late cenozoic kinematics of the calabrian arc, southern italy

TECTONICS, VOL. 10, NO. 6, PAGES 1164-1172, DECEMBER 1991

LATE CENOZOIC KINEMATICS OF THE CALABRIAN ARC, SOUTHERN ITALY

Steven D. Knott1 Department of Earth Sciences, Oxford, England

Eugenio Turco Dipartimento di Scienze della Terra, Universita della Calabria, Cosenza, Italy

Abstract. Based on structural analysis, seismicity, and paleomagnetic data a model is presented for the late Cenozoic kinematics of the southern Italian

mountain belt. The model predicts that late Cenozoic deformation of the internal part of the belt involved extension and lateral bending of the mantle and lower crust and that in the upper crust faulting was accompanied by semirigid block rotation. In the external part of the belt thrusting dominated. In the north where the arc joins up with the southern Apennines regional shear was dominantly left lateral and distributed between left-slip along WNW trending cross faults, a component of right-slip on north to NNE trending extensional faults, and counterclockwise rotation of upper crustal blocks, including the basin fill and north to NNE trending basin margin faults. In the southern part of the arc, overall right lateral shear was accommodated by right-slip on WNW trending cross faults, a component of left-slip on NE trending extensional faults, and clockwise rotation of upper crustal blocks, basins, and NE trending faults. These styles of deformation were probably confined to the upper plate of the Tyrrhenian subduction system. They are considered to have been active from the beginning of extension in the Tyrrhenian Basin (circa 11 Ma) and are still active today (based on recent seismicity).

INTRODUCTION

The southern Italian mountain belt (Figure 1) has been the subject of various tectonic studies [e.g., Ogniben, 1969, 1973; Haccard et al., 1972; Amodio- Morelli et al., 1976; Dubois, 1976; Ghisetti and Vezzani, 1981; Wezel, 1982; Malinverno and Ryan, 1986; Meulenkamp and Hilgen, 1986; Meulenkamp et al., 1986; Oldow et al., 1990]. Of considerable interest within these studies has been the arcuate

shape of the mountain belt. Carey [1958] considered the curvature to have arisen from bending of the orogenic belt within a sinistral megashear. Ghisetti and Vezzani [ 1981], on the other hand, proposed that curvature was due to buckling, in a horizontal plane,

1 Now at B.P. Exploration, Glasgow, Scotland.

Copyright 1991 by the American Geophysical Union.

Paper number 91TC01535. 0278-7407/92/91TC-01535510.00

that gave rise to an external tensile field (probably analogous to the tensile field in a bending elastic beam) generating two sets of graben distributed radially and parallel to the arc, respectively.

Eldredge et al. [1985] considered the Calabrian arc an example of the bending of an originally straight zone as it indents a rigid, irregular margin. In this mechanism the Calabrian nappe pile formed the rigid-block against which the southern margin of Neotethys collided in early Miocene time, producing an orocline. They considered lateral shear a small- scale effect localized along the north and south margins of the advancing arc. Rotations of palcomagnetic declinations with respect to the adjacent forelands, clockwise in Sicily and counterclockwise in the southern Apennines, were explained by rotation of thrust sheets during the collision (sensu lato).

Wezel [ 1982] considered the curved shape of the arc to be predominantly the result of vertical tectonics, whereas Malinverno and Ryan [1986] proposed that northeastward subduction beneath Calabria of an oceanic seaway caused the Calabrian arc to migrate passively through the seaway bending the arc as it collided with the continental blocks on either side.

Reapproaching the problem of the postcollisional (i.e., post-11 Ma) kinematics of the Calabrian arc, we note, as shown in Figure 2, the following characteristics: (1) its curved shape, (2) rhomb- shaped graben with long axes oriented transversely and longitudinally with respect to the arc, (3) tectonic rotations of crustal blocks about a vertical axis, (4) thrust and extensional tectonics, and (5) seismicity.

In the following we briefly outline the geologic setting of the southern Italian mountain belt, summarize published earthquake fault plane solutions and palcomagnetic data on tectonic rotations, and present kinematic analyses of faults we studied in the field (1985 field season) or identified from remotely sensed images. We then present an integrated model of kinematic development for the last 5 m.y. and discuss implications for inversion of the Plio- Quaternary basins.

The Calabrian Arc

The Calabrian arc is the central segment of the curved southern Italian mountain belt (Figure 2). The northern part of the arc can be divided vertically into three tectonostratigraphic units. The lowest unit comprises predominantly Mesozoic carbonate rocks that originally formed the continental margin of Africa (including Adria) and that were detached from their basement during late Cenozoic time and now form part of the Africa-verging Apennine and Maghrebide fold-thrust belts of mainland Italy and Sicily, respectively [Dewey et al., 1973; Scandone et al., 1974; Ogniben et al., 1975]. The middle unit comprises Mesozoic to Cenozoic metasedimentary and ophiolitic rocks (Liguride and Sicilide complexes) considered to be the remains of an ancient accretionary wedge [Knott, 1987; Hill and

Knott and Turco: Calabrian Arc'Kinematics

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Fig. 2. Plio-Quaternary tectonics of the Calabrian arc. The position of inferred cross faults is based on analysis of onshore mapping, Landsat tmage interpretation, and offshore seismic interpretatIon by Moussat [1983].

Hayward, 1988]. The uppermost element comprises Paleozoic igneous and metamorphic rocks and a Mesozoic to Cenozoic sedimentary cover (Calabride Complex) considered to be a fragment of the former European margin of Neotethys [Ogniben, 1969; Bouillin, 1984; Bouillin et aI., 1986; Knott, 1987; Dewey et aI., 1989](see Amodio-Morelli et ai. [1976] and Chalouan and Michard [1990] for alternative interpretations).

Each of the three units contain kinematic indicators showing directions of pre-Tortonian (11

Ma) thrusting from roughly west to east in the northern part of the arc [Dietrich, 1988; Faure, 1980; D.W.H. Hutton and E. Turco, manuscript in preparation, 1991; Knott, 1987]. In Sicily the direction of thrusting at this time was from roughly north to south with variable amounts of clockwise rotation of thrust sheets during emplacement [Oldow et aI., 1990].

Amodio-Morelli et aI. [1 976] proposed the NE trending Sangineto Line (Figure 1) as the northern boundary between the three central allochthonous units of the Calabrian arc and the southern Apennines (sensu stricto). These authors consider the lineament to be a transcurrent fault with left lateral displacement. Scandone [1982] has since cast doubt on the transcurrent nature of this lineament, and up to now no additional data have been published on the sense of displacement along the Sangineto Line.

Another major lineament recognized in the northern sector of the Calabrian arc, the WNW trending Pollino Line (Figure 2)[Bousquet and Gueremy, 1969], is considered by Ghisetti and Vezzani [1982] to be a post-Tortoninian right lateral wrench zone. In contrast we present data that shows the Pollino Line is part of a southern Apennines subvertical left lateral shear zone (see also Moussat [1983] and Boccaletti et ai. [1984]). Our interpretation is supported by evidence that the central portion of the Calabrian arc was translated southeastward from early Miocene time onward [Haccard et aI., 1972; Alvarez, 1976; Scandone 1979; Ghisetti and Vezzani, 1981; Malinverno and Ryan, 1986], which implies that the Pollino Line should have a left lateral not a right lateral sense of displacement. The WNW trending Taormina Line in northeastern Sicily (Figure 2) was proposed by Amodio-Morelli et aI. [1976] as the thrust contact

1166 Knott and Turco: Calabrian Arc Kinematics

between the central allochthonous elements of the

Calabrian arc and the adjacent Cretaceous-Eocene flysch. The lineament in detail consists of irregular tectonic contacts between the upper crystalline basement nappes of the Calabrian arc and the underlying flysch. Amodio-Morelli et al. [1976, Figure 1] considered the Taormina Line to have a fight lateral strike-slip component, but no evidence has yet been published which demonstrates conclusively the sense of shear or the timing of movement along the lineament.

The Tyrrhenian Basin

The Tyrrhenian Basin (Figure 2) is a WNW trending elongate trough, over 3 km deep in places, considered to be the result of back-arc extension [Boccaletti et al., 1971; Malinverno and Ryan, 1986; Rehault et al., 1987], counterclockwise rotation of the Italian peninsula [Scandone, 1979; Boccaletti et al., 1984], vertical tectonics [Wezel, 1982], or delamination of the lithosphere [Channell, 1986]. Extension started in late Tortonian time (circa 11 Ma) and is probably continuing at the present day in the southeastern part of the Tyrrhenian Basin and onshore Calabria. Leg 107 Shipboard Scientific Party [ 1986] of the Ocean Drilling Program stated that pelagic deposits overlying basalts drilled by Leg 107 were dated at about 3.5 Ma. In the southeastern

part of the basin, the basalts are overlain by younger, i.e., late Pliocene, ooze, and oceanic crust has been identified in the central part of the basin. The southeastern margin of the Tyrrhenian Basin has a markedly curved shape, following the curvature of the Calabrian arc.

The Aeolian Islands (Figure 1) are a series of emergent Quaternary volcanoes, the surface expression of a more extensive region of submerged volcanic seamounts [Colantoni et al., 1981]. The islands occur above a Benioff zone of intermediate to

deep seismicity which dips steeply (> 45') northwestward beneath the Tyrrhenian Basin [McKenzie, 1972].

A number of onshore and offshore sedimentary basins containing thick (up to 6 km) sequences of post-Miocene terrigenous deposits are distributed around the Calabrian arc. The long axes of most of the basins are oriented either parallel or transversely to the curvature of the arc (Figure 2). The offshore basins beneath the Tyrrhenian Sea are floored by marine sedimentary rocks of Messinian age overlain by a particularly thick Pliocene section [Colantoni et al., 1981]. The onshore basins in Calabria contain thick sequences of Pliocene to Pleistocene sedimentary rocks which pass upward from marine to continental deposits. Both onshore and offshore basins show evidence of tectonic activity throughout sedimentation [Bousquet and Gueremy, 1968; Fabbri et al., 1980; Colella et al., 1987; S.D. Knott and E. Turco, manuscript in preparation, 1991].

We recognize a variety of basin-forming mechanisms in the Calabrian arc that include back-arc

extension, strike-slip fault linkages (rhomb-shaped graben), and small (a few kilometers wide) basins at the edges of rotating blocks. Our model predicts that rotation of the basins and certain basin-bounding faults will occur during deposition (Figure 3). Large rotations (more than 45') of rhomb-shaped graben will eventually result in contraction of the original basin structure and its fill. This will give rise to inversion of preferentially oriented structures (e.g., reverse reactivation of normal dip-slip faults) and the development of contractional structures in the basin fill (Figure 3). Inversion can occur solely by block rotation irrespective of any change from extension to compression in the regional stress field [cf. Meulenkamp and Hilgen, 1986]. Any regional change will be superimposed on inversion caused by block rotation.

10km 0

Fig. 3. Block diagram showing structural inversion of a sedimentary basin due to rotation of bounding blocks, margin faults, and the basin itself. Extension faults in the top diagram are reactivated as reverse faults during the terminal stages of rotation. The width of the basin and the basin fill contract during inversion. An example of this type of structural inversion is the Crati Valley (see Figure 2 for location).

A basin which provides a natural test area of this hypothesis is the Pliocene to Pleistocene Craft Valley basin. The Crati Valley occupies a north trending rhomb-shaped graben bounded north and south by two left lateral cross faults. Back-arc extension

[Malinverno and Ryan, 1986] occurred throughout deposition therefore the basin formed in response to linkages between the cross faults and to back-arc extension. It is proposed here that the present geometry is due to counterclockwise rotation about a vertical axis of the entire basin and the extensional

basin-bounding faults by roughly 45' in a similar mode to that proposed by Eyal et al. [ 1986] for the Bir Zreir rhomb-shaped graben in eastern Sinai.

Post-late Pleistocene contractional structures have

been reported from the Craft Valley basin [Carobene and Damiani, 1985] and also in the Mercure Basin to

Knott and Turco: Calabrian Arc Kinematics 1167

the north (Figure 2)(S.D. Knott and E. Turco, manuscript in preparation, 1991). These contractional structures are related here to basin inversion predicted by the model for large rotations of blocks bounding sedimentary basins. The origin of other sedimentary basins located around the Calabrian arc (Figure 2) also appears to be related, in part, to linkages between cross faults. These basins, which include the Paola Basin, the Gioia Basin, the Sibari Trough, the Crotone Basin, the Catanzaro Trough, the Mesima Valley, the Strait of Messina and the Cefalu Basin, may also show the effects of rotation and inversion. Further, we propose that the mechanisms of lateral bending, heterogenous shear, and block rotation during extension affected the entire southern Italian region, from north of Naples through to the Sicily Channel.

External Calabrian Arc

Outboard of the Calabrian arc, toward the Ionian Sea, and extending for approximately 150 km from the shoreline, there is a discontinuous ridge of imbricated Pliocene and Pleistocene sedimentary rocks (External Calabfian arc, Figure 1) that follows the curvature of the arc. This ridge is considered to be a submarine accretionary wedge [Kastens, 1981; Rossi and Sartori, 1981] and links to the north with the Apennine chain and to the west with the Maghrebides of Sicily. The external arc is dissected by WNW trending lineaments into a number of segments.

Foredeep Basins

External to the southern Italian mountain belt, extending from the southern Apennines to Sicily, are asymmetric foredeep basins containing Pliocene to Recent deposits. The thickness of the foredeep deposits increases toward the mountain belt attaining up to 7 km of post-Miocene fill in places [Royden et al., 1987]. The foredeep basins are considered to have a flexural origin and to be the result of retreat toward the foreland of a subducted slab [Royden and Kamer, 1984; Royden et al., 1987]. The forelands of the southern Italian mountain belt in the southern

Italian mainland (Apulia) and the Sicilian Maghrebides (Iblei) are considered to be flexurally maintained outer highs of Mesozoic carbonate rocks still attached to their continental basement [Royden et al., 1987].

DATA

Seismicity

Southern Italy has been designated an area of high seismic risk [De Vivo et al., 1979], occasionally experiencing crustal earthquakes of up to M = 7.

PALERMO

Seismicity

0 200km I I I I I

Fig. 4. Most reliable fault plane solutions obtained from Recent seismicity in south Italy. These are lower hemisphere equal-area stereographic projections, where black is contractional and white is dilational. Data are after Cello et al. [1982 and references therein]. See text for discussion.

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Fig. 5. Distribution and kinematics of possible active faults in south Italy derived from fault plane solution data and selection of nodal plane on the basis of local geology. See text for discussion.


Numerous earthquakes and microearthquakes have been catalogued (over 200 epicenters located in Calabria alone (I. Guerra personal communication 1987)). Recent syntheses of instrumental seismicity include those by Cello et al. [ 1982], Gasparini et al. [1982], Cristofolini et al. [1985], Westaway et al. [ 1989], and Westaway [1990]. Of particular interest here are the kinematics of the fault plane solutions; only those solutions we considered most reliable have been represented in Figure 4. The kinematics can be summarized as follows: in the southern

Apennines, fault planes striking NE have a dominant left lateral component of slip (a in Figure 5) and NW striking fault planes (b and c) have extensional dip- slip and left lateral strike-slip components of displacement. In north Calabria north to NNE striking faults (d, e, and f) have dominant normal dip-slip components of displacement with a minor component of fight lateral slip. Near the Catanzaro Trough, WNW striking faults (h and i) are inferred to have fight lateral strike-slip displacements [Cello et al. 1982]. In the Messina Strait and in northeastern Sicily, NNE striklng faults (j and k) have left lateral strike-slip and normal dip-slip components of displacement.

Pale oma g netic Studies

The majority of paleomagnetic studies of southern Italy (Figure 6) have been in a regional context [Manzoni, 1975; Catalano et al., 1976; Channell et al., 1980] with the exceptions of the detailed study of the Lagonegro basin in the southern Apennines by Incoronato et al. [1985] and southern Calabria by Alfa et al. [ 1988].

Most of the previous interpretations of paleomagnetic declinations around the Calabrian arc [Catalano et al., 1976; Ghisetti and Vezzani, 1981; Eldredge et al., 1985; Incoronato and Nardi, 1987] agree that the variation in directions is due to tectonic rotations related to thrusting during collision and emplacement of the Calabria-Peloritani nappes onto the African (Adriatic and Sicilian) continental margin from early Miocene time onward. In contrast, we present evidence below which indicates that the major part of these tectonic rotations took place during extension and strike-slip deformation from late Miocene time onward. The rotations are

clockwise in Sicily and counterclockwise in the southern Apennines relative to the adjacent forelands of Iblei and Apulia, respectively (Figure 6).

Structural Lineaments

All lineaments described in this section have been

identified as major faults of demonstrable post- Messinian age. They are located either on the margins or within offshore and onshore post- Messinian sedimentary basins. The lineaments were identified from remotely sensed data (Landsat 2 MSS band 7 images 202/33 16 JUL 78; 203/32 15 JUL

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Paleomagnetism

Fig. 6. Paleomagnetic data for south Italy. Rotations of paleomagnetic declinations with respect to the declinations in the "African" foreland (large arrows). Clockwise rotations, in map view, occur in Sicily, and counterclockwise rotations in the southern

Apennines. For clarity, only data from late Cretaceous samples are shown. Data are after Catalano et al. [1976], Eldredge et al. [1985], and Incoronato and Nardi [ 1987].

75; 203/304 30 JUL 79; 202/32 30 MAR 78 and aerial photographs at 1:33,000 scale), from published geological maps [Carta Geologica della Calabria at 1:25,000; Cassa per il Mezzogiorno; Meulenkamp et al., 1986], the unpublished mapping by the authors (1985) and reflection seismic data of Moussat [1983].

Lineaments, first identified from remotely sensed data, were confirmed in the field as fault zones, and their relative age and sense of displacement were determined in the field. In the southern Apennines these lineaments were often found to be subvertical

fault zones, up to 100 m wide, of cataclastic dolomite. These fault zones truncate all

polydeformed ductile structures related to the early Miocene Apennine fold-thrust belt. In south Calabria and Sicily lineaments, and their sense of displacement, were identified from remotely sensed data and published maps. These identifications, and other predictions of the hypothesis presented here, are yet to be tested In the field.

Four well-defined lineament trends can be recognized around the Calabrian arc; these are WNW, north, NE and SE. As shown in Figure 7 major WNW trending faults occur along the Pollino ridge, at the noah and south terminations of the Craft Valley, in the S ila massif, the Crotone Basin, the Catanzaro Trough, Capo Vaticano, the Mesima Valley, and northeastern Sicily. The main lineaments


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Fig. 7. Post-Messinian structural lineaments of the Calabrian arc obtained from remotely sensed data (Landsat images and aerial photographs), published geological maps, and the authors' mapping at 1:10,000 and 1:25,000 scales.

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Fig. 8. Structural map based on authors field work showing part of the southern Apennine left lateral shear zone. The inset shows the geometric model for block rotation of Dibblee [ 1977] and the geometry of the Riedel shear model [after Wilcox et al., 1973] in left lateral shear. The stippled area is Plio-Pleistocene nonmarine sediments; the white area is mainly Apennine carbonate rocks and Liguride Complex metamorphic rocks. Abbreviations are R, Rotonda; MC, Monte Cerviero; MP, Monte Pollino; LF, La Falconara; LFF, La Falconara fault; TSL, Timpa San Lorenzo; TSLF, Timpa San Lorenzo fault; and MS, Monte Sellaro. AB and CD are the locations of cross

sections shown in Figure 9.

of the north trending set occur on the eastern and western margins of the Crati Valley, in the S ila Massif, and on the western margin of the Crotone Basin. The NE trending set occurs mainly in south Calabria, on the northwestern border of the Se:'re massif, throughout the Aspromonte massif, and in northeast Sicily. The SE trending set occurs only in north Calabria, near the Pollino ridge, in the Sila massif, and near Amantea (Figure 7).

An approximate bilateral symmetry of these fault zones about a line drawn parallel to the length of the Catanzaro Trough (Figure 2) provides the basis for dividing the Calabrian arc into two sectors north and south of this line (see also Meulenkamp and Hilgen [1986]). The detailed geometry and kinematics of the southern Apennine lateral shear zone (Figure 8) have been studied using slikenline data and gouge fabrics observed on fault planes, whereas kinematic interpretation of most lineament sets in other areas of the arc is based, in part, on comparisons with structural models by Cloos [1955], Riedel [1929], and Tchalenko and Ambreseys [ 1970].

Along the Pollino ridge (Figure 8), which lies parallel to the classical Pollino Line, two main fault sets are recognized. A set of faults with a major left- slip component, striking WNW, bounds the ridge to the north and south. The other set comprises faults with a major fight-slip component striking NNE that lie between the WNW trending faults. The two sets divide the ridge into various fault-bounded blocks. The WNW trending faults continue westward along strike, through the Mercure Basin (Pliocene to Recent), and form the northern and southern bounding faults of the Monte La Spina ridge (Figure 8). This ridge is also cut into blocks by a series of NNE striking faults with a fight lateral component of

displacement. Curvature of WNW striking faults toward the NW within the Mercure Basin is associated with a push-up of Mesozoic carbonate rocks which protrude through the basin sediments (Figure 9; section AB).

In the vicinity of Civita curvature of the same fault system toward NW has produced a thrust of Mesozoic carbonate rocks over the allochthonous Cretaceous to Paleogene flysch (Figure 8). North of Civita the Timpa San Lorenzo fault (TSLF) and the La Falconara fault (LFF) are well-exposed left lateral strike-slip faults that truncate all ductile (Apennine) structures. The TSLF strlkes roughly NW, whereas the LFF strikes roughly WNW. Monte Sellaro (MS)

A ROTONDA NB E $w MERCURE BASIN

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Fig. 9. Cross sections across parts of the southern Apennine left lateral shear zone showing positive flower structures at Rotonda and Cerchiara di

Calabria. See Figure 8 for location of the cross sections. Open circle pattern is Plio-Pleistocene nonmarine sediments; brick pattern is mainly Apennine carbonate rocks, asymmetric folds above carbonate rocks are Liguride Complex, and s.1. is sea level. Note different scales. Vertical scale equals horizontal scale in both cross sections.


near Cerchiara di Calabria is bounded north and

south by two large, curved, WNW to NW trending faults. Monte Sellaro is intepreted to be a push-up of Mesozoic carbonate rocks which in cross section has

the geometry of a positive flower structure (Figure 9; section CD).

The Pollino rldge fault system continues northwestward and is contiguous with faults in the Valle di Diano area (Figure 2). The same system continues southeastward and extends offshore the

Ionian margin of Calabria. The north trending lineaments on the western and eastern margins of the Crati Valley, in the Sila massif, and the westem margin of the Crotone Basin (Figure 7) are normal dip- to oblique-slip faults. The occurrence of left stepping en echelon faults located on the western margin of the Craft Valley (Figure 7) suggests a component of fight lateral strike-slip for these faults.

The WNW trending lineaments near the Craft Valley and elsewhere in the northern Calabrian sector are interpreted to be left lateral cross faults (sensu Sengor [1987]) for the following reasons: (1) the sense of offsets of north trending extension faults in the northeast part of the Tyrrhenian Basin (see also Rehault et al. [ 1987]) and (2) their parallelism with left lateral offsets along the Pollino ridge.

The possible origin of the SE trending lineaments will be discussed later in combination with the

tectonic synthesis. In south Calabria we have no data on the sense of

shear along lineaments. The kinematic interpretation of these lineaments is based on (1) Riedel shear models, (2) the approximate bilateral symmetry of lineaments noah and south of the Catanzaro Trough, (3) the lateral variation in extension around the Tyrrhenian Basin, and (4) paleomagnetic evidence for fight lateral shear in the southern sector of the arc [Channell et al., 1980].

In south Calabria and northeast Sicily the WNW trending lineaments, south of the Mesima Valley (Figure 7), are interpreted to be fight lateral cross faults. The NE trending set generally form the margins of extensional basins (e.g., Mesima Valley) and have a dominant normal dip-slip component of displacement. A minor component of left lateral strike-slip related to block and fault rotation is also present; this is described below.

SYNTHESIS

The kinematic model proposed here (Figure 10) bears some marked differences and also some

similarities to previous structural models for the southern Italian mountain belt. Moussat [1983] proposed that lateral shear remains left lateral around the belt whereas in our model shear varies from left

lateral in the southern Apennines to fight lateral in south Calabria and north Sicily. Variation of lateral shear around the arc is strongly supported by paleomagnetic data. Our kinematic analysis differs fundamentally from the stress analysis of Boccaletti et al. [1984]. We believe the rotation of crustal blocks and their bounding faults in the region will

•!•'•.Cat •a ro

IONIAN SEA

150 km

Fig. 10. Kinematics of the Calabrian arc based on the analysis of post-Messinian faults. Motion of faults continued at least until late Pleistocene time. Some

faults are still active. Stippled area is post-Tortonian sedimentary basins; dashed lines are faults inferred from reflection seismic data of Moussat [ 1983]. See text for explanation.

introduce a large degree of error in the analysis of paleostress (see also McKenzie and Jackson [ 1986]). Further, there are inconsistancies in the analysis of Boccaletti et al. [1984, Figures 7a and 7b] where faults show a shear sense contradictory to their proposed model. Whereas Meulenkamp et al. [ 1986] largely based their analysis on stratigraphic arguments, the model we propose is based on integration of different data sets. It can be summarized as follows.

Late Cenozoic deformation of mantle and lower

crust in the internal part of the southern Italian mountain belt and the southeastern part of the Tyrrhenian Basin, above the Tyrrhenian Cenozoic subduction zone, proceeded mainly by ductile extension and lateral bending accompanied by heterogeneous shear along subvertical shear zones in the middle and upper crust (e.g., southern Apennines lateral shear zone and Taormina lateral shear zone). The bending can be seen to be roughly symmetrical about an imaginary line drawn parallel to the length of the Catanzaro Trough. Thrusting dominated in the extemal Calabrian arc, Apennines, and Maghrebides.

In the upper crust of the Calabrian lithosphere, deformation occurred (and is still occurring) by faulting (including motion across lateral shear zones) and semirigid block rotation. The lateral shear zones in our model include the WNW trending cross faults in the Tyrrhenian Basin [Rehault et al., 1987] and those onshore (Figure 10). The WNW trending faults separate segments that experienced different amounts of upper crustal extension (and concomitant contraction in the external arc) within the Tyrrhenian back-arc system.


Generally, the amount of block rotation in the upper crust depends on (1) the geometry of the fault system [Luyendyk et al., 1980; Ron et al., 1984; Woodcock, 1987], (2) the degree of coupling between the upper crustal blocks and the lower lithosphere across a regional midcrustal detachment zone [McKenzie and Jackson, 1983, 1986; Nicholson et al., 1986a, b; Dewey et al., 1986; Webb and Kanamori, 1985], and (3) the amount and sense of displacement along the cross faults controlled by deformation in the lower lithosphere.

Heterogenous lateral shear across the Calabrian arc requires that extension varies between each of the segments of upper crust separated by a major cross fault. Variation in the amount of extension around the

southeastern comer of the Tyrrhenian Basin is visible from bathymetric maps [e.g., Rehault et al., 1987, Figures 1 and 2]. Deformation between the cross faults is controlled by (1) accommodation of the upper crustal blocks to the bending and heterogeneous extension and lateral shear of the lower lithosphere and (2) the position of the deforming area with respect to the Tyrrhenian back- arc basin and the External Calabrian arc. Extensional structures dominate the former region, contractional structures the latter.

DISCUSSION AND CONCLUSIONS

In previous models "oroclinal bending" and "buckling" were considered to be the main mechanisms for producing the curved shape of the Calabrian arc. On its own, the mechanism of bending of the lithosphere is inadequate to explain the present crustal structure of the arc. It can explain only the presence of transverse graben (e.g., Catanzaro Trough) considered to have formed by extension in the outer arc of the bending mountain belt [Ghisetti and Vezzani, 1981]. The presence of the graben aligned parallel to the curvature of the arc is not explained by this mechanism. The bending mechanism also predicts N-S compression within the internal part of the arc. This is not supported by any structural evidence.

The combination of mechanisms we present predicts that rotations of crustal blocks occurred mainly from Tortonian time (circa 11 Ma) until the present day during the opening of the Tyrrhenian Basin. The rotations are considered here the product of distributed lateral shear related to the lateral

variation in extension in the Tyrrhenian Basin caused by pinning of the advancing Calabrian arc between two continental promontories (Adria and

Sicily)[Malinverno and Ryan, 1986]. Therefore some of the observed rotation of paleomagnetic declinations occurred in areas undergoing late Cenozoic extension and not just during thrusting [cf. Channell et al., 1980].

Most paleomagnetic data from southern Italy do not allow unequivocal dating of tectonic rotations since Tortonian time. However, the data from southern Calabria of Aifa et al. [ 1988] document up to 30' clockwise rotation of paleomagnetic declinations since early Pliocene time in sediments from the Reggio Calabria area. Although used to support an alternative interpretation [Moussat, 1983] these data also support the hypothesis presented here. In the southern Apennines rotations occurred sometime in the Tertiary [Incoronato and Nardi, 1987] and in Sicily after Tortonian time [Channell et al., 1986]. These data by themselves are insufficient to discriminate between deformation produced either by (1) bending of an initially straight mountain belt or (2) bending accompanied by upper crustal block rotations. However, when combined with the fault kinematics and arguments on basin development presented above, the combination of mechanisms in the latter is strongly favored.

Upper crustal blocks rotated counterclockwise and are bounded by normal faults with a right-slip component in the southern Apennlnes and north Calabria. In south Calabria and north Sicily, blocks rotated clockwise and are bounded by normal faults with a left-slip component. WN•N trending cross faults compartmentalize the block-bounded basins. These styles of deformation are confined to the upper plate of the Calabrian subduction system. They were active from the beginning of extension in the Tyrrhenian Basin (late Tortonian, 11 Ma) and are still active today (based on Recent seismicity). Our combined analysis of data from seismicity, paleomagnetics, and fault kinematics provide strong evidence to support our model.

Acknowledgments. A Natural Environment Research Council (UK) studentship is gratefully acknowledged (to S.D.K.). This work was financially supported by the Ministero della Pubblica Istruzione, Italy (40% and 60%, 1984/1985). Comments by Joe Cartwright, John Dewey, Marc Helman, John Platt, and in particular Leonardo Seebet improved an early version of the manuscript. Critical and constructive comments by Tectonics reviewers and especially Asger Berthelsen clarified our arguments. Claire Carlton is thanked for skillfully drafting the figures.

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(Received August 29, 1989; revised May 18, 1991; accepted June 6, 1991.)

late cenozoic kinematics of the calabrian arc, southern italy

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