Structure of the Mediterranean Ridge accretionary complexfrom seismic velocity information

J. Fruehn �, T. Reston, R. von Huene, J. BialasResearch Centre for Marine Geosciences ^ GEOMAR, Wischhofstrasse 1^3, D-24148 Kiel, Germany

Received 15 May 1997; received in revised form 4 May 1999; accepted 19 November 2001

Abstract

Seismic velocities obtained from ocean-bottom hydrophone, expanding spread profile and multi-channel seismicdata were used to compile a velocity model for the Mediterranean Ridge along a 220-km-long transect extending fromthe Sirte Abyssal Plain to the Cleft region near the Hellenic Trough. A 200^300-m-thin layer of Plio^Quaternarysediments with velocities of 1800^2200 m s31 covers the whole Ridge. The Messinian evaporites (4000^4500 m s31)occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the Ridge. In theintervening region however, the evaporites appear absent and the seismic velocities are generally lower. Archedreflectors, imaged in the depth-migrated section, suggest that the sediments beneath the Ridge crest belong to a Pre-Messinian accretionary wedge. Beneath the Messinian evaporites a northeastward-thinning layer of probable Tertiarysediments shows laterally increasing velocities from 3300 m s31 to 4600 m s31. Assuming that the layer thinning iscaused by compaction due to increased overburden alone, we have calculated a porosity reduction from 15% to 4%and an associated fluid expulsion of 10 km3 km31 along the trench axis. This corresponds to c. 60% of the initial fluidvolume of an undeformed sediment column from the abyssal plain. The almost impermeable evaporitic cap over thesesediments leads to high fluid pressures at the base of the evaporites, likely to make this horizon the basal de¤collementof the modern accretionary system. A 2.5-km-thick unit of probable Mesozoic carbonates with velocities of 4500^4600m s31 is inferred at c. 8 km depth. The top of the oceanic crust occurs at a depth of about 10 km. The results fromthis study have widespread implications for the understanding of the regional geological history. ; 2002 ElsevierScience B.V. All rights reserved.

Keywords: Mediterranean Ridge; Seismic velocity; Messinian evaporites

1. Introduction

The convergence of Africa with the Aegean re-gion has led to the development of an accretion-ary complex as sediments deposited on the Afri-

can plate have been scraped o¡ and transferred tothe overriding Eurasian plate. This complex is be-lieved to form a large part of the MediterraneanRidge, a broad bathymetric high that de¢nes anarc to the south of Greece and Crete.

The International MEditerranean Ridge Seis-mic Experiment (IMERSE) investigated the struc-ture of the Ridge, focussing on a transect betweenthe Sirte Abyssal Plain and the southeastern tip ofthe Peloponnesus, parallel to the local conver-

0025-3227 / 02 / $ ^ see front matter ; 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 5 - 3 2 2 7 ( 0 2 ) 0 0 1 7 2 - X

* Corresponding author. Present address: GX TechnologyEAME Limited, Lawrence House, 45 High Street, Egham,Surrey TW20 9DP, UK.

E-mail address: jfruehn@gxt.com (J. Fruehn).

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gence direction (Le Pichon et al., 1995). Alongthis transect, the Mediterranean Ridge risesfrom over 4 km below sea level at the deforma-tion front in the Sirte Abyssal Plain, to about 2.5km at the crest of the Ridge. To the northeast ofthe Ridge crest a series of deep basins, the so-called Cleft region, are thought to mark thenortheastern limit of the accretionary complex.A backstop of Hellenic basement has been iden-ti¢ed further to the northeast based on wide-angledata and gravity modelling (Tru¡ert et al., 1993;Lallemant et al., 1994).

Although it is generally accepted that the fron-tal portion of the Ridge represents an accretion-ary wedge, the internal structure of this complexis unclear. In particular, the extent and thicknessof the Messinian formation within the wedge aredisputed. For instance, Tru¡ert et al. (1993) pro-posed that the entire Ridge was covered by a thinveneer of Messinian evaporites, whereas Ryan etal. (1982) suggested that evaporites may be largelyabsent on the crest of the Ridge, as the Ridge wasalready a topographic high at the time of theMessinian salinity crisis. While Chaumillon andMascle (1997) proposed that the frontal portionof the wedge consists predominantly of evaporitesaccreted since the Messinian, Lallemant et al.(1994) suggested that even near the deformationfront, only a thin layer of deformed evaporitescovers accreted Pre-Messinian sediments. Thesequestions have implications for Messinian palae-oceanography and for the nature of accretion atthe front of the wedge. Furthermore, if the evap-orites do form a continuous layer over a substan-tial portion of the complex, it is likely that theywill have hindered the expulsion of £uid fromdeeper sediments on the down-going plate andwithin the wedge.

In this contribution, we examine evidence fromseismic velocities for the structure of the Mediter-ranean Ridge accretionary complex. We derive avelocity model for the wedge and use this andother velocity information to constrain its inter-pretation, thereby focussing on the questions ofthe distribution and thickness of the evaporitesto the southwest of the Cleft region, and the de-watering of Pre-Messinian sediments. We use thevelocity model to depth-migrate the seismic re£ec-

tion data, providing an improved image in depthof the internal structure of the wedge. Our resultscomplement those of other papers presented inthis volume.

2. Data and analytical methods

The data used in this study come from threemain sources (Fig. 1b): ESP (expanding spreadpro¢le) data collected in 1988 during the Pasiphaecruise (de Voogd et al., 1992), OBH (ocean-bot-tom hydrophone) data collected in 1993 duringthe Meteor cruise 25/4, and 4.5-km streamerdata collected in 1994 as part of the IMERSEproject.

2.1. Expanding spread pro¢les (ESP)

Prior to IMERSE, the velocity structure of thewedge had largely been deduced from a series ofwidely spaced ESP, published by Tru¡ert et al.(1993). These provide a series of deeply penetrat-ing one-dimensional velocity models at the ESPmidpoints, but unfortunately, several of theseare substantially o¡set from the pro¢le (5A)that we consider most representative of the ac-cretionary tectonics of the wedge. However, theESP data do provide the best constraints on thevelocities within the deeper section, which weincorporate into our integrated velocity model be-low.

2.2. Ocean-bottom hydrophone (OBH) data

In 1993 during the Meteor 25/4 cruise, a 200-km-long wide-angle pro¢le was collected from theregion of the backstop over the accretionary com-plex out onto the Sirte Abyssal Plain. OBH werespaced every 11^12 km along Pro¢le 5A (Fig. 1)and recorded shots made every 1 or 2 min by asingle 32-l airgun operating at 140 bar. Althoughdata quality was disappointing (Fig. 2), with littlepenetration beneath the evaporites, these data doprovide some information on the shallow velocitystructure of the accretionary part of the Ridge,and in particular on the presence of Messinianevaporites in the sub-surface (Fig. 2). These can

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Fig. 1. (a) Regional tectonic setting of the IMERSE experiment. (b) Close-up showing IMERSE pro¢les (thin lines), Pro¢leIM94-5A and the locations of ESP midpoints and ocean-bottom hydrophones deployed during Meteor 25/4. Swath bathymetry(Foucher et al., 1993) shows sea-£oor morphology within the IMERSE transect.

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be identi¢ed as refractions with an apparent ve-locity of 4^4.5 km s31 and observed at o¡setsbetween 3 and 15 km (Fig. 3) on most of thestations across the accretionary complex, but sig-ni¢cantly not on stations 12, 14 or 15 from the

crest of the Ridge, suggesting that the evaporitesmay be absent there. Events from the sea£oor andshallow sediment layers could not be observed asthey did not separate well enough from thestrong, direct water wave arrival (Fig. 3).

Fig. 2. Seismic records from OBH deployed (a) near the deformation front (OBH 33) and (b) near the cleft region (OBH 5);travel-time axis is reduced with 4.5 km s31. The dominant event is an arrival of 4.5 km s31 apparent velocity. It is interpreted asrefraction from the top of the Messinian evaporites. The data recovery along the pro¢le varied signi¢cantly. The reduced qualityof OBH 5 is typical for instruments deployed over the Ridge crest.

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2.3. Multi-channel seismic data (MCS)

During 1994, 20 re£ection pro¢les (total lengthover 2500 km) were collected with the OGSExplora. The source was a tuned array of 32 air-guns with a total volume of c. 5000 cu in (80 l),¢red at 120 bar every 50 m. All but three of thepro¢les used a 4.5-km streamer, consisting of 180hydrophone groups spaced 25 m apart, giving anominal CMP spacing of 12.5 m. The other threepro¢les used a 3-km streamer and will not beconsidered here. We extract velocity informationfrom the 4.5-km streamer data in three ways:from refracted arrivals, depth-focussing analysis(DFA) and semblance analysis.

2.3.1. Refracted arrivalsThe refracted arrival (turning wave) from shal-

low high-velocity layers (such as the Messinianevaporites) can be observed on the relatively smallo¡set range (4.5 km) of seismic re£ection data byits linear move-out. However, the 3-km streamerwas too short to record this arrival. We used thisobservation to map the evaporite distributionalong the lines, thereby complementing the ¢nd-ings from OBH analysis. Fig. 4 shows CMP gath-ers from an area around the Ridge crest, where norefraction is observed, and from a basin (Fig. 5)to the northeast of the Ridge crest, where evap-orites have been identi¢ed by other methods

(OBH and DFA). The refracted arrivals appearabove the sea£oor on the far o¡sets: beforeNMO, they follow a straight line with an appar-ent velocity of c. 4^4.5 km s31. We have mappedthe lateral extent of both the evaporitic break be-neath the Ridge crest and the evaporitic basin byanalyzing CMP gathers from all IMERSE tran-sects (Reston et al., 2002b).

2.3.2. Velocities derived from depth-focussinganalysis (DFA)

DFA was performed on three line segments aspart of an iterative prestack depth migration al-gorithm. The migration velocities give detailed in-formation on the sedimentary section and the ac-cretionary structure to intermediate depth levels(5000^8000 mbsf). Prestack depth migration, asperformed on Pro¢le IM94-5A, uses a ¢nite-dif-ference scheme performed in the frequency-o¡setdomain. During DFA the depth error (Denelle etal., 1986) is mapped and translated into an up-dated velocity model. Migration and DFA arerepeated in a top-to-bottom approach until thiserror is minimal for all re£ectors.

Fig. 6 shows the result of this procedure ap-plied on a segment of Pro¢le IM94-5A. The ve-locity model (Fig. 6a) shows a shallow high-veloc-ity layer (4250 m s31) followed by a layer of lowervelocities (3500^4000 m s31). Between 4.5 and6.0 km considerable lateral variation is observed,

Fig. 3. Travel-time picks from all OBH stations. For clarity, only Messinian evaporite events are displayed and travel-time axis isunreduced. No 4.5 km s31 arrivals were identi¢ed on stations 11^15, deployed on the crest of the ridge.

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which corresponds well with the area of thearched re£ections imaged in the depth section(Fig. 6a, shotpoints (SPs) 4900^5200; 4^6 kmdepth). The velocities in the lower part of themodel are laterally uniform and increase down-wards from 4200 m s31 to 4500 m s31. Here thedepth section images strong continuous re£ections(B, K, O in Fig. 6a). Re£ection B has negativepolarity, which is an essential observation forthe tectonic model presented by Reston et al.(2002b).

In contrast to other data sets processed withthis migration package (e.g. from the Iberian mar-gin: Reston et al., 1996, or from the Gulf of Alas-ka: Fruehn et al., 1999), focussing quality wasvariable, probably due to the scattering e¡ectsof the rough sea£oor and the occurrence of thethick Messinian evaporites. Focussed energy fromre£ections below the evaporites was generallyweak. The accuracy of velocities from focussingerror analysis varies accordingly with depth,structural complexity, and the distance from the

Fig. 4. Selection of CMP gathers to show high-velocity refraction as evidence for shallow evaporites or, where it is not observed,the lack of evaporites. (a) No refraction is observed on gathers from the Ridge crest. (b) Refraction cuts sea-£oor re£ection ongathers from the evaporitic basin near to the Cleft region.

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trench axis (thickness of the evaporites). For theundeformed Messinian and post-Messinian sedi-ments of the Sirte Abyssal Plain average errorsbetween 2 and 5% were estimated from DFA.For the Pre-Messinian sediments and for the ac-cretionary wedge the errors may be greater than15%.

2.3.3. Stacking velocityInterval velocities deduced from stacking veloc-

ities have no direct lithologic signi¢cance, butthere is a correlation between variations in thesevelocities and variations in true interval velocities,provided that the imaged structure is simple (hor-izontal horizons) and the lateral velocity varia-tions are smooth. Furthermore, this techniquehas low resolution in the deep section. However,it can be used as a background velocity to inter-polate between other results, e.g. between regions

investigated by prestack depth migration. In prac-tice, we use the migrated section to identify andtrace major boundaries from one velocity controlpoint to the next, using stacking velocities to pro-vide an additional check on the validity of thecorrelation so made.

3. Integrated velocity model along IMERSE-5A

We used the di¡erent types of analyses outlinedabove to generate local velocity information (Fig.7a) and subsequently combined this informationwith the constraints provided by the seismic imageto provide an integrated velocity model (Fig. 7b)for the Mediterranean Ridge accretionary system.

The stacking velocities reveal a shallow high-velocity layer at about 200 ms beneath the sea-£oor that ends abruptly on the crest of the ridge

Fig. 5. Local deep evaporite basins (hatched) on the crest of the Mediterranean Ridge. The basin ¢ll has a velocity of c. 4 kms31, consistent with evaporites. These basins appear to be controlled by northeast-dipping structures, probably faults. We inter-pret the basins as piggyback basins in¢lled during the Messinian event.

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being replaced laterally by a layer of generallylower velocities (Fig. 7a). These observations areconsistent with the refraction analysis of OBHand streamer data and indicate that the Messinianevaporites may be absent on the crest of the Med-iterranean Ridge. However, around SP 3000 alocal high occurs in the stacking velocities : thiscorresponds to the high-velocity evaporite basinalso identi¢ed by DFA, and refracted arrivals(OBH and streamer).

Three line segments were analysed with DFA.The ¢rst segment (SPs 6050^6600) consists of asequence of four velocity layers: a layer of uncon-

solidated sediment velocities (1800^2200 m s31), ahigh-velocity layer with values between 4000 and4250 m s31 and two deeper layers having averagevelocities of 3300 m s31 and 4100 m s31. The ¢rstlayer (unit 5 in Fig. 7a, corresponding to unit 5 inReston et al., 2002b) represents the Plio^Quater-nary cover, which extends over the whole Medi-terranean Ridge. The high-velocity layer (unit 4)can be attributed to the Messinian evaporites, andthe deep layers (units 3 and 2) are the Pre-Messi-nian sediments, probably consisting of Tertiaryclastics and Mesozoic carbonates as suggestedby Finetti (1976), Kastens et al. (1992) and Res-ton et al. (2002b). The velocities from ESP-19,with a midpoint at 25 km SE of SP 6250, tie invery well in the upper three units, but haveslightly higher velocities for unit 2 (average of4500 m s31). Since focussing energy from thisunit, and hence the accuracy in velocity determi-nation was generally low, we favoured the ESPvalues in the construction of the integrated model(Fig. 7b). The ESP data also gave the velocitiesfor the basement (unit 1), which is believed to beoceanic in nature.

From the migration velocities alone, a correla-tion of velocity units between the ¢rst and secondDFA segments (SPs 4820^5480) is di⁄cult, sincethe number of layers increased to six from previ-ously four. The cross-section presented by Restonet al. (2002b) shows that the re£ections M and B(cf. Fig. 6a) delimit the evaporites (unit 4), and Kis the top of the Mesozoic carbonates (unit 2).Using this information, we can identify unit 4 inthis segment and ¢nd that the Messinian evapor-ites have doubled their thickness (1000 v. 500 msin the ¢rst segment) and appear strati¢ed (averagevelocities of 4250/3750/4300 m s31). The lowerevaporitic layer is followed laterally by lower ve-locities (3700 m s31) along a sharp boundary,which corresponds to the ¢rst arched re£ectionimaged in Fig. 6a. The sudden velocity decreaseindicates a change in lithology rather than in po-rosity. This supports the interpretation of Restonet al. (2002b), that the arched re£ections do notbelong to the evaporites but to a di¡erent unit,believed to be Pre-Messinian clastics, subcreted tothe Messinian wedge. The velocity of unit 3 is c.4500 m s31, which is considerably higher than

Fig. 6. Result of prestack depth migration and DFAs. (a)Depth section shows arched re£ectors between horizons M(top Messinian) and B (base of evaporites), interpreted byReston et al. (2002b) as duplex structures of Pre-Messinianunits, subcreted beneath the evolving accretionary wedge. Kis believed to be the top of the Mesozoic carbonates and Othe top of the oceanic basement. (b) The detailed migrationvelocities (DFA every 500 m) show considerable lateral var-iation at intermediate levels. A clear velocity boundary fromevaporitic to sediment velocities between 4.5 and 5 km depthsupports the existence of a Pre-Messinian accretionary bodyin the area of the arched re£ections.

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in segment 1. ESP-6, the midpoint of which un-fortunately lies o¡ the pro¢le discussed here,shows a somewhat di¡erent velocity strati¢cationin the evaporites: two thin layers of 4500 m s31

and 4000 m s31 are followed by a thick layer(2500 ms) with velocities ranging from 3500 ms31 to 4100 m s31. We note, however, that theaverage velocity in the evaporitic units is similarin both data sets. As for the previous segment, wefavoured in the set-up of the integrated model(Fig. 7b) the migration velocity for the upperlayers (units 5^3) and the ESP data at depth(units 2 and 1).

The focussing quality of the migration opera-tors was lower in the third segment (SPs 2650^3150) than in the previous segments. Lateral ve-locity variations and a very thick high-velocitylayer might be the reason for this. Reston et al.(2002b) have interpreted the upper part of thissegment as a basin which was probably isolatedduring Messinian desiccation events, allowing theaccumulation of more than 3 km of evaporites.The midpoint of ESP-8 is located at about 35 kmSE of SP 2320 in a tectonically complex area ^ theCleft region.

The integrated velocity model (Fig. 7b) was ob-

Fig. 7. The integrated velocity model and its components. (a) Interval velocities derived from semblance analysis show a discon-tinuous, shallow high-velocity layer, DFA and ESP velocities give a more detailed image of the velocity structure and allow cor-relation of the velocity units in the southwestern part. (b) The integrated velocity model was obtained by editing and interpola-tion along the structures imaged in the time migration of IM94-5A (Reston et al., 2002b). The Messinian evaporites (M. evp.)are thickening towards the Ridge crest and show strati¢cation. The Pre-Messinian accretionary wedge is a body of sedimentaryvelocities as suggested in Fig. 6. Tertiary clastics (T. cl.) show lateral velocity increase. The velocities for Mesozoic carbonates(M. cb.) and the oceanic crust were taken from ESP measurements.

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tained by editing and interpolation of the datashown in Fig. 7a. Editing of the migration veloc-ities consisted in changing the velocities of certainlayers according to the ESP data (as mentionedabove), and in averaging over areas where thescattering of the values was high. Interpolationbetween the data sets was performed along thestructures imaged in the time migration of Pro¢leIM94-5A (Reston et al., 2002b; Figs. 5^10). Themodel shows (1) that the thin layer of Plio^Qua-ternary sediments covers the whole ridge, (2) theMessinian evaporites thicken as they are incorpo-rated into the wedge and (3) they do not occurover the whole Mediterranean Ridge. An evapor-itic basin exists beyond the crest of the ridge. Themodel also indicates (4) a strong velocity increasein the Tertiary clastics and (5) the existence of abody beneath the crest of the ridge (Pre-Messinianaccretionary wedge), with lower velocities thanthe evaporites. While the data coverage is good

in the southwest and the northeast, there is a largegap in the central part of the pro¢le.

4. Depth migration, dewatering and gravity

Before discussing the implications of the modelit is important to bear in mind the strengths andweaknesses of the model, which re£ect those ofthe di¡erent data and methods used to constructit. The OBH data provide valuable informationon the presence and velocity structure of evapor-ites within the upper few kilometres of the Ridge,but do not contribute velocity information deeperthan about 5 km below the sea£oor because of thelow quality of the arrivals from these depths.However, this data set delivers valuable informa-tion on evaporite velocity and distribution alongthe line, which as discussed below has implica-tions for the palaeoceanography of the region

Fig. 8. (a) Post-stack depth migration and (b) simpli¢ed, depth-converted velocity model used for migration. PQ: Plio^Quater-nary; ME: Messinian evaporites; TC: Tertiary clastics; MC: Mesozoic carbonates; OC: oceanic crust; pre-/post-M AW are pre-and post-Messinian accretionary wedge respectively. See Reston et al. (2002b) for a detailed tectonic interpretation.

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during the Messinian. The MCS data contributemost substantially to the model through three dif-ferent methods of analysis (semblance, DFA, re-fraction). Velocities from DFA are considered

most reliable for shallow to intermediate depths(down to 5 km below the seabed). For areas be-tween the DFA segments, the semblance velocitiesindicate lateral and vertical trends. The analysis of

Fig. 9. Compaction and dewatering of the Tertiary clastics. (a) Post-stack depth migration showing the region of interest (grey-shaded). (b) The sediments lose about 60% of their initial £uid content as they are compacted by the thickening overburden.Since the £uid cannot escape through the almost impermeable evaporitic cap, it is very likely that £uid pressures increase at thebase of the evaporites. The seismic expression of this overpressure may be the negative polarity of horizon B, the de¤collementzone (SPs 5000^5500).

Fig. 10. Gravity map, complied by OGS Trieste, showing free air anomalies in the IMERSE area. The thick line marks Pro¢leIMERSE-5A. The trend of the anomalies is roughly perpendicular to the IMERSE transects, and therefore allows us to use two-dimensional gravity modelling.

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evaporite refractions on CMP gathers constrainsthe evaporite distribution along the line. Finally,we have limited the contribution of the ESP datato great depths (for which they provide the bestconstraints) mainly because two of the ESP mid-points are located tens of kilometres o¡ the lineand because of the one-dimensional character ofits velocity information.

4.1. Depth migration from deformation front toRidge crest

We have used the integrated velocity modelto carry out a post-stack depth migration of thefrontal portion of Pro¢le 5A. The velocity modelfrom the Sirte Abyssal Plain to the crest of theridge is well constrained by migration, ESP, andstacking velocities. Fig. 8b shows a simpli¢ed,depth-converted version of the model, whichwas used to produce the post-stack migrationimaged in Fig. 8a. A smooth velocity modelis essential for migration. We therefore neglect(1) the observed strati¢cation of the evaporitesand (2) the vertical gradient applied to the Pre-

Messinian units, and use average velocities in-stead.

The quality of the resulting image not only is acon¢rmation that the velocity structure is reason-able, but also, in combination with the constraintsprovided by the velocity model, it aids the inter-pretation of the frontal portion of the Ridge. Thedepth migration shows a 200^300-m-thick Plio^Quaternary cover that is heavily deformed acrossthe frontal slope, producing the rough ‘cobble-stone topography’ often observed in this area.The Messinian evaporites have an undeformedthickness of 1 km, which increases towards thecrest to 3 km at SP 5500, where the onset ofarched re£ectors marks a change in structure.From velocity information and the seismic char-acter of the re£ections, we infer that the wedgefrom the deformation front to the arched re£ec-tors consists of Messinian evaporites and youngersediments, and thus is a post-Messinian wedge.Beyond this point however, we interpret the dip-ping structures within the wedge as imbricatethrusts (partly based on their sub-evaporiticvelocity), of probable Pre-Messinian sediments.

Fig. 11. Gravity modelling of free air anomaly along IMERSE-5A. (a) Observed and calculated values show a very good ¢t.(b) The model parameters (density in g cm33 and boundaries) were derived from the seismic velocity model presented in this pa-per. The quality of the ¢t implies that the velocity model and the lithological interpretation of those velocities are at least reason-able.

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These either represent Pre-Messinian units sub-creted since the Messinian or structures developedwithin a Pre-Messinian wedge (Reston et al.,2002a). The top of the Pre-Messinian section, cor-responding to the base of the Messinian evapor-ites (as identi¢ed from the velocity model), is arelatively continuous re£ection from the deforma-tion front to the central part of the segment (SP4750). Above this re£ection, the wedge thickens tothe northeast, whereas below it, the Pre-Messinianunits thin and increase in velocity in the samedirection. We interpret this re£ection as the basaldetachment to the wedge (de¤collement).

4.2. Dewatering of the Pre-Messinian sediments

The sediments beneath the Messinian evapor-ites (Tertiary clastics, Fig. 9a) show a velocityincrease from 3300 m s31 to 4600 m s31 betweendeformation front and the crest of the ridge (Figs.7b and 8b). Assuming that this increase representsthe e¡ect of compaction due to the increasingoverburden, we can relate it to porosity reduction,which equals the £uid loss from these sequences.Bulk saturated densities were obtained from theseismic velocities using an empirical relationship(Hamilton, 1978) and grain and pore water den-sities as found in other accretionary wedges (2700kg m33 and 1050 kg m33 respectively: Bangs etal., 1990). These assumptions could lead to sys-tematic errors in porosity, but relatively small er-rors in porosity variation. The average porositiesdecrease from 15% to 4% between SPs 6500 and5000, suggesting an expulsion of 60% of the £uidpresent in the undeformed rock (Fig. 9b). Thisyields a total £uid loss of about 10 km3 km31

along the trench axis. As the sediments enteredthe trench within the last 2.5 Ma (Reston et al.,2002b) we can assume that the observed compac-tion and £uid expulsion has taken place in thistime. This results in a mean rate of Darcian £uidexpulsion over the whole frontal region of thewedge of about 0.05 mm yr31, which is far smallerthan for other accretionary wedges. However, ourvalue is compatible with the results of Westbrookand O’Neill (2002), although they point out thatmost of this £ow is likely to be concentratedalong faults and other conduits, where the £ow

rate will consequently be much higher. We notethat since the evaporites are practically imperme-able, high £uid pressure at the base of the evap-orites is likely to build up (von Huene et al., 1997;Westbrook and O’Neill, 1996).

4.3. Gravity modelling

Gravity modelling can be used to check thevalidity of the velocity model and to further con-strain its interpretation. Along-track gravity datawere collected during the IMERSE Explora cruiseand have been merged (Cernobori and Nicolich,personal communication) with other marine grav-ity data, acquired by OGS Trieste and the CNR(Consiglio delle Ricerche), to produce a detailedgravity map of the study area (Fig. 10). Thisshows that Pro¢le 5A runs virtually normal tothe main gravity trends and thus is suitable fortwo-dimensional gravity modelling.

The gravity model was constructed by convert-ing the velocities of the integrated model (Fig. 7b)into density using published relationships (deVoogd et al., 1992; Tru¡ert et al., 1993) for thedi¡erent lithological units interpreted from thedepth migration. Oceanic crust was assigned adensity of 2800 kg m33, mantle 3400 kg m33,the Hellenide crust forming the backstop 2750kg m33, the Messinian evaporites (undi¡erenti-ated) 2200 kg m33, and the Pre-Messinian withinthe wedge (either subcreted beneath the Messinianor forming the Pre-Messinian accretionary wedge)a density of 2350 kg m33. This relatively low den-sity (consistent with the observed velocity struc-ture of this unit) re£ects the inferred high £uidpressure beneath the evaporites and the presenceof mud diapirs rising from depth beneath thecentre of the Ridge. Sediments within the subduc-tion channel are modelled as two layers: theupper one with a density of 2300 kg m33, thelower with density 2550 kg m33. The increasingvelocity toward the northeast within this channelis simulated by pinching out the upper layer (italso physically pinches out as the detachment cutsdown to the Aptian shales : Reston et al., 2002a).

The ¢t between modelled and observed gravityis very good (Fig. 11), with a maximum mis¢t ofless than 5 mGal. While we accept that gravity is

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inherently non-unique, the quality of the ¢t im-plies that the velocity model and the lithologicalinterpretation of those velocities are at least rea-sonable.

5. Discussion

In the preceding sections we have derived avelocity model for the Mediterranean Ridge ac-cretionary complex, used this model to producea depth migration of the re£ection pro¢le, inter-preted this depth section and the velocity model interms of di¡erent lithological units and supportedboth the model and the interpretation throughgravity modelling. We have also used the velocitymodel to constrain the amount of dewatering oc-curring at depth beneath the basal detachment. Inthis section, we discuss the implications that ourresults have for an understanding of the evolutionof the Mediterranean Ridge and for the mechan-ics of current accretion.

5.1. The structure of the Ridge during theMessinian

The apparent absence of evaporites on the crestof the Ridge may mean that it was already a topo-graphic high during the Messinian salinity crisisas suggested by Ryan et al. (1982). This is notaltogether surprising, as subduction beneath theAegean region is estimated (on the basis of thelength of the slab imaged tomographically) tohave been continuous over the past 26 Myr(Spakman et al., 1988) and is thus likely to havecreated a Pre-Messinian accretionary wedge (dis-cussed further below). More surprising however isthe presence of thick evaporitic basins on the crestof the Ridge (Fig. 5 and Tay et al., 2002). It isunlikely that these evaporite units have been em-placed tectonically onto the crest of the Ridge, asthe Plio^Quaternary is little deformed. Instead,we suggest that the evaporites ¢lled local basins(piggyback basins?) that were present on the crestof the Ridge during the Messinian and were iso-lated during the fall of sea level. As the basins arefull to the brim with evaporites, we infer thatthese basins must have been repeatedly charged

with highly saline £uids during the crisis. Thisimplies either that the sea level £uctuated at closeto the level of the basin rim, re¢lling the basin byoverspill, or that highly saline £uids could seepinto the basin through the surrounding wedge.In both cases, the distribution of evaporites im-plies that during at least part of the Messinian,sea level must have been close to the depth of theRidge crest, which, after correction for the iso-static response to the lowering of sea level, wouldbe c. 2 km below present sea level. We thus inferat least partial desiccation during the Messinian.

5.2. Evolution of the accretionary complex

Although we thus infer that the crest of theRidge is a Pre-Messinian wedge covered by rela-tively undeformed Plio^Quaternary sediments andlocal evaporitic deposits, the toe of the Pre-Mes-sinian wedge is now buried beneath the subse-quently accreted Messinian units. Both the veloc-ity and gravity models allow us to identify Pre-Messinian units within the wedge. These modelscannot easily distinguish between Pre-Messinianunits accreted prior to the salinity crisis (i.e. thePre-Messinian wedge) and units subcreted be-neath the deforming evaporites, as discussed byReston et al. (2002a). However, based on the seis-mic image we tentatively locate the toe of the Pre-Messinian wedge at between SPs 4600 and 4700where there is a slight change in the geometry ofthe sub-Messinian structures.

We thus propose that the wedge has evolvedfrom a Pre-Messinian wedge with a basal detach-ment at the top of the Mesozoic carbonates to acurrent situation with a wedge of dominantlyMessinian evaporites. This post-Messinian wedgeis characterised by a basal detachment at the baseof the evaporites, and is superimposed on a deep-er wedge where the basal detachment follows thetop of the carbonates. The landward deepening ofthe basal detachment has led to the subcretion ofPre-Messinian clastics beneath the deformingevaporites.

5.3. Wedge mechanics

The depth migration shows that the frontal

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slope of the accretionary wedge is c. 1‡ to thesouthwest, and that the basal detachment dipsequally gently to the northeast. The overall taperof the wedge is thus only c. 2‡, making the Med-iterranean Ridge one of the £attest submarinewedges known. Whether a Coulomb or a plasticrheology is assumed for the wedge, such a lowtaper implies £uid pressures within a few percent of lithostatic at the level of the basal detach-ment (Reston et al., 2002b). We suggest that thesehigh £uid pressures result from the dewatering ofthe Pre-Messinian clastics beneath the basal de-tachment at least over the frontal portion of thewedge. As described above, we can deduce fromthe velocity structure of these clastics that up to60% of the pore water present at the deformationfront has been expelled 50 km to the northeastbecause of increasing compaction. However, theimpermeable nature of the overriding evaporiteswill have hindered escape of these £uids, resultingin high £uid pressures along the basal detachment(Westbrook and O’Neill, 1996). We note that thebasal detachment re£ection has a negative polar-ity (Chaumillon et al., 1996) even where the over-lying units have a lower velocity than the com-pacted underlying units, and interpret this asevidence for a thin, not completely resolved low-velocity channel characterised by high £uid pres-sure. Similar negative polarity detachments havebeen identi¢ed from other accretionary wedges(e.g. Barbados: Shipley et al., 1994).

5.4. Pre-Messinian stratigraphy

As noted above, the Pre-Messinian clastics be-neath the basal detachment thin towards thenortheast as the overlying Messinian clastics with-in the wedge thicken in the same direction. Thethinning can in part be related to the loss of porespace, as can be deduced from the velocity varia-tion within the layer. This alone would implycompaction to perhaps 90% of the original thick-ness, assuming that the volume reduction occursdominantly in the vertical direction, which is con-siderably less than the thinning observed on thedepth section. The di¡erence probably represents

stratigraphic thinning away from the Africanmargin, implying that the dominant source forthese clastics is from that margin, which probablyprograded out over the ocean^continent transi-tion of the passive margin.

6. Conclusions

The compiled velocity model for the Mediterra-nean Ridge accretionary complex has helped (1) tode¢ne structure, velocity and distribution of theMessinian evaporites and (2) to constrain the na-ture of underlying units (sediments, accreted sedi-ments, oceanic crust). The observed velocity in-crease in the Pre-Messinian sediments has beenused (3) to estimate compaction-related dewater-ing. The evaporites of the Sirte Abyssal Plain, a1-km-thick high-velocity layer of 4200^4600 m s31,are entrained in modern accretion against a cen-tral body of probable Pre-Messinian origin. Theyreach a maximum thickness of 3 km near theRidge crest, where the postulated Pre-Messinianwedge is indicated by the onset of arched re£ec-tors. This study has demonstrated that the Ridgecrest is evaporite-starved, a conclusion with manyimplications for our understanding of the regionalgeological history. The results also support theinterpretation given by Reston et al. (2002a,b),in showing that the sediments beneath the Ridgecrest have generally lower seismic velocities thanthe Messinian evaporites, and that the most likelylocation for the basal de¤collement is the base ofthe Messinian evaporites. Sediments underlyingthe Messinian evaporites, Tertiary clastics andMesozoic carbonates, are constrained by seismicre£ectors and sedimentary velocities. The Tertiaryclastics experience compaction due to the in-creased overburden of the growing wedge, asdocumented by a thinning from 2.5 to 1.5 kmand a lateral velocity increase (3300^4600 m s31).The £uid expulsion associated with this compac-tion (c. 10 km3 km31 trench axis) is likely to leadto £uid overpressure at the base of the almostimpermeable Messinian evaporites as shown byWestbrook and O’Neill (1996).

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Acknowledgements

The IMERSE project in general and J.F. inparticular were supported by the EU MAST IIprogramme through Contract CT93-0062. Wethank Licio Cernobori, Rinaldo Nicholich, IginioMarson and Marko Stoka for permission to usetheir gravity data and modelling in this paper.

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