennines when compared to the heavier oceanic

nature of the material to the south.

The continental lithosphere has a lower temperature for the

brittle±ductile transition (300±400 uC) than the oceanic crust

(500±650 uC). The paucity of deep seismicity along the southern

Apennines and Sicily could be attributed to the more ductile

rheology of the quartz±feldspar-rich continental lithosphere

with respect to the olivine±pyroxene-rich Ionian Sea subduct-

ing underneath Calabria with a more brittle behaviour, and

generating higher seismicity.

The differences in the subducting lithosphere, that is,

continental below the central-northern Apennines and oceanic

below Calabria, are also supported by the magmatism, which

clearly shows different sources (Peccerillo 1985; Serri et al.

1993).

It is well established that the Tyrrhenian Basin is larger in

its southern part, where the deeper Ionian Basin occurs in the

# 2001 RAS, GJI 144, 49±64


foreland (Malinverno & Ryan 1986; Doglioni 1991; Faccenna

et al. 1997). This is another indirect piece of evidence that the

Ionian Sea has a different lithosphere in comparison to the

Sicily and southern Apennines lithospheres.

I O N I A N S E A A N D T E T H Y S

In this paper we mainly support the idea of the oceanic nature

of the Ionian sea. However, there are still several points of

uncertainty, for example, the age of the opening of the basin,

the anomalous geophysical signatures that are used to support

the continental nature, etc. Nevertheless, there is general

agreement that the Ionian Basin mainly developed during the

Mesozoic, that is, as a branch of Tethys.

The reconstructions made of Tethys (Biju-Duval & Dercourt

1980; Dercourt et al. 1986; Ziegler 1988 and references therein)

and the related Atlantic opening show that the so-called Tethyan

realm was a strongly fragmented area, with isolated pieces of

continental lithosphere, sometimes separated by oceanic crust,

passively moving along a general E±W trend (following the

transform faults of the Atlantic opening) in the western Tethys,

and along a NE±SW trend in the eastern Tethys (along the

Vardar subduction zone or the Cimmerian suture). When

mapping the normal faults that controlled the Atlantic and

Tethys opening, we note a strong coherence of data: N±S striking

faults (for example, the Malta escarpment), with a variable

range of 15u west or east for the Adriatic margin (Bernoulli et al.

1979 and Lemoine et al. 1986 for the European margin; Dal

Piaz et al. 1995 and Channell & Kozur 1997 for the western

Tethys; Masson & Miles 1986 for the Atlantic; Ziegler 1987 for

the general area).

The Cimmerian and Vardar suture zones with a NE or NNE

sense of subduction (WNW or NW trending thrust belts) have

been documented as active throughout the entire Mesozoic,

sometimes since the Palaeozoic (SengoÈr et al. 1984; SengoÈr 1984).

This coincides with Tethys (both palaeo- and neo-Tethys)

extensional tectonics.

The palaeogeographic reconstructions based on stratigraphic

correlations have different solutions for the Ionian Sea. There

are hypotheses connecting this basin to the palaeo-ocean sub-

ducted and obducted in Oman (Catalano et al. 1988; Bernoulli

et al. 1990). There are interpretations of the Ionian as an

embayment, closed towards the northwest; various other theories

connect the Ionian Mesozoic Basin to the Ligure±Piemontese

Ocean, later involved in the Alpine orogen. Some authors debate

the presence of an ocean between Sicily and the southern

Apennines due to the occurrence, in both areas, of the Miocene

Numidian sands sourced from Africa (Patacca et al. 1992).

The Mediterranean orogens involved oceanic branches of the

Tethys. Ophiolitic rocks of Triassic and Jurassic age and/or

coeval deep-water sediments are widely distributed throughout

the Mediterranean area, from Turkey into the Carpathians

area, and in Sicily and the southern Apennines. The trace of

the Mesozoic oceanic basins is to ®rst approximation marked

by the present shape of the Mediterranean orogens because

subduction and collision zones strictly followed the pre-existing

shape of the passive continental margins. Rare parts of the

Tethys, not yet involved in subduction processes, are good

targets for analyses of how the oceans involved and their

margins were created before being lost in subduction zones or

deeply transformed by metamorphism and shortening in the

orogens. The Mesozoic oceanic basins probably represented

extensions into the Mediterranean area of the Palaeotethyan

Ocean located to the east. The rifting of Africa from North

America/Europe in Triassic times gave rise to terrigenous

basins and the subsequent development of evaporites and wide

areas of shallow-water carbonate deposition. Accelerated sub-

sidence in Liassic times caused foundering of carbonate plat-

forms throughout the western Mediterranean area. The onset

of oceanic rifting between Africa and North America/Europe is

dated as Toarcian±Callovian by the age of the oldest sediments

above the basaltic basement in the western Atlantic (Stamp¯i

et al. 1998). The event is marked by accelerated subsidence

throughout the western Mediterranean.

The motion of Africa relative to Europe can be established

from the Atlantic spreading history (Dewey et al. 1989; Olivet

1996). For Jurassic and Early Cretaceous times, a sinistral

transtensional motion of Africa relative to Europe is expected.

This is consistent with the subsidence history and the style

of faulting in the western Mediterranean, where the Triassic/

Jurassic rifting between Africa and Europe occurred in an

east±west sinistral transtensional kinematic framework. North

Africa records the same pattern as detected in the whole of

southern Europe, with north±south (N20uW±N20uE) striking

normal faults and east±west (N70u±100u) strike-slip or transfer

faults. In Morocco, from Late Permian until Cretaceous times,

sinistral transtensional tectonics occurred along N70u±90uEbasins (Gibraltar, Pay des Horst, High Atlas), whilst graben

and half-graben (pull-aparts) developed en eÂchelon to these

features (i.e. the Middle Atlas) during the relative eastward

motion of Africa with respect to Europe. The dextral relative

motion of Europe relative to Africa during Late Cretaceous

and Tertiary times produced the inversion of the previous

structures. Former east±west-trending negative ¯ower structures

have been inverted as positive ¯ower structures (for example,

Hyblean offshore and Sicily Channel, Antonelli et al. 1991;

Casero & Roure 1994). The greatest inversion occurred where

the crust was more stretched by the earlier tectonics.

The Mediterranean region is characterized by a great lateral

variability in the thickness and composition of the litho-

sphere (Calcagnile et al. 1982; Suhadolc & Panza 1988, 1989).

This background is something which evolved during the

`Alpine' cycle, with a variety of tectonic regimes that affected

the region throughout post-Palaeozoic times, with the opening

and closing of several oceanic branches, grouped together as

Tethys. It is well known that the thickness and composition

of the lithosphere is a key point in controlling the rate and

possibility of subduction (Cloos 1993). Consequently, the extreme

lateral variability of the lithosphere of the Mediterranean,

which has persisted since at least the Mesozoic up to the

present, strongly controlled the relative microplate motions in

the whole area (Doglioni et al. 1994). Note, for instance, how the

Ionian subduction rapidly changes and decreases northwards

and westwards, corresponding to the different compositions

of the Apulian and Sicilian lithospheres. In the Adriatic and

Ionian zones, the lithosphere has an average thickness of

90 km, with positive and negative oscillations of about 20 km

(Calcagnile & Panza 1981; Calcagnile et al. 1982). The lateral

variations are responsible for the irregular pattern of Tethys

itself, located in between two major continental blocks (Eurasia

and Africa).

The different thicknesses and compositions of the Ionian,

Adriatic and African lithospheres determined the asymmetry

between the northern and southern Tyrrhenian sea and southern


# 2001 RAS, GJI 144, 49±64

Apennines and Sicily. The opening of the southern Tyrrhenian

sea and the shortening in the southern Apennines are much

larger than those of their northern counterparts, in particular

at the 41u latitude transition; this structural variation occurs

where the Ionian oceanic lithosphere in the foreland to the south

and the Adriatic thick continental lithosphere to the north are

subducting. In this regard, the Tyrrhenian sea represents a

powerful laboratory in which to investigate different styles and

amounts of back-arc extension as a function of the composition

and thickness of the subducting lithosphere of the foreland,

that is, the Ionian, Adriatic and Sicily lithospheres.

The larger expansion of the Neogene and Quaternary

Apennines arc in the Ionian Sea (Fig. 1) con®rms how the

Mesozoic Ionian Basin subducted and retreated faster and more

easily than the neighbouring areas of Sicily and the southern

Apennines (Malinverno & Ryan 1986; Patacca & Scandone

1989; Doglioni 1991; Faccenna et al. 1997).

C O N C L U S I O N S

The Ionian Sea appears to be an oceanic basin of Mesozoic

age (Fig. 9). It mimics several morphological and geophysical

signatures which have been described in the central Atlantic

Ocean (DanÄobeitia et al. 1995). However, there are still some

anomalies in its geophysical signatures (compare Calcagnile

et al. 1982 and De Voogd et al. 1992). Whatever its origin, the

Ionian Sea is a rifted area whose stretched conjugate margins

are the Malta escarpment to the southwest and the Apulian

escarpment to the northeast (Fig. 9). The oceanic nature of the

Ionian Sea would imply an aborted middle oceanic ridge

located halfway between the two conjugate passive margins

(Fig. 10). However, the topographic elevation of the ridge axis

should have been lost by thermal cooling after ocean abortion

and the later burial of Tertiary sediments. The Apennines

accretionary wedge also overrode the ocean ridge zone (Fig. 6).

Based on the undulating strike of the two conjugate passive

continental margins, the direction of the Mesozoic extension

could have been oriented NE±SW (Fig. 10).

The age of the basin is not yet well constrained. Based on the

deep sea¯oor (>4000 m in the abyssal plain), low heat ¯ow

(30±40 mW mx2, Della Vedova & Pellis 1989) and stratigraphy

of the rifted margins, the rifting could be Permian±Triassic at

the continental stage, evolving to oceanic spreading during the

Late Jurassic±Early Cretaceous. During the Late Tertiary, the

Ionian Sea looks like an aborted trapped oceanic basin.

Based on inland constraints, that is, the Sicilian and southern

Apennines carbonate platform margins and adjacent basins,

the Ionian Ocean continued to the northwest (Fig. 10). Since

the main deformation of the Ionian area started in the late

Neogene, the reconstruction at early Miocene±late Oligocene

times of Fig. 10 is considered as a view of the Mesozoic

palaeogeography.

The seismicity of the Apennines slab underneath the southern

Tyrrhenian sea, which implies downgoing oceanic lithosphere,

also supports the palaeogeographic prolongation of the Ionian

Ocean towards the northwest (Fig. 10). The absence or paucity

of seismicity to the north in the southern Apennines and in

Sicily and their Tyrrhenian margins does not imply the absence

of the Apennines slab continuity in those areas. These changes

in seismicity and in several other geological and geophysical

signatures may be explained by the inherited Mesozoic crustal

and lithospheric differences of the downgoing foreland, that is,

oceanic in the Ionian, and continental in the southern Apennines

and Sicily.

A C K N O W L E D G M E N T S

The paper bene®ted from critical reviews by M. A. Khan,

G. Panza and an anonymous referee. Many thanks to A. Bally,

M. Bello, D. Bernoulli, M. B. Cita, J. Channell, M. Gaetani,

E. Gueguen, F. Mongelli, P. Pieri and M. Tropeano for helpful

discussions. The Italian MURST (Co®n 99) and CNR supported

this study (grant 98.00228.CT05).

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AAPG HEDBERG CONFERENCE “Deformation, Fluid Flow and Reservoir Appraisal in Foreland Fold and Thrust Belts”

May 14-18, 2002, Palermo – Mondello (Sicily, Italy) THE IONIAN CRUST AND THE CALABRIAN ACCRETIONARY WEDGE

Catalano R.*, Doglioni C.**, Merlini S.° & Sulli A.* * Dipartimento di Geologia e Geodesia, Università di Palermo, via Archirafi 26, Palermo

** Dipartimento di Scienze della Terra, Università “La Sapienza” di Roma, P.le A. Moro 5, Roma ° ENI-AGIP, San Donato Milanese (MI)

E-mail: [email protected] (corresponding author) Introduction

The interpretation of deep seismic lines has revealed the structure of the Pelagian passive continental margin and the related Ionian ocean (Catalano et al., 2000), highlighting the evolution of the Mesozoic continental rifting and oceanic spreading (Catalano et al., 2001). Crustal pattern and geometries of the continental margin-to-ocean transect is an important constraint to study subduction processes of the Ionian lithosphere beneath the outer Calabrian Arc (Fig. 1).

The unmigrated crustal lines were calibrated by using geological and geophysical data. Interpretative techniques attempted to distinguish several seismic facies to estimate the lithology and geometry of the reflecting units and to discriminate between sedimentary and crystalline units. Previously published magnetic, heat flow, gravity and bathymetry, as well as seismic reflection and refraction data (ESP, DSS, EGT and WARR) were used to constrain, at a regional scale, the crustal structures interpreted from the seismic reflection lines. General setting The passive continental margin and the related Ionian ocean

The passive continental margin extends from the Iblean-Malta shelf, through the Malta Escarpment, to the Western Ionian Sea. Rifting events started in pre-Late Triassic time, but major extensional features appear to dissect the top of the Triassic carbonate platform and the late Jurassic-early Cretaceous pelagic deposits.

The continental margin crust becomes progressively thinner eastwards. In addition to early Mesozoic block-faulting of both the basement and sedimentary cover, the interpreted large igneous intrusions (Catalano et al., 2000) support the “transitional” nature of the crust flooring the Malta slope and the western Ionian sector. Several Triassic to Neogene mafic volcanic levels, sandwiched in the 6 to 9 km thick sedimentary strata (Antonelli et al., 1991), and a magnetically postulated major mafic body, located in the upper crust east of the Malta Escarpment, support the hypothesis of a magmatic underplating of the thinned continental crust (volcanic continental margin) previously postulated by Della Vedova & Pellis (1992), Bonatti & Seyler (1987).

The lateral continuity of the sedimentary facies across the Malta Escarpment and the depositional relationships between the carbonate platform and basinal deposits enable the locating of the original edge of the Mesozoic continental margin in the western Ionian well beyond the Escarpment (Scandone et al., 1981; Catalano et al., 2000). The Malta Escarpment owes its morphogenesis to the reactivation caused by more recent vertical and/or transtensional tectonics (Casero et al. 1984; Doglioni et al., 2001).

The Iblean-Malta continental margin has recently been described by Cantarella et alii (1997), Catalano et alii (2001) as the conjugated margin of the Apulian swell in the other side of the Ionian, which is considered as a remnant of the Mesozoic Tethys ocean.

The Ionian abyssal plain and its eastern side are floored by a crust already interpreted as oceanic (Finetti, 1982; De Voogdt et al., 1992; Catalano et al., 2000, 2001) or thinned continental (Cernobori et al., 1996). The seismic characteristics of the Ionian crust strongly differ from the adjacent western and eastern sectors.

AAPG Search and Discovery Article #90011©2002 AAPG Hedberg Conference, May 14-18, 2002, Palermo - Mondello, Sicily, Italy

Catalano, Doglioni, Merlini & Sulli

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When correlated to the sedimentary body lying on the thinned continental crust, the “oceanic” sediments appear to be more recent than the lower Mesozoic carbonate platform: one could argue that the age of initial oceanic spreading cannot be further than Late Jurassic.

Despite the fact that tectonic subsidence analysis, deep sea floor (>4000 m in the abyssal plain), low heat flow values (Della Vedova & Pellis, 1992) support a late Jurassic-early Cretaceous age, the absence of borehole stratigraphy impedes the possibility of defining the true age of the ocean formation. The study area The Ionian subduction zone and the Calabrian accretionary wedge

A CROP seismic line crossing the Ionian abyssal plain towards the southern Calabria offshore images a well developed SE vergent accretionary wedge and the NW dipping oceanic basement (Fig. 2).

The most impressive signature of the Ionian abyssal plain is a couplet in the form of a highly reflective layered body and a transparent and unstratified band with overlapping hyperbolae (Catalano et al., 2000); the oceanic Moho deepens northward from 9 to more than 10 s/TWT in few kilometers (Fig. 2). The resulting sharp lateral discontinuity is here interpreted as the result of a near WNW-ESE transform (?) paleofault offsetting the Ionian crust. The crystalline crust is still coupled with the oldest sedimentary layers. The younger sedimentary layers are deformed and chaotically arranged along a decollement plane overlying the oldest sediments. They lack a coherent tectonic bodies organization.

In the intermediate sector the crust progressively deepens with a complex trajectory appearing offset by a WNW-ESE lateral (?) discontinuities (Fig. 2). Moho develops at about 11 s/TWT. The sedimentary cover is still deformed in its upper part appearing as coherent thrust ramps corresponding to the tip of the socalled External Calabrian Arc (Morlotti et al., 1982; Cernobori et al., 1996).

Towards the North the deformation reaches the lower and older sediments that form a 4 s/TWT thick tectonic wedge detached from its crust. Moreover the crystalline crust, well seismically imaged at 9 to 12 s/TWT depth interval is a gently arched body offset by reverse faults with variable fold wavelength.

Approaching SE Calabria offshore the oceanic crust becomes steeper as the faintly recognized Moho discontinuity occurs at more than 14 s/TWT. The upper crust (Layer 2 ?) is strongly deformed in several tectonic slices (Fig. 2). Above it a 2 s/TWT thick imbricated wedge composed of rocks correlatable to the older portion of the sedimentary cover is piled up. It underlies a 3 s/TWT thick seismically transparent to chaotic body physically correlatable with the continental crystalline portion of the adjacent outcropping Calabrian units.

Discussion

The seismics images that both sedimentary and crystalline Ionian crustal bodies are progressively detached from their substrate more deeply and markedly towards NW. As a consequence sedimentary and oceanic crust units appear embricated to form the SE verging accretionary wedge.

The subduction hinge zone is seismically imaged in the area where the Calabrian crystalline units overthrust the deformed oldest sediments deposited on the Ionian crust.

Thickness of the Ionian crust with respect to the adjacent continental areas, petrological-physical characteristics and relic structures could have influenced the geometry of subduction and generated processes in the shallow levels.

Occurrence of oceanic crust certainly favours subduction in this area, generating the Aeolian volcanic arc and the deep seismicity in the southeastern Tyrrhenian, as well as the ascent of the Etna magmas (Doglioni et al., 2001).

The geometry of the downgoing slab recalls articulated topography due to ancient morphostructures as the supposed medioceanic paleoridge (Cantarella et al., 1997; Catalano et al., 2001) and the transform faults formed before the subduction time. These morphostructures can make difficult the subduction and/or complicate its geometry (Font et al., 2001).

The seismic reflection study of the southern Tyrrhenian along the Cefalù basin-Solunto Mount sector (Agate et al., 2001) images a northward inflection of the Sicilian (African) continental crust below the



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submerged interpreted Kabilian-Calabrian continental crust (Figs. 1, 3), confirming a north-directed continental subduction as suggested by Doglioni et al. (1998).

The comparison with the crustal setting of the Calabria-Ionian sector highlights the importance of the crustal and lithospheric heritage of the downgoing foreland. The convergence of two continental crusts causes more difficulty in the subduction of the Sicilian crust respect to the Ionian sector where major convergence rate facilitates both a southward advancing of the deformation front (arcuate shape of the Apenninic front) and a vertical separation between the Ionian and Sicilian crusts.

The surface expression of this behaviour is the shorter propagation of the Sicilian frontal accretion and the building of a chain with major topographic relief respect to the accretionary wedge of the Calabria-Ionian sector.

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