Spatial transition from compression to extension in the Western Mediterranean Ridge accretionary...

TECTONOPHYSICS

ELSEVIER Tectonophysics 234 (1994) 33-52

Spatial transition from compression to extension in the Western Mediterranean Ridge accretionary complex

S. Lallemant, C. Truffert, L. Jolivet, P. Henry, N. Chamot-Rooke, B. de Voogd *

Laboratoire de Giologie, CNRS CJRA 1316, Ecole Normale Supt%eure, 24 rue Lhomond, 75231 Paris, France

(Received July 29, 1992; revised version accepted January 5, 1994)

Abstract

The Mediterranean Ridge has been described since 1982 as an accretionary prism related to the Hellenic subduction zone. However, little attention has been paid to the actual mechanical boundary between the sedimentary wedge and the crustal backstop of the outer Hellenic arc. Using seismic velocity structures from recent ESP’s (Pasiphae cruise), we propose a new interpretation of existing geological and geophysical data around the western Hellenic trenches. The velocity structure of the inner unit of the Mediterranean Ridge suggests that the Late Neogene to Quaternary sequences were deposited above a high-velocity rigid body. We propose to correlate the boundary of this rigid body (backstop of the accretionaxy wedge) with a series of oblique reflectors dipping to the southwest, near DSDP sites 126 and 377. The backstop extends further south than previously thought and decreases by one third the total width of the compressional wedge; it has thus major implications on the volume of accreted material. The short-wavelength folding of Plio-Quaternary sediments observed on the outer wall of the South Matapan trench is probably a consequence of gravity-gliding layers above the high velocity body, gently dipping (2”) to the northeast using the Messinian evaporite as a dtkollement. On the other hand, the Hellenic trenches appear to be narrow half-grabens affected by the widespread extensional deformation active on the outer Hellenic arc. They are situated on the landward boundary of a wider and poorly sedimented fore-arc basin. We conclude that extensional deformation extends at least over the width of the Hellenic trenches, on the continental side of the backstop, whereas compressional deformation stops near the edge of the backstop itself, 100 km to the southwest.

1. Introduction

Eastern Mediterranean active tectonics is dominated by slow convergence (about 1 cm/ a in a N-S direction according to Argus et al., 1989) of Africa and Europe and lateral westward es- cape of Turkey (e.g., McKenzie, 1972; Dewey et

* Now at: IPRA (GCophysique), Avenue de KJniversitC, 64000 Pau, France.

al., 1973). The Aegean arc and trench system has been recognized as a subduction zone with a 30”-40”-dipping Wadati-Benioff seismic plane (Papazachos and Comninakis, 1971) on top of a deeper cold slab (Spakman et al., 1988) and a volcanic arc (Fytikas et al., 1984). Using the focal mechanisms of major interplate earthquakes, Le Pichon and Angelier (1979) have shown that slip vectors along the Aegean subduction zone fit small circles about a close Eulerian pole (40”N, 18”E). They concluded that, despite distributed

0040-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0040-1951(94)00005-T

34 .S. Lallemant et ul. / Trctonoph.vsics 234 (19941 .73-52

35”

30”

25”

Fig. 1. Map of the studied area with location of ESPs, MS33 seismic line and main structural boundaries. Dashed line in Sirte Abyssal Plain represents the boundary between the continental and the oceanic crust.

extensional deformation, the kinematics of the Aegean external arc can be described as rigid body thrusting over the Mediterranean basins causing an almost frontal subduction to the west in the Ionian trench system and a nearly strike-slip oblique convergence in the complex Eastern trench system (Pliny and Strabo trenches; see Huchon et al., 1982).

The Mediterranean Ridge, the most prominent submarine relief of the Eastern Mediter- ranean, is the site of active compressional tectonics (e.g., Rabinowitz and Ryan, 1970; Finetti, 1976) and is now interpreted as an accretionary prism built up by offscrapping and piling of the upper part of the sediments deposited on top of the downgoing plate (Biju-Duval et al., 1982; Le Pichon et al., 1982a,b; Ryan et al., 1982; Kastens, 1991; Truffert et al., 1993) (Fig. 1).

On the other hand, the most active tectonic process in the Aegean area is an intense and

distributed continental extension, but the mechanisms of extension are still a matter of debate (e.g., McKenzie, 1978; Le Pichon and Angelier, 1979; Le Pichon et al., 1982a; Mercier et al., 1989; Jackson and McKenzie, 1988a,b). Various estimates have been made of the present rate of extension, based on kinematic arguments (Mc- Kenzie, 1978; Le Pichon and Angelier, 1979), seismic moment (e.g., Jackson and McKenzie, 1988b; Ekstrom and England, 1989) and more recently by geodetic techniques (Billiris et al., 1991). All lead to the conclusion that the extension rate is considerably higher (40-70 mm/a from seismic deformation and preliminary geodetic measurements) than the convergence between Africa and Europe (about 10 mm/a>.

This paper describes the transition from compression in the western Mediterranean Ridge to extension in the Aegean domain. The morpho- logic depression of the Hellenic trench system is

S. Lallemant et al. / Tectonophysics 234 (1994) 33-52 35

Mediterranean Ridge Hellenic Outer Hellenic Arc Trench NE

Mediterranean Ridge Hellenic Outer Hellenic Arc NE

b)

Mediterranean Ridge Hellenic Outer Hellenic Arc Trench NE

I

Fig. 2. Three schematic sections illustrating the main hypotheses concerning the Hellenic Trench System (HTS). (a) The HTS considered as real subduction trenches and the Ridge as a stack of crustal material (modified from Rabinowitz and Ryan, 1970). (b) The HTS in a position of fore-arc basins (after Le Pichon et al., 1982a, modified). Cc) The HTS as extensional basins within the backstop of the Ridge, considered as a part of the Hellenic arc (Truffert et al., 1993 and this study). Symbols: MR, Mediterranean Ridge; HT, Hellenic Trench; HA, Hellenic Arc; I, crust of the upper plate; 2, crust of the downgoing plate; 3, deformed sediments (accretionary wedge); 4, indurated sediments of the lower plate.

2w 220


thus a key area to discuss this transition (Fig. 1). We schematically describe in Fig. 2 successive interpretations of these features. The trenches have been first considered as subduction trenches where the Mediterranean basins were actively underthrust below the Aegean arc (e.g., Genthon and Souriau, 1987), the compressive system of the Mediterranean Ridge being a compressional system independent from the subduction processes (e.g., Rabinowitz and Ryan, 1970 or Fig. 2a). With the recognition of the Mediterranean Ridge as an accretionary prism (Le Pichon et al., 1982a; Ryan et al., 1982; Kastens, 1991), the Hellenic trenches were interpreted as narrow fore-arc basins (Fig. 2b), becoming the major mechanical boundary between compressional deformation of the Mediterranean Ridge and extensional tectonics of the Aegean arc (Huchon et al., 1982; Le Pichon et al., 1982a,b). Le Pichon et al. (1982a,b) reported Sea-Beam data suggesting that extensional faulting dominates the tectonics of the inner wall of the Hellenic trench, whereas the outer wall is apparently characterized by compressional deformation, expressed as short wavelength folding (see also, Lyberis et al., 1980; Hu- chon et al., 1982). Le Pichon et al. (1982a) interpret the Hellenic trench inner slope as a rigid buttress (called “backstop” by Suppe, 1981) in front and above which the upper part of the sediments are stacked by folding and faulting. Truffert et al. (1993) propose a new crustal section across the subduction complex based on recently acquired seismic data (Pasiphae cruise), gravity modelling, distribution of seismicity and bathymetry. They have shown that the backstop

of the accretionary complex actually extends 100 km seaward of the Ionian branch of the Hellenic trench (Fig. 24. Within this new framework, we shall show that the Hellenic trenches are half- grabens similar to those described further ups- lope in the Ionian margin of the Peloponnese (Papanicolaou et al., 1988).

We present a detailed structural section across this transition from the Mediterranean Ridge to the southern part of the Ionian Hellenic trench system. We focus our attention on the southwestern part of the system (South Matapan trench), integrating existing seismic reflection profiles, compiled bathymetric data, results of DSDP drilling and diving observations during the HEAT program. We show that the South Matapan trench actually consists of a series of asymmetrical extensional basins (with a typical fan-shape in cross-section) affected by the widespread extension of the Aegean arc. We also show that the folding on the outer wall of the Trench can be related to gravity gliding of the upper sedimentary sequences above a flat and gently dipping backstop. Therefore, the short-wavelength folding is not related to a backthrust system and does not necessarily imply generalized compression.

2. Origin of data

Over the last two decades, numerous geophysical investigations were performed in the western Mediterranean Ridge and in the Hellenic trench, from the Sirte abyssal plain to the South Mata- pan trench. In order to constrain a crustal section

Fig. 3. Bathymetric map (from ISC/IOC, isobaths every 200 m) of the western part of the Eastern Mediterranean Sea with the location of ESP (Truffert et al., 1993), MS33, HEAT Sea-Beam maps (the five thin-lined boxes refer to Huchon et al., 1982) and earthquake locations (ISC catalog) and micro-earthquakes from a one-month survey (Hatxfeld et al., 1989). Dense dotted pattern: outer and central units of the Mediterranean Ridge (accretionary wedge); loose dotted pattern: inner unit of the Mediterranean Ridge (rigid backstop of the wedge). The thick-lined box locates the detailed map of Fig. 10 and a dotted broken line shows the deeps of the Hellenic Trench system.

crossing the western Mediterranean Ridge. we used available geophysical and geological data. These are reported on the bathymetric map re- drawn from the International Bathymetric Chart of the Mediterranean recently compiled by the Intergovernmental Oceanographic Commission (IBCM/ IOC, 19811 (Fig. 31.

From August to October 1970, Leg 13 of the Deep Sea Drilling Project was carried out and holes (sites 125, 126, 127 and 128, located in Fig.

31 were drilled along a line crossing the western Mediterranean Ridge from the Sirte abyssal plain to the South Matapan trench (Ryan et al., 1973). A few years later, during DSDP leg 42A (Hsii et al., 19781, holes (site 377) were drilled very close to site 126 (Leg 13, Ryan et al., 1973) situated in a deep cleft, between the central and the inner- most part of the western Mediterranean Ridge (see Fig. 3-6 for detailed location).

A vertical reflection seismic line (MS331 was

LOWER PLATE ACCRETIONARY PRISM UPPER PLATE lonian Basin

>< Mediterranean Ridge Aegean arc

DSDP

SW 1.9 8 126l377 9 11 NE

40.

50.

60.

70.

km L

I I I

0 4 8 km/s

0 Miwismicly (Hat&Id 81 al. 1969).

. ISC tismicity

0 Earthquakes wilh focal mechanisms

I\ Velodty model of ESP’t

1

m Messinian to Quaternary sediments

m pre-messinian accreted sediments

m oceanic crust

high velocity material (Aegean arc)

Fig. 4. Crustal cross-section through the western Mediterranean Ridge deduced from ESP velocity structures and gravity modelling (simplified from Truffert et al., 1993). Horizontal scale for the EsPs in km/s, horizontal scale for the crustal cross-section in km, vertical scale for both ESP and cross-section in km. Earthquake locations were projected on this crustal cross-section from a 240-km wide band. White squares show the distribution of micro-earthquake from a recent survey (Hatzfeld et al., 1989) and small dots are earthquakes from ISC catalog. The large dots represent the location of earthquakes with known focal mechanisms.

S. Lallmant et al. / Tectorwphysics 234 (1994) 33-52 39

CENTRAL UNIT _ _ ESP8

I 1261377

_ INNER UNIT

10 20 30 c 40 50 60 70 90 km

INNER UNIT I

_ HELLENIC TRENCH c

ESP9 ESPll

90 I:0 110 120 130 140 16( I I I I I I I

I km _

s

Fig. 5. Part of an unmigrated vertical multichannel seismic reflection line (MS33 shot by OGS, Trieste, see Finetti, 1976). ESP from Pasiphae cruise and DSDP drilling site 126/377 are located.

shot by the Osservatorio Geofisico Sperimentale (OGS) of Trieste in 1971 (Finetti, 1976, 19821, very close to sites 125, 126, 127, 128 and 377 (Fig. 3). From southwest to northeast, this seismic line crosses the Sirte abyssal plain, the deformation front, the western Mediterranean Ridge and the South Matapan trench. In this paper, we present only the northeasternmost part of this seismic section running from the top of the Mediter- ranean Ridge to the South Matapan trench (Fig. 5).

Within the framework of the HEAT program (Hellenic Arc and Trench), Seabeam detailed

bathymetric data were acquired on R.V. “Jean Charcot” in the South Matapan trench area in 1978. In 1979, a field study using the submersible Cyana was conducted in this area (Le Pichon et al., 1982bI.

Our new interpretation concerning the western Mediterranean Ridge and its backstop (Fig. 2c) comes primarily from the results of a new seismic data set collected during Pasiphae cruise (Truffert et al., 1993) which provided new velocity structures. Pasiphae cruise was a two-ship refrac- tion and oblique reflection seismic cruise in the Eastern Mediterranean sea. Fourteen Expanding

CENTRAL UNIT INNER UNiT TRENCH SW NE

a) ESP 9 ESP 11

100 150 km

NE

km

b)

Fig. 6. Correlations between our ESP velocity models and the reference multichannel seismic line MS33. (a) Velocity model used

for the deph conversion of the line drawing with the ESP velocity profiles. Vertical exageration: 2.5. fb) Depth converted line

drawing from Fig. 5 converted in depth. Vertical exageration: 2.5. (c) Close-up view of MS33 seismic line around the edge of the

backstop. Arrows show oblique reflectors (see text for discussion).


ESP 8 DSDP 126/377

I 1 Line MS33 ] I NE

7

T)

50

Fig. 6 (continued).

60 70 km

Spread Profiles (ESP) were shot along three lines crossing the Ionian basin and the Calabrian prism (ESP 1, 2, 4 and 5), the Sirte abyssal plain and the Matapan trench (ESP 19, 6, 8, 9 and 11: very close to MS33 seismic line and DSDP sites), and the Herodotus plain and the Strabo trench (ESP 14, 16, 17 and 18) (for location: see Fig. 1 in de Voogd et al., 1992; Truffert et al., 1993). This recent seimic cruise has supplied velocities down to Moho depth in the Sirte abyssal plain (ESP 19>, on the western Mediterranean Ridge (ESP 6, ESP 8 and ESP 9) and in the South Matapan trench (ESP 11). Seabeam data acquired on board R.V. “Jean Charcot” during Pasiphae cruise completed the existing bathymetric data of HEAT cruise.

3. Structural section of the inner part of the western Mediterranean Ridge from seismic data

In a previous paper (Truffert et al., 19931, we have presented a crustal cross-section from Sirte

abyssal plain to the Hellenic trench (Fig. 4) and we have divided the western Mediterranean Ridge in three main units: the outer, the central and the inner unit. This cross-section constrained by gravity modelling, ESP results, distribution of seismicity and bathymetry confirms Le Pichon et al.? (1982a) assumption that the backstop extends below the summit of the Mediterranean Ridge and locates it roughly below the inner unit of the western Mediterranean Ridge. We now discuss more precisely its location based on a structural interpretation of the MS33 seismic section using velocity models from ESP data (Truffert et al., 1993).

Fig. 5 shows the northeasternmost part of the vertical reflection seismic line (MS33). A first layer with numerous reflectors is seen just below the sea-floor (average thickness of the layer around 0.2 s two-way travel time or twt) overlying a relatively transparent seismic sequence (average thickness of the layer around 0.3 s) which corre- lates nicely with a high velocity layer (4-4.5 km/s) on both ESP 8 and 9 in Fig. 6a. This acoustically

transparent layer is obvious between km 10 and km 50 in Fig. 5 and can be traced northeastward. It is reasonable to intrepret this sequence as the Messinian evaporites. The top of this layer was named horizon M and its age had been first estimated to be about 4.3 Ma (Ryan et al., 1973) and more recently about 4.8 Ma (Kastens, 1992). The base of the Messinian evaporites is locally outlined in Fig. 5 (see also line drawing in Fig. 6) by several reflectors in the central unit (e.g., around 4.5 s between 30 and 40 km), in the inner unit (around 5 s) and in the Matapan trench (between 140 and 160 km, around 7.5 s).

The main difference between velocity structures of the ESPs 8 and 9 is located in the pre-Messinian layers. ESP 8, in the central unit, is characterized by a well defined velocity inversion beneath the high velocity evaporites. A similar velocity inversion at the base of the Messinian has a wide distribution over the Eastern Mediter- ranean and is typically attributed to relatively poorly-compacted sedimentary sequences (e.g.,

ESP 6, 19 in Fig. 4 or ESP 1X in De Voogd et al., 1992). From ESP 9 data, conversely, the Messinian cvaporite sequence is directly underlain by high velocity layers (4.75-6.6 km/s). These velocities are far too high to be attributed to a normally compacted sedimentary pile or even to a slightly overconsolidated accretionary complex. Accord- ing to Truffert et al. (19931, we propose to interpret this 6.5~km thick high velocity body as a dense material acting as a backstop. We can thus explain the presence of a velocity inversion at the base of ESP 9 (6.6 km/s above 4.3-5 km/s) as due to a 2.6~km thick sedimentary sequence underthrust on top of the oceanic basement below the continental margin. On ESP 8, a slightly thicker sequence with a similar velocity (4.2 km/s) overlies the oceanic crust. Consequently, this deep velocity inversion at ESP 9 outlines the major d&ollement between the upper continental plate and the downgoing oceanic slab.

Using velocities derived from ESPs 8, 9 and I 1, and the boundaries between major sequences,

ESP 11

km

01 c232 m3 El4 KY5 a6

Fig. 7. Synthetic section of the South Matapan trench locating the following close-up views (Figs. 8 and 9).


we have drawn a velocity model displayed in Fig. 6a (in this figure, the velocity model itself has been converted into depth). Averaged velocities derived from ESP data were attributed to each sequence. Fig. 6b displays a depth-converted line drawing of the line MS33. Fig. 6c displays a series of reflectors, between 50 km (around 7 s) and 130 km (around 5 s), with an apparent dip toward the southeast whereas some others reflectors at similar depths in the inner unit appear to be flat (Figs. 5 and 6a). We propose that the dipping reflectors are due to lateral velocity variation along the seaward edge of the backstop (Fig. 6a), thus supporting the existence of a structural dis- continuity (Fig. 6b).

4. South Matapan trench as a series of half- grabens

The northeastern extremity of MS33 profile (Figs. 5, 7 and 8) shows two asymmetrical basins

SW

6

7

6

Grl

(with a fan shape in the vertical plane) separated by a small topographic high. Only the beginning of the second basin is present in Fig. 4 because this seismic line had not reached the inner wall of the South Matapan trench. The high has been interpreted as a diapir (Lyberis et al., 1980) by comparison with the diapiric areas of the Pliny and Strabo trenches, but an interpretation in terms of diapiric origin does not account for the asymmetry of these fan-shaped basins (the west- ernmost basin is filled by more than one second twt of sediments). We interpret the fan shape in vertical plane as indicating a syntectonic filling of these basins. Because fan-shaped basins can be produced either by compressional deformation (e.g., piggyback basins) or by extensional tectonics (tilted blocks), we need geological constraints to discriminate between both origins.

Fig. 9 displays a synthetic SW-NE geological section based on four DSDP holes (Three at site 127 and one at site 128) drilled along a transect from the South Matapan trench northernmost

NE

130 140 150 160 km

Fig. 8. Close-up view of MS33 multichannel seismic line MS33 around the southernmost fan-shaped basin of the South Matapan trench.

deep basin to the inner wall (see location in Fig. 8). The single hole of site 128, located near the seaward edge of the trench floor, has crossed the Quaternary sequence from upper to lower Pleis- tocene. The DSDP shipboard Party (Ryan et al., 1973) divided this sequence in four units. At site 127. three holes have been drilled closer to the trench inner wall along a roughly SW-NE oriented line parallel to our reference seismic profile. The basement has been reached at the three locations and is made of dolomitic rocks, Early Cretaceous in age, partially to highly brecciated with sparite cemented fractures suggesting a tectonic breccia. Because the tectonic breccia is deepening toward the southwest, whereas the Quaternary layers are thickening northeastwards, we interpret the whole system as a tilted basin bounded by a large scale active normal fault. Some Pliocene foraminiferas were found in the cement of the basement breccia in hole 127B

suggesting that the system was active at least since the Pliocene. This is in good agreement with our interpretation of the southwesternmost deep basin where most of the thickening occurs in the post-Messinian sedimentary sequences (Fig. 7).

However, the detailed stratigraphy of DSDP Hole 127 appears to contradict this model. The contact between Quaternary sediments and the basement is complex there: below the Quater- nary, there are about 8 m of Cretaceous indurated limestones and dolomites overlying one meter of Upper Pliocene marls and Cretaceous dolomite again (detail in Fig. 9). The first contact between Pleistocene marls and Cretaceous dolomites was not recovered but the second con-

tact between the dolomites and the Pliocene marls is a 50” dipping fault, intersecting layers in both the hanging wall and the footwall. Does this reverse sequence (Cretaceous above Pliocene)

SW NE

South Matapan Trench Floor

4640

4840

5040

m

0 zcioo 4000 6000 m

Quatamary sediments

Basement

El

Tectonic (Early Cretaceous) Breccia

Fig. 9. Small-scale geological section of the northernmost basin of the South Matapan trench according to our interpretation of the

DSDP results from sites 127 and 128. I, Pleistocene soft sediments; 2, Cretaceous indurated limestones; 3, tectonic breccia reworking Pliocene material. Vertical exageration about 5.


imply the existence of a reverse fault? To explain the presence of one meter of Pliocene marls inserted between Cretaceous breccia and Creta- ceous limestones, we propose an alternative extensional tectonic evolution in two stages. First,

during Pliocene times, some marls were deposited above a relatively irregular normal fault surface. Second, a reactivation of the former nor- ma1 fault during Pleistocene times lead to a deepening of the basin. During this second stage

229 23O

Fig. 10. Detailed bathymetric map compiled from IBCM conventional bathymetry and SeaBeam data from various cruises (mainly HEAT and Pasiphael. The lobe seen in Fig. 3 and the trench axis are outlined. Outside SeaBeam swaths, IBCM bathymetry is contoured every 200 m (dashed lines with solid lines every 1000 ml. The dotted line along the trench axis is the limit between two contouring of Sea Beam data (to the southwest, isobaths every 20 m and every 100 m to the northeast). Midpoints of ESP 9 and 11 as well as MS33 seismic line are located.

Pliocene sediments deposited just above the fault have been dragged along the new fault surface (with a locally lower angle). This interpretation in two successive extensional stages fits the observation that the southwestward basin of the seismic line (Figs. 7, 8 and 9) is filled in two distinct post-Messinian episodes (both with tilted strata suggesting a syn-tectonic origin).

5. Extensional pattern in the South Matapan trench

Based on Sea-Beam data (Fig. lo), seismic profiles (Finetti, 1976; Mascle and Le Quellec, 1980) and diving results (Le Pichon et al., 1981), we propose in Fig. 11 a crustal section, in Fig. 12 a schematic structural map of the South Matapan trench area and a synthetic tectonic block diagram of the same area in Fig. 13. The Ionian branch of the Hellenic trench system is not a continuous feature over hundreds of kilometers, but instead it is a set of ponded basins that we propose to interpret as fan-shaped isolated basins

SYNTHETIC CRUSTAL SECTION OF THE

controlled by normal faults. We can estimate the trends of these normal faults from bathymetric features on the Sea-Beam map (Fig. 10). We recognize three groups of faults: most of the faults have a NW-SE trend parallel to the trench axis, but one can identify a group of nearly E-- W-trending faults and a prominent isolated N- S-trending lineament crossing all reliefs from the inner wall (to the north) to the ridges of the outer wall (to the south). Although tectonic observations are scarce, we try to evaluate the geometry of active deformation.

First, N-S lineament Fl is probably an active fault as it is responsible of a small uplift in the south Matapan Quaternary trench floor Figs. 12 and 13). The northern part of this scarp has been investigated during a submersible dive (Le Pichon et al., 1981) near the crossing with the extension of MS33 line (Fig. 12). Although no direct evidence for active faulting has been observed during the dive, it has been shown from a slicken- sided fault plane that the scarp appeared to be due to slightly oblique (dextral) normal faulting. This fault can be traced northward up to the

MEDITEWRAMEAW RIDGE INNlEW UsdlT

>4

SW CENTRAL UNIT INNER UNIT UWENCH NE

16

km

Fig. Il. Interpretative crustal section of the Mediterranean Ridge inner unit (based on the velocity model of Fig. 5a). I. Accreted

sediments of the prism. 2, Messininian sequence (mostly evaporites); 3, indurated sediments of the Hellenic nappes; 4, crustal

basement of the Peloponnesus; 5, underthrust sequence attached to the downgoing plate; 6, oceanic crust of the downgoing plate.

The deep d6collement level (top of the lower Cretaceous shales, according to Ryan et al., 1982) is the bold line outlining the top of

the underthrust sequence.


Kythira strait where E-W extensional deformation along en tkhefon N-S normal faults prevails (Lyberis et al., 1982) from Southern Peloponnisos (Lallemant, 1984; Lyon-Caen et al., 1988; Armijo et al., 1992a) to western Crete (Lyon-Caen et al., 1988; Armijo et al., 1992b).

On the other hand, the most prominent active extensional regime is the one related to the fan- shaped basins of the trench. The probable normal fault bounding the two fan-shaped basins deduced from MS33 seismic line (Figs. 7 and 8) has a very large throw but a surprisingly poor morphological expression (Figs. 7, 8 and 10). It coin- tides with an E-W-trending neck in the middle

of the deep. The normal fault bounding the northernmost basin (not resolved in the seismic data but mostly deduced from drilling results, Fig. 9) has also a roughly E-W trend. This geometry, although surprising, is not unique and a similar but smaller asymmetrical basin (at 35”30’N and 22”40’E) bounded by a prominent E-W- trending fault has been recognized on a previously published line in the same area (Mascle et Le Quellec, 19801. We have no direct evidence of the direction of slip along these faults, but the basins have a limited lateral extension and are bounded by NW-SE-trending faults which probably act locally as transfer faults. Our interpreta-

36”oo

Fig. 12. Structural sketchmap of the Hellenic trenches area showing the relationships between Hellenic Trench half-grabens and active faults. The large solid arrows indicate the direction of extensional stresses we infer from this system.

40 22O 00 20 40 23” oo

tion would account for the recent report of several micro-earthquakes at the southern tip of the South Matapan trench (Hatzfeld et al., 1993) showing pure extension on N35” to N70”-trending faults. We consequently suggest that the present- day extensional regime in the South Matapan trench is dominated by along arc extension (NW- SE-trending minimum principal stress a,). This along-arc extension can be predicted from kinematic considerations since Peloponnese and Crete

diverge significantly. Peloponnisos suffered a clockwise rotation whereas Crete drifted southward (Laj et al., 1982) and the azimuth of the Crete/Africa relative motion (from focal mechanisms see Taymaz et al., 1990; Hatzfeld et al.. 1993) is about N25” whereas the azimuth of the Peloponnese/ Africa relative motion is closer to N50” (Hatzfeld et al., 1993). Note also that this along-arc extension is known onland since Ly- beris and Lallemant (1985) have shown that the

ANTIKYTHIRA

Fig. 13. Synthetic tectonic block diagram of the South Matapan trench area. Vertical exaggeration about 10.


20

km

Fig. 14. General block diagram of the western Mediterranean ridge subduction complex with our interpretation of the stress distribution.

a, axis is oriented roughly N-S on the whole western side of the Peloponnesus since at least the Early Pleistocene, overprinting older NW-SE trends in the southwestern part. Fig. 14 synthe- sizes the stress distribution we infer from our study in a simplified block diagram crossing the subduction complex from the Syrte abyssal plain up to the Hellenic outer arc (Kythira strait).

6. Conclusion

Our structural interpretation of seismic line MS33 constrained by the ESP velocity structures is consistent with the hypothesis of a backstop extending up to 100 km seaward of the western Hellenic Trenches (Truffert et al., 1993). We

locate more precisely the seaward edge of this backstop along southwestward dipping reflectors at the boundary between the two morphological provinces of the Mediterranean Ridge named central and inner units. The inner unit is characterized by a flat topography at wavelengths greater than few kilometers. We have further shown that the short-wavelength folding on top of this inner unit of the Mediterranean Ridge can be related to gravity gliding of the upper sedimentary sequences over the Messinian evaporite layer caused by the gentle northeastward slope (2”) of the backstop. This superficial folding, therefore, does not imply a generalized compression in the inner unit. Finally, the observation of fan-shaped basins on the MS33 seismic line allows us to propose a new interpretation of the South Matapan deep

50 S. Lallemant et al. / Tectcmophysic.s 234 (1994) X-52

basins, taking into account DSPD drilling and Sea-Beam data, as half-grabens, controlled by nearly E-W-trending and southward dipping normal faults active since the Pliocene. The subsi- dence of these fan-shaped basins and, therefore, the Hellenic trenches filling are post-Upper Miocene in age.

Considering the Mediterranean Ridge inner unit as a backstop of continental origin, we end up with a paradox: why are they the deepest sea floors in the Eastern Mediterranean Seas? Kas- tens (pers. commun., 1992) proposes that the Hellenic Trenches could represent the former limit of the Middle Miocene erogenic belt. The Aegean continental mass experiencing the same post-erogenic extension could result in a thin continental slice at the present-day Mediter- ranean Ridge inner unit where initial thickness was normal. On the other hand, Truffert et al. (1994) suggest that the Hellenic subduction zone could have been non-accretive (and thus probably erosive) prior to the recent accretion of the Mediterranean Ridge (not earlier than 19-14 Ma). The thinning in an erosive subduction zone is known to decrease rapidly from the plate boundary (subduction trench) to the upper slope of the margin (e.g., Von Huene and Lallemand, 1990). During Pliocene and Quaternary times, an important uplift of the Aegean outer arc probably due to a massive underplating of sediments beneath the continental upper plate (Le Pichon and Angelier, 1981; Truffert et al., 1993) could have led to a steepening of the slope between the toe of the upper plate and the Aegean outer arc. There are not enough constraints yet to choose between these two main hypotheses which are not incompatible.

The outer wall of the South Matapan trench cannot be interpreted as a major backthrust system and the boundary between the recent accretionary complex and the rigid backstop is located about 100 km seaward of the western Hellenic trenches. This interpretation implies that any mass-balance of the western Mediterranean accretionary complex considering the western Hel- lenic trenches as the edge of the backstop (Kastens, 1991) significantly overestimates the volume of the accreted material. We shall con-

sider this mass-balance problem in a separate paper.

Acknowledgments

Thanks to R. Nicolich (Univ. of Trieste) and to OGS (Trieste) for providing us with seismic reflection data (line MS33) and for the discussion on this line. We are grateful to X. Le Pichon for his constructive criticism on early versions of the manuscript. We are also indebted to K. Kastens and an anonymous reviewer for their helpful comments and suggestions. This work was partly supported by INSU OcCanoscope program. This paper is the contribution 248 of the “Laboratoire de GCologie de 1’Ecole Normale SupCrieure”.

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