a traverse of the ionian islands front with coincident normal incidence and wide-angle seismics

E L S E V I E R Tectonophysics 264(1996) 35-49

TECTONOPHYSICS

A traverse of the Ionian islands front with coincident normal incidence and wide-angle seismics

Alfred Him a,*, Maria Sachpazi a,b,c, Risto Siliqi d, John Mc Bride e, Fedon Marnelis f, Licio Cernobori g, the STREAMERS-PROFILES group

a Laboratoire de Sismologie Expdrimentale, Dpt. Sismologie, UA 195 CNRS, Institut de Physique du Globe, 4 pl. Jussieu, 75252-Paris 05, France

b Seismological Institute, National Observator), Athens, Greece c Institute of Earth Sciences Jaume Almera, CSIC, Barcelona, Spain ,t Ecole et Observatoire de Physique du Globe, Strasbourg, France

e Bullard Labs., University of Cambridge, Cambridge, UK f DEP-EKY, Public Petroleum Corporation, Athens, Greece

g CNR-Ist. Talassografico and DlNMA-University, Trieste, Italy

Received 27 March 1995; accepted 30 March 1995

Abstract

Marine vertical reflection profiling with a powerful airgun source, augmented with a few wide-angle seismometer stations on land, has been applied along a 180-km line across the presently active deformation front of the subduction of the Ionian Sea plate beneath the western Hellenides. East of the Ionian islands, there is limited evidence for reflectivity down to 23 km interpreted as the base of a rather thin continental-type crust. Under the western slope of the islands, a major normal-incidence reflector dips eastward, first gently to the west of the islands, then more steeply under them. This reflector may be extrapolated southwestwards beneath a bulge that is thought to represent the modern pre-Apulian front located over the subduction zone. The continuity, signature, and geometry of this reflector suggest that it may act as the lower limit of the western Hellenides where they override the Ionian Sea quasi-oceanic crust, rather than just an intracrustal interface of a pre-Apulian crust. The location of the previous deformation front, the Ionian thrust proposed in previous models, can be constrained by the new seismic data. The new data raise the possibility of a larger area of evaporite mobility than previously considered, insofar as active block motion apparently related to halokinesis is recognized west of the Zakynthos anticline. Diapirism, drcollement, and westward-directed over-thrusting in the pre-Apulian crust may have been brought on by Late Pliocene-Quaternary reactivation of regional extension associated with the separation of the Peloponnesus from northern Greece as it was captured by the Anatolian-Aegean rotation and associated also with the fast clockwise rotation of the Ionian islands as they were sheared off the Apulian domain to the southwest by the initiation of the Kefallinia transform.

Keywords: Reflection seismic; Greece; Subduction

* Corresponding author. Fax: -t-33 144 274-783.

0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 9 5 1 ( 9 6 ) 0 0 1 16-3

36 A. Him et al./Tectonophysics 264 (1996) 35 49

1. Introduction: geological background and seismic methods

The evolution of the Ionian islands region (Fig. 1) and its relation to the Hellenide fold and thrust belt, which developed by progression of the deformation front towards the foreland can be summarized from numerous investigations in geology (e.g., Brooks et al., 1988; Underhill, 1989; Sorel, 1992), in paleo- magnetism (Laj et al., 1982), and in seismotectonics (Hatzfeld et al., 1990). Since the Middle Miocene, the Ionian thrust (IT) has formed the convergent limit of the Hellenides towards the Apulian platform in the northern part of the Ionian islands. South of the islands, the Hellenic arc and trench system is thought to have absorbed convergence by subduction since the Early Pliocene. In the intervening central Ionian islands the motion, which occurred previously at the Ionian thrust over the southern edge of the Apulian platform, may have been taken up since the Early Pliocene by a pre-Apulian subduction front offshore, to the southwest of the islands. These south-central islands have hence been sheared off the main Apu- lian realm by the active fight-lateral NNE-SSW- directed transform fault west of Kefallinia (e.g., Le Pichon and Angelier, 1981). Earthquake activ-

ity characterizes the islands. Diapiric intrusions and aseismicity mark a broad zone of Plio-Quaternary subsidence east of these islands. The zone of subsidence continues into the Gulf of Patras where recent extension is expressed in the formation of seismi- cally active grabens (e.g., Makropoulos and Burton, 1984; Melis et al., 1989; Amorese, 1993).

To investigate the relationships between these geological structures and their tectonic regimes, the STREAMERS project shot the seismic profile ION- 7 running from southwest to northeast for 180 km from the deep Ionian basin into the western Gulf of Patras. This was part of a broader survey of more than 500 km of profiles further west (Cerno- bori et al., 1996). The seismic signals were produced by Geco-Prakla's M/V Bin Hai 511 which towed a 36-airgun tuned array with a capacity of 7118 inch 3 (about 120 1) (see also McBride et al., 1994). Signals were recorded by a 4.5-kin, 180-channel streamer to produce a 30-fold normal-incidence reflection profile, and by fixed stations on the islands and mainland to give single-fold, variable-offset refraction wide-angle profiles. In order to constrain a structural model for the area, we have applied partial post-stack migrations, water-layer replacement, amplitude recovery, tau-p coherent reflection picking

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Fig. 1. Schema t i c m a p and b a t h y m e t r y o f the cent ra l I on i an is lands o f wes te rn Greece. S u p e r i m p o s e d S T R E A M E R S l i ne I O N - 0 7 w i t h

d is tance i den t i f i ca t i on . KD = K e f a l l i n i a d iap i r , MF = M o u n t a fau l t , ZA = Z a k y n t h o s anticline, FB = f ron ta l bu lge. Th ree - l e t t e r codes:

land recorders.

A. Him et al./Tectonophysics 264 (1996) 35-49 37

(e.g., Siliqi, 1994), as well as integrated the wide- angle data in addition to the data we presented earlier (Kamberis et al., 1994)•

2. Faults and blocks: evidence for diapirs and the Ionian thrust

The two bathymetric highs located at km 132 and km 143 along the reflection profile (Fig. 2) have been previously surveyed with single-channel seismics by Brooks and Ferentinos (1984) who interpreted both as diapiric structures bounded by thrust faults, with the western one, called Hydra bank, being the eastern limit of the Zakynthos basin and the other one being the eastern limit of the Kefallinia basin. The frontal thrust of the Hellenide belt which replaced the Pin- dus thrust and was active from the Middle Miocene to the Early Pliocene, and is called the Ionian thrust, has been identified either with the western high (e.g., Brooks and Ferentinos, 1984; Stiros et al., 1994) or with the eastern high (e.g., Underhill, 1989). Our seismic multichannel line penetrates through the Neogene sequence that unconformably overlies Mesozoic to Palaeogene sediments composed of carbonates, which correspond to the very strong reflections in the 1 to 2.5 s TWT (two-way traveltime) range. These reflections show a clear interruption and depth change at km 143 by the easternmost structure. At this point, the observed image is that of a positive flower structure which we interpret as

the Kefallinia diapir (KD) which involves Triassic evaporites. In contrast, the western high at km 132 shows a thick, sloping and layered Plio-Quaternary sequence with no interruption of the Mesozoic sequence under its eastern slope. Thus, no evidence is observed for the upwarp of evaporites beneath the western high. However, slightly to the west of this high, at km 132, Kamberis et al. (1994) suggest that a sharp, localized area of structural relief in the top of the carbonates as well as in the sea bottom indicates strike-slip motion which they correlate with the Mounta fault (MF) on southeastern Kefallinia. Kamberis et al. (1994) also suggest that the IT is located at least another 7 km further west of the MF at km 125 on the basis of their interpretation of the seismic data which shows the tectonic su- perposition of the Mesozoic carbonate sequence and associated evaporites over the pre-Apulian platform, which is a structural style conventionally attributed to the Ionian thrust sheet over its foreland.

In fact, the migrated tau-p coherency filtered section shown in Fig. 3 may be interpreted as showing evidence of thrusts which are situated even further west, such as at km 120. Here, the local true dip of the thrust is about 40 ° beneath the thick pl io- Quaternary cover. This point lies just east of the point along the seismic line that corresponds to the northern extrapolation of the Zakynthos anticline which culminates at km 115. The steeply dipping thrust surface beneath km 120 seems to flatten east-

km 60 FB km 100 ZA MF km140 KD km 180

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Fig. 2. Normal incidence stack section.Tuned array, 7118 inch 3 (about 120 1), 4.5 km long, 180-channels streamer, 30-fold coverage stack. Coherence-weighting and trace decimation for scale compression. Distances and main features as in Fig• 1.

38 A. Him et al./Tectonophysics 264 (1996) 35~19

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Fig. 3. The profile segment spanning the islands. Portion of the stack with tau-p coherency reflection extraction and constant velocity migration at 3.5 km/s.


ward into the base of a thrust sheet that branches into a level of strong, coherent reflectivity at around km 130 at 3-3.5 s TWT, dipping slightly to the west. Hence, the same type of structure usually taken to mark the Ionian thrust sheet of the Mesozoic sequence over the pre-Apulian carbonate platform is also observed as a deep seismic reflection image as far west as km 120. Previous interpretations of the Ionian thrust placed it further east, at km 132 or km 143, where the new seismic data identify features which could be related to strike-slip or diapirism as well as to thrusting.

East of the KD (Fig. 2), from km 145 to the end of the line at km 178 in the Gulf of Patras, the shallow sea bottom is underlain by a Plio-Quaternary sequence with a thickness reaching as much as 2 km, suggesting that sedimentation may be nearly keeping pace with subsidence. This sequence is resolved into two units. The lower one, probably Late Pliocene, onlaps westwards and is thickest at the edge of the KD. The Plio-Quaternary sediments are horizontally layered and onlap to the west over the east-sloping boundary next to the KD. The continued subsidence of the underlying Mesozoic carbonate sequence is not accompanied by any extensional features in the plane of the section, except for the major disruption by the KD. This subsidence may have occurred, at least in part, in piggyback fashion on the hanging wall behind a thrust front which jumped westward from its previous location as the Ionian thrust to a pre-Apulian position and was modified by the evaporite mobilization accompanying the extension that developed between the Peloponnesus and central Greece. A possibly significant amount of wrenching perpendicular to the plane of section cannot be discussed from our single line.

3. The islands and western slope: evidence of active block motion

Although the western parts of Zakynthos and Kefallinia are considered to be typically Apulian, the nature of the central part of Kefallinia from north of km 120 to km 135 is still debated (Accordi and Carbone, 1992). Evaporite diapirism is generally recognized as a feature characteristic of the Ionian zone, i.e. east of the IT (Underhill, 1988). In this context, the suggestion of halotectonic behaviour in

the central part of Kefallinia as inferred from the spatial distribution of co-seismic displacements that occurred during the major 1953 earthquake (Stiros et al., 1994) would imply either that central Kefallinia is part of the Ionian zone or that evaporites also underlie the Mesozoic carbonate platform of the pre- Apulian zone. They could then also accommodate southwestward transport along a drcollement.

Variable-offset recording of the airgun shots along the profile on land, including both Zakynthos and Kefallinia, will be discussed later, but it is useful to mention now some of the first-order features apparent on the record sections. On the eastern side of the islands (Fig. 4), a strong local wavefield anomaly appears at shot points near km 142 which matches the location of the KD. It corresponds to a locally earlier arrival of waves, analogous to a velocity pull-up in normal-incidence seismics and thus supports the presence of a high-velocity body, as a diapir may appear in contrast to recent sediments. From west of the stations on the islands, a similar pattern is observed, corresponding to shot points at km 110. Thus by comparison, we may infer that a distinct, high-velocity body also pierces the Neogene and possibly the surrounding Mesozoic sediments at this location. The interpretation of this feature as a salt diapir, however, requires confirmation by additional data.

The high block corresponds to a peculiar image on the normal-incidence section (Fig. 5). From km 117 to km 110, the culmination of the Zakynthos anticline is cut by short-wavelength, small-amplitude faults and draped by recent sediments as it steeply deepens to the west. Monopolis and Bruneton (1982) noted that the steeper western flank of Zakynthos is devoid of Neogene sediments and is affected by major normal faulting. However, much further west, west of km 106, concordant stratification over 1 s TWT in thickness immediately beneath the sea bottom is conspicuous and from its non-tectonized appearance might be interpreted tentatively as Plio- Quaternary deposits. Between these two areas, the seismic section is markedly different: a sharply de- fined reflection appears just beneath the sea bottom, followed by a multiple train. From its location this reflector may correspond to the top of the high- velocity body observed from the refraction data. It is interesting to note that on the reflection time section,

40 A. Hirn et al . / Tectonophysics 264 (1996) 35~t9

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60km - 50 - 40 - 3'0 - 20 - 10 0 10 20 30 40 50 60 km

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Fig. 4. (a) Zakynthos station. (b) Kefallinia station. Variable offset record sections obtained on the islands slightly broadside to the profile. Linear move-out plot with reduction velocity of 6 km/s. Split profile, offsets increasing to both sides of the receiver. To reduce data to a common datum plane at sea-level, times have been corrected by replacement of the water layer by sediments with 4 km/s velocity. Travel-time curves discussed in Section 6 are indicated on the sides by their apparent velocity.

A. Him et al./Tectonophysics 264 (1996) 35~t9 41

0 0

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km 100 km 110 ! !

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Fig. 5. The western slope of the islands. Detail of the stack after water velocity migration.

42 A. Him et al./Tectonophysics 264 (1996) 35-49

this reflector overlies an apparent velocity pull-up of the strong reflection at 5.5 s TWT beneath km 107, which further attests to the upper reflector marking a high-velocity boundary. The western flank of the reflector corresponds to an anomalous contact. Even though the interpreted Plio-Quaternary layering to the west of the contact is tilted to the east, it does not seem to be a westward-dipping normal fault since the layering is observed fight up to just beneath the high-velocity block which shows a positive relief on either side. Because the sea bottom is already several hundred metres deep on either side and is underlain by Quaternary sediments, one may interpret the en- tire western slope to be subsiding; however, from its higher position, the high-velocity block would then have to be at least partially decoupled on either side so as to subside less rapidly. The fact that the eastern side of the block corresponds to a normal fault marked by a scarp at the sea bottom suggests that the relative motion of the block is presently active; in this case, the western margin of the block could be a steep reverse fault interpreted as a separation fault rather than a compressional feature. Such a vertical block motion with respect to the surrounding sediments and basement may be similar to the piston-like model advanced for Kefallinia by Stiros et al. (1994). This model would suggest a decou- pling zone at depth. On the basis of the high-velocity character of the block and its relative upward motion along two faults which display a thrust and normal character respectively to the west and east, one is tempted to attribute the displacement to upward vertical motion that is decoupled above a possible salt d6collement.

This discussion, like the others presented in this paper, will remain tentative until a grid of profiles is surveyed which would provide three-dimensional control. The mere fact that islands exist on either side of the ION-7 profile is evidence enough of along- strike complexity. Even more basically, although we discussed above mainly the nature of faults in the plane of the section available, it is clear that a component of strike-slip on them may exist and is likely with the present direction of deformation being at an angle with the seismic line.

4. The deeper reflections: contact of the Hellenides and the Ionian basin crust, interfaces of an intracrustal layering in the pre-Apulian or Moho

Deep reflecting elements appear in different places within the 4-7 s TWT interval. One of the more conspicuous examples appears around 5 s TWT below km 90 to km 110 (Fig. 6). It is expressed as a single-cycle arrival which does not show characteristic lithological stratification and may be hence suggestive of a tectonic contact. In this and other cases, large variations in bathymetry distort the time section. After applying a water layer replacement by sediments with a velocity of 3.5 km/s, the reflector is restored with a slight eastward dip beneath the seaward margin of the Ionian islands, between km 95 and km 115. If we then project this reflector westward under the basin located between the islands and the frontal bulge at km 65, it appears to continue into a level of low-frequency reflected energy beneath the bulge. This reflected energy beneath the bulge is distinct from the second water-bottom multiple arriving at about 4 s TWT beneath the 1.4-km-deep sea bottom. With the water replacement, the reflector would be at a depth of roughly 11 km beneath the bulge and dip at 3 ° to the east. East o f k m 115 under the islands, where it would be on the order of 15 km deep, its dip steepens to 10 °. East of km 130 where the reflection reaches over around 6.5 s TWT, it can no longer be followed without ambiguity. The reflection we have detected may represent a discontinuity or tectonic contact which could be of significance in the particular regional geodynamics. Mapping of its regional distribution can be achieved by additional profiling.

In the deep sea portion of ION-7, a rough, tectonized sea-floor is indicated over a seamount at km 40 (Fig. 1) which appears to be asymmetric and may indicate strike-slip along the Kefallinia transform fault or, due to its possible NW-SE orientation, at the limit between divisions of the Mediterranean ridge, and also over the broad bulge at km 70 (FB) that stands out above the 3000-m-deep Ionian basin and 30 km in front of the Ionian Islands. The shape and size of this bulge, as well as the roughness of its surface, would suggest that it marks the front of active subduction. Although the position of such a


km 80 km 140 i

o

1

2

3

4

5

6

7

8

s

lO

Fig. 6. (a) Stack, uncorrelated. (b) Stretch of the stack to explore the recovery, in presence of bathymetric variation, of true dip of deep interfaces by replacing propagation through the water layer by that through a sediment layer (note that the sea-bottom reflection time is now meaningless). Even with a low 3.5 km/s replacement velocity, the true dip of the 5.5 s TWT (km 90-115) is already eastwards and the time shift of the frontal bulge makes energy beneath to line up with the westwards extrapolation of this smoothly dipping event. Main elements correlated.

44 A. Him et al./Tectonophysics 264 (1996) 35~19

front is debated even in places where it ought to be more clearly expressed, as in the Hellenic arc-trench system, its development as the limit of a continental backstop located well in front of the island arc, which is similar to the case discussed for the South Matapan trench by Lallemant et al. (I 994), does not seem unlikely here. There are no intermediate earthquakes to give any evidence of a subducting slab at latitudes north of Zakynthos. South of Zakynthos any possible slab that may exist beneath the Pelopon- nesus would have an anomalously small dip of 10 ° (Hatzfeld et al., 1989, 1993). We feel that the sharp, continuous, progressively eastward dipping reflector described herein is a good candidate for the limit between the top of the crust of the Ionian basin and the overriding western Hellenides. Of course we cannot exclude other possible locations of such a boundary. For instance, if our flat reflector was an intracrustal interface of the pre-Apulian, then the top of the subducting slab should dip much steeper at the frontal bulge. However, in such a case, it would have no structural expression in reach of our seismic section.

5. The steep events in the crust

Within the depth interval between the top of the Mesozoic platform and the deep interface around 5 s, both of which have an eastward dip, the strongest events in the section (Fig. 3) between km 120 and 135 show westward dip. We have suggested that the upper events around 3-3.5 s might be the base of a thrust sheet which connects upslope westwards with the edge of the Zakynthos anticline. Deeper on that section, there is a possible, very steep (about 30°), westward dipping reflection which joins the deep reflector at 6 s. Caution should be used in interpreting steeply dipping events on reflection sections, due to the possibility of diffracted noise with low apparent velocity, particularly from out-of-plane water diffrac- tions. The only sure remedy is to have more than one seismic line. Any possible tectonic interpretation of these steeply dipping features is not readily apparent from this section alone. The westward-dipping reflection seems to limit a wedge of material that extends to greater depth to the east. On top of it more mobile material might have been displaced. At both km 100 and 115, in the 1.5 s above the 5.5 s flat reflector, there are indications of local 20 ° east

dipping reflections, which might correspond to local deformation on top of the flat reflection. There is a remarkable change with depth of the reflectivity to a low frequency character with reduced lateral coherency. It contrasts with the sharpness and layering of the Mesozoic platform and overlying Neogene sequence, and might indicate less competent or de- formed material beneath. This variation occurs at largely different depths in different places: 1.6 s at km 120 and much deeper, around 3.5 s at km 105, indicating that corresponding material has quite different thicknesses above the 5.5 s flat reflector.

6. Crustal velocity contrasts from single-coverage variable-offset recording on the Ionian islands

For the stations on the Ionian islands, the wave- fields vary markedly from one side of the islands to the other (Fig. 4). To the west, strong first arrivals propagate to 35 km and 65 km offset with apparent velocities of 6 km/s and over 7 km/s, and with reduced times less than 1 s. To the east, first arrivals are weak and appear only late in the section. For example, only after 2 s reduced time does strong energy appear with a high apparent velocity of over 8 km/s at 40 km offset.

These images require a strong variation in the deep structure of the crust across the islands under the Neogene sediments. Because of the very strong lateral variations of the Neogene sediments and the lack of reversed profiles, a detailed velocity structure cannot be recovered from these data alone if dipping interfaces are present. Assuming a flat-layered model to the west of Zakynthos, at km 112, we would have an upper layer with 3.7 km/s slightly over 2 km in thickness. Its lower limit may then correspond to the base of the flat-layered sequence on the reflection section, at 1.3 s TWT beneath the sea bottom (Fig. 5). The bottom of the underlying 6 km/s layer would lie around 8 km depth if we take the apparent velocity of 7 km/s for the first arrival around 50 km offset as equal to the true velocity beneath that level. Its image on the reflection section should lie at 3.2 s TWT; however, it is not clearly visible but could correspond to the transition of high- to low-frequency signal at 4 s TWT at km 105. A further branch is suggested in the record section which would correlate the first arrivals after km 70 and secondary arrival at


shorter distance with an apparent velocity of 7.5-8 km/s. This branch would then represent arrivals that have been turned back from 13 km depth and correspond to 4.8 s TWT. This is close to the position in time of the deep reflection in the vertical-incidence section.

To the east of station ZAK, the Ionian thrust zone and Kefallinia diapir (KD) disturb the propagation of waves in the crust. For station KEF situated on Kefallinia further east, at km 127, intracrustal waves undershoot these near-surface perturbations, and four travel-time branches may be distinguished, although with a rather low signal-to-noise ratio. The first branch gives the same 3.7 km/s velocity as for ZAK. The next branch corresponds to weak first arrivals after 12 km, with a velocity of 5.3 km/s. The corresponding depth is 2 km, and the corresponding TWT is 1.1 s. On the normal-incidence reflection section in Fig. 6, which accounts for the water layer, this corresponds to the top of the Mesozoic carbonate sequence. Beyond 25 km, this travel time branch almost vanishes and is followed by a later one, which is also weak but has a higher velocity of 6.4 km/s. The corresponding refractor would lie at 8 km depth. On the normal-incidence section the corresponding 3.5 s TWT would be situated beneath km 130 under the strong reflections, which might form the flat continuation of the basal thrust of the Ionian portion of the Mesozoic platform carbonates above Triassic salt. At larger offsets, over 30 km, strong late arrivals are noted with an apparent velocity in excess of 8 km/s, probably wid~-angle reflections which imply a further interface with a strong velocity contrast although they do not tightly constrain the underlying velocity. The corresponding reflector lies at 15 km depth or 5.5 s vertical TWT.

7. Evidence of deep interfaces from wide-angle recording on the mainland

At the stations on the mainland the wavefield is unusually complicated. The arrival times and amplitude distribution are likely to be dominated by the rapid lateral succession of low-velocity sediments and high-velocity intrusions. However, by ray-tracing through the shallow 2-D model that was derived from the vertical reflection section down to a dummy interface at 7 km, we may use these wide-

angle data to test whether there are more interfaces at depth (Fig. 7).

From station MES, situated in-line on the northern shore of the Gulf of Patras, near Messolonghi, the computed times of waves travelling through the upper part of the model fit the observed average 1.5 s RT (reduced time to 6 km/s) at 30 km offset. Testing a variety of models shows that, although rays graz- ing at less than 10 km depth in a 6.2-km/s velocity basement arrive not far from the observed times at 60 and 80 km offset, their apparent velocity is much smaller than that observed between 35 and 60 km offset. In order to fit such an observed phase, late and with high apparent velocity at short offset, a reflector is needed deeper in the model.

We may test three families of models. First we set the crustal material under the dummy interface to have high velocity (6.8 krn/s) like lower crust or oceanic crust. Although the apparent velocity in the model is too high, arrival times can be grossly fitted with a 25-km depth to the interface which could then be Moho. The assumption of having a layer of lower continental or oceanic crust directly beneath the upper 7 km hence leads to a thickness of 18 km for it, which is too large to be consistent with the nature of this model itself. Alternatively, a second extreme model would be to take 5.8 km/s, which is a lower reasonable velocity for continental crust, beneath the upper 7 km. The late high-velocity arrivals from 40 to 60 km offset then constrain the deep reflector to be 15 km in depth. If mantle material was beneath it, the computed critical distance would be 30 km. One might also consider the energy observed after 40 km to be subcritical. The 15-km deep interface may then be of smaller velocity contrast than that across the Moho. Also there are indications of more later arrivals between 65 and 80 km with high apparent velocities. An additional deeper interface beneath the one at 15 km could then be the Moho. Taking a lower crustal velocity of 6.8 km/s beneath 15 km, this Moho would have to be placed at 23 km depth so as to fit the arrival time at 80 km. However, the apparent velocity is not high and thus indicates a westward Moho up-dip under the Ionian islands.

Station RIO situated at 24 km south of the eastern end of the normal-incidence line shows a complex pattern of early arrivals just after 1 s RT at small offsets, presumably due to lateral velocity variations.

46 A. Him et al./Tectonophysics 264 (1996) 35-49

I I I I I t 4s

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Fig. 7. (a) MES station in-line to the east of the profile. Record section with reduction velocity 6 km/s, offset increasing to the left. (b) Arrows mark picked arrivals and continuous lines mark computed travel times in model (c). (c) Ray-tracing in model of continental crust. The Kefallania diapir (KD) is at - 6 2 km. Its topography and that of sedimentary layers are inferred from normal incidence seismics. Lower crustal interface between 5.8 and 6.8 km/s at 15 km depth and possibly base of the crust at 25 km. (d) RIO station, broadside south. Early energy at 80 km offset could indicate shallower position of mantle material west of the islands.

A. Him et aL /Tectonophysics 264 (1996) 35~19 47

It has clear signals around 80 and 100 km, which allows us to extend the sampling of crustal structure to the west since waves on the shot side penetrate the crust under and to the west of Zakynthos. The correlation of these waves is quite flat in the reduced time section due to the strong increase in water depth westwards. Because of early arrival times at large offset, their propagation has to include a path through a high-velocity medium, i.e. beneath deep crust-Moho interfaces which were suggested from station MES to dip upwards towards west under the islands.

The record section of station OBL situated further east, south of the Gulf of Patras, shows a delay that is more pronounced than those for the two other stations at 40 km offsets or more for shots at the eastern end of the reflection line. This is attributable to lower velocity near the surface. Taking this into account, the deeper part of the model east of KD, which fits observations of MES, is not inconsistent with the OBL data.

Time conversion of the MES wide-angle ray- traced model gives, east of KD, 6.2 s and 8.6 s TWT, respectively, for the 15 and 23 km deep interfaces, which correspond to the top and bottom of inferred lower crust, respectively. However, only scant indications of energy at the time for the later one are found on the vertical reflection stack section even after amplitude balancing.

8. Discussion

Powerful marine vertical-incidence reflection profiling images a major reflector at about a depth of 13 km slightly dipping to the east under the western slope of the Ionian islands then dipping steeply under them. It may reach westwards to the present front of the pre-Apulian over the subducted Ionian basin crust. From its continuity, signature, and geometry, this reflector may be a fiat tectonic contact rather than an intracrustal interface of the pre-Apulian crust, and so mark the lower limit of the western Hellenides. The seismic image of disruptions of the Mesozoic sequence suggests a more westerly position for the Ionian thrust than previously assumed, and suggests the development of block tectonics west of the islands, both of which would indicate a broader extent of evaporite mobility than generally accepted.

The description of deep structural elements given in this paper leads toward an interpretation of a thin- skinned tectonic model which is one end member of a range of possible models for the Ionian-pre- Apulian thrust sheet. Our model implies a significant amount of horizontal transport at a relatively shallow crustal level of pre-Apulian material over the Ionian basin. The lithosphere of the Ionian basin would extend with a slight dip, to reach 13 km depth under the islands, then probably dip more steeply under a more complete, although thin, continental-type crust underlying the Gulf of Patras east of the Ionian islands where the Ionian thrust sheet is superposed on the pre-Apulian. Diapirism of Triassic evaporites, drcollement, and westward overriding in the pre- Apulian may have been brought on by late Pliocene- Quaternary reactivation of regional extension. The amount of probable wrenching cannot be derived from the single section available.

Three main causes for deformation generally invoked to various degrees in order to account for Aegean extension or its acceleration are: counterclockwise extrusion of Anatolia-Aegea (and the corresponding clockwise rotation of northwestern Greece) under boundary forces exerted on its eastern limit by the northward migration of Arabia; roll-back of the convergence by retreat at the. Hellenic trench system due to slab pull; and gravitational spreading of the high elevation land mass over the adjacent oceanic basins. These three possible causes may be examined in light of the present-day tectonics and observable geologic structure.

In the case of western Greece, rapid changes in stress regime over time during the Plio-Quaternary have been related to the pull of the subducted slab, that is, its growth or detachment (Sorel et al., 1988). The jump of the deformation front from convergence at the Ionian thrust to subduction in front of the pre- Apulian may be regarded as a case of finite trench retreat, which changed the boundary force on western Greece. Correlative piggyback subsidence on the hanging wall (Ori and Friend, 1984) following the westward jump of the frontal thrust may account for the Pliocene episode of stratified deposits before diapirism, as evidenced near the Kefallinia diapir (KD).

In order to account for a diapiric rise through a brittle overburden, Vendeville and Jackson (1992) have recently advocated the primary necessity of a

48 A. Him et a l . / Tectonophysics 264 (1996) 35-49

regional thin-skinned extension regime, which is also more generally supported for basins by the numerical experiments of Daudr6 and Cloetingh (1994). This challenges the usual view, also held for the Ionian zone (Underhill, 1988), that, independent of stress regime, the trigger is buoyancy, which could here be due to subsidence or late out-of-sequence thrust loading. Hence, if it was not confined to a particular depth level by buoyancy, evaporite mobility may occur more generally and foster drcollement, transport, and relative block motions when the present extension became established regionally. If evaporite mobilization indeed occurred rather later than the onset of piggyback subsidence as suggested by the structural setting of the KD, it may indicate the onset of reactivated regional extension, in the framework of Vendeville and Jackson's (1992) model.

An increase in slab pull is not easily invoked for this present-day accelerated extension, since seismicity shows that the slab is actually flat. Gravity nappe-like spreading has been repeatedly suggested (e.g., Hatzfeld et al., 1989) but if the gradient in elevation is large from central Greece to the Ionian Sea, it is not clear that it has changed significantly and may serve as a triggering mechanism.

Alternatively, a major change in regional tectonics has been recently documented by Le Pichon et al. (1995) as imposing relative extension over a broad zone through west-central Greece with respect to northern Greece due to the recent capture of the Peloponnesus by the counterclockwise rotation of Anatolia-Aegea in relation to the collision of the Mediterranean ridge accretionary wedge with the African continental margin. Accelerated clockwise rotation of the Ionian islands (also Laj et al., 1982) and their southwest shearing off the Apulian domain (also Kahle et al., 1993) by the Kefallinia transform relate to this extension. Thus, the activa- tion of diapirism, the localization of drcollement and horizontal transport, and the internal vertical block motions on top, will all be related to a thrust sheet overriding the pre-Apulian subduction zone, sheared laterally along the Kefallinia transform.

Although large earthquakes are mostly considered to be strike-slip on the transform fault off west Ke- fallinia or to be flat thrusts related to subduction near Zakynthos (e.g., Anderson and Jackson, 1987), microearthquakes on Kefallinia show the full range

of focal mechanisms, thrust, strike-slip, and normal faults (Amorese, 1993). Rather than indicating compressive stress, the earthquake and neotectonic reverse faults might correspond to relative block motions, which then also account for normal fault mechanisms on corresponding separation faults, both being types of mechanisms occurring in relation to a drcollement level. The ddcollement may be first in the evaporites sandwiched under the Ionian thrust sheet and, if they exist, further down or westwards in the pre-Apulian material, or at even deeper strati- graphic levels of the sedimentary or intracrustal pile and finally at the contact with the overridden Ionian c rus t .

Acknowledgements

Funds for the data acquisition have been mainly provided by the Commission of the European Com- munities, DGXII, programme JOULE, Deep Ge- ology contract STREAMERS, JOU2 90-CT-0013 (contractors: A. Him, IPG de Paris; E. Banda, C.S.I.C. Barcelona; D. Blundell, RHBN Univ. of London; L.A. Mendes Victor, Univ. of Lisboa; R. Nicolich, DINMA Univ. of Trieste; J. Drakopoulos, Univ. of Athens; N. Lalechos, DEP-EKY Athens) and for the data processing by contract PROFILES, JOU2-CT93-0313. Additional funds were also provided by authors' institutions and national research programmes. In addition to the contractors, cited, the STREAMERS-PROFILES group for the present research comprised M. Loukoyannakis (DEP-EKY, Athens); R. Hobbs (BIRPS, Cambridge); J. Diaz and J. Gallart (CSIC, Barcelona); L. Cernobori (DINMA, Triest); A. Nercessian, M. Sapin and J. L. Veinante (IPG, Paris). Thanks are due to the Geco-Prakla staff and crew of the vessel Bin-Hai 511 supervised by J. McBride (Univ. of Cambridge). We are indebted to M. Loucoyannakis and DEP-EKY staff, E Maltezou, A. Zoulou, D. Eugenis and G. Sousounis for the multiple coverage processing. We acknowledge con- structive reviews of C. D. Reuther and anonymous referees.

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a traverse of the ionian islands front with coincident normal incidence and wide-angle seismics

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