Reston Fontalaccretionmediteranian Marinegeology 2002

Frontal accretion along the western Mediterranean Ridge:the e¡ect of Messinian evaporites on wedge mechanics

and structural style

T.J. Reston �, R. von Huene, T. Dickmann, D. Klaeschen, H. KoppGEOMAR Research Centre, Wischhofstrasse 1^3, D24148 Kiel, Germany

Received 6 May 1997; received in revised form 7 May 1999; accepted 19 November 2001

Abstract

In the context of the IMERSE project, several crossings of the deformation front of the western MediterraneanRidge were made in the region of the Sirte Abyssal Plain, the Messina Abyssal Plain and the intervening region. Inthis paper, we present seismic images and interpretations across the deformation front, with particular emphasis onthe role the Messinian evaporites have played in controlling the accretionary tectonics of the thin frontal portion ofthe wedge. The seismic images show that the basal detachment generally is located at the base of the evaporites. Froma consideration of the mechanics of the wedge, for both Coulomb and plastic rheologies, we show that the low wedgetaper (c. 2‡) requires that the detachment is characterised by extreme fluid overpressuring (within 2% of lithostatic inplaces) and that the basal yield stress (less than 1 MPa) is lower than that of a wet salt de¤collement zone. Thissupports the seismic interpretation that the detachment occurs in overpressured sediments beneath the impermeableevaporites. Lateral variations in the accretionary style can be related to lateral variations in evaporite thickness, theeffectiveness of the evaporite as an impermeable seal and to local relief on the subducting plate; these factors controlthe escape of fluids from beneath the evaporites and hence fluid pressure and basal yield stresses. ; 2002 ElsevierScience B.V. All rights reserved.

Keywords: Mediterranean Ridge; accretionary wedges; basal detachment; £uid pressure; Messinian

1. Introduction

It is near the deformation front that the depthto the basal detachment and hence the stratigra-phy of much of an accretionary wedge are estab-lished. As it is in the ¢rst 20 km or so of thewedge that frontal accretion takes place, thestructure of this region provides information on

the mechanics of accretion. Furthermore, it hasgenerally been acknowledged that most pore £u-ids are mobilised not far from the deformationfront (e.g. Moore and Vrolijk, 1992). Hence astudy of the deformation front and areas immedi-ately arcward provides information vital for anunderstanding of wedge evolution and mechanics.The Mediterranean Ridge has formed by con-

vergence of Africa and the Aegean region and thesubduction of the African Plate beneath Eurasia:sediments have been scraped o¡ the subductingAfrican plate and piled up in front of the Aegean

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 3 - 1

* Corresponding author.E-mail address: [email protected] (T.J. Reston).

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www.elsevier.com/locate/margeo

region. However, the Ridge is a unique accretion-ary complex being unusually wide (c. 250 km) and£at (Ryan et al., 1982), perhaps re£ecting thepresence of thick sequences of Messinian evapo-rites within the wedge. For instance, Kastens et al.(1992) proposed that the wedge is £at because ofthe presence of a weak shallow de¤collement at asalt layer within the evaporites. However, otherworkers have proposed that the main de¤collementlies in the lower evaporites (Chaumillon et al.,1996) or at their base (Chaumillon and Mascle,1997).In 1994, the International MEditerranean

Ridge Seismic Experiment (IMERSE) collectedwith the seismic vessel OGS Explora over 2500km of near-vertical re£ection data over the Med-iterranean Ridge (Fig. 1). The source was a tunedarray of 32 airguns, total volume 80 l (4880 cuin); the streamer was 4500 m long, with 180 chan-

nels. Processing was carried out at the di¡erentIMERSE partner institutions (see Acknowledg-ments), but generally followed a standard se-quence of edit, despike, ¢lter, resample to 8 ms,deconvolution before stack, velocity analysis,NMO correction, CMP stack, deconvolution afterstack, time-variant frequency ¢lter, and time mi-gration.The IMERSE data set provides crossings of the

deformation front in three di¡erent regions (Fig.1): the Messina Abyssal Plain (Pro¢les 0A, 0Band 1), the Sirte Abyssal Plain (Pro¢les 5A, 6,16 and 17) and between the two abyssal plains(Pro¢les 3 and 4). In this contribution, the styleof deformation at the front of the wedge withinthese regions is compared and the role that theMessinian evaporites have played in the evolutionof the wedge, and in particular in controlling theform of the wedge, is discussed. The results have

Fig. 1. Map showing the location of the IMERSE experiment in relation to the main tectonic features of the Ionian Sea region.The data set provides several crossings of the deformation front, which are discussed in this paper. The inset shows the locationsof the portions of data studied (swath bathymetric data from Foucher et al., 1993), as well as the location of the Bannock struc-ture, a major morphological feature near the deformation front.

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implications for the spatial and temporal develop-ment of the wedge, for its £uid £ow history andfor its internal structure.

2. Seismic observations and interpretations

2.1. Stratigraphy of incoming section

A key to the understanding of the style of de-formation at the front of a wedge and hence themechanics of accretion is the geology of the in-coming section (Fig. 2). In the case of the Medi-terranean Ridge, the section consists of Plio^Qua-ternary sands and muds (labelled here as Unit 5)overlying the Messinian evaporitic sequence (Unit4). The top and base of the evaporites can gener-ally be identi¢ed on the seismic pro¢les (Ryan etal., 1973; Finetti, 1976) as the M and B re£ectionsrespectively. The acoustic thickness of the Messi-nian beneath the abyssal plains varies (Fig. 2)from about 200 ms (Pro¢le 1 ^ Messina AbyssalPlain) to more than 500 ms (Sirte Abyssal Plain).As we shall see, this along-strike variability mayexert an in£uence on the style of frontal accretion.Beneath the abyssal plains, the seismic pro¢les

reveal a thick strati¢ed sequence beneath the Mes-sinian evaporites (Fig. 2). However, the lack ofborehole information in the vicinity of the IM-ERSE experiment means that knowledge of thegeology of the Pre-Messinian section is limitedto exposures on land some distance from theabyssal plains, and inferences that can be madefrom the seismic data (Finetti, 1976; Polonia etal., 2002). Following these authors, we interpretthe sequences underlying the Messinian as earlierTertiary (and perhaps some Mesozoic) clastics (la-belled here as Unit 3), separated by the K re£ec-tor from underlying Mesozoic carbonates (Unit2). Unit 3 is internally only weakly re£ectivewhereas Unit 2 is a layered sequence of high-am-plitude re£ections (Fig. 2). On one of the sections,a locally thick sub-unit (Unit 3b) can be identi-¢ed: as discussed below this may be a large olis-tostrome.Beneath the carbonates of Unit 2 lies basement

(Unit 1), as identi¢ed by seismic refraction studies(e.g. Tru¡ert et al., 1993), and marked by a re-£ection labelled ‘O’. Although in general we shallassume that basement is Tethyan oceanic crust, itis possible in some places, particularly along Pro-¢les 3 and 4, that the basement beneath the abys-

Fig. 2. Stratigraphy and lithology of the incoming sections based on the seismic characteristics and on limited core information.Stratigraphy modi¢ed after Kastens et al., 1992 and Polonia et al., 2002, seismic interpretation detailed in this paper. Note thatthe top of the Mesozoic carbonates may not be the top of the Mesozoic, but may rather lie within the Cretaceous. Also notethat the stratigraphic thicknesses of the various units is laterally quite variable. A possible olistostrome is present on Pro¢le 3.

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sal plain is thinned continental crust of the Juras-sic rifted margin of North Africa. Local basementrelief might therefore in places represent continen-tal fault blocks rather than oceanic seamountsand abyssal hills.In the following sections, we describe the main

features of the frontal part of the accretionarywedge (the frontal slope, Reston et al., 2002),and the implications for the mechanics of accre-tion and dewatering. We start with the SirteAbyssal Plain, which represents one end-memberstyle of deformation, although probably the mostimportant within the main IMERSE transect, be-fore moving on to describe results from elsewherein the study area.

2.2. The Sirte Deformation Front(Pro¢les 5A, 6 and 16)

2.2.1. Pro¢le 5ABeneath the Sirte Abyssal Plain, the Plio^Qua-

ternary section (Unit 5) is approximately 200 msthick and is underlain by 500 ms (about 1 km) ofthe Messinian comprising Unit 4 (see Pro¢le 5A,Fig. 3). The top of the Messinian is marked by apositive polarity re£ection (M), the base by a neg-ative polarity re£ection (B), as discussed for in-stance by Chaumillon et al. (1996). The depth toand thickness of the Messinian agrees well withthe wide-angle data (Tru¡ert et al., 1993; DeVoogd et al., 1992).About 1.2^1.3 s beneath the base of the evap-

orites a strong low-frequency re£ection (K) marksthe base of the Tertiary (+Mesozoic?) clastic sec-tion of Unit 3. The underlying unit is character-ised by strong relatively continuous re£ections,interpreted as coming from the Mesozoic carbon-ates (Unit 2). This sequence (Fig. 3) is approxi-mately 1^1.3 s thick: its base (the O re£ection) isbelieved to correspond to top basement. Thisagrees well with the travel-times deduced fromthe wide-angle data (e.g. Le Meur, 1994).

The deformation front is marked by a changefrom the £at abyssal plain to the rough ‘cobble-stone topography’ characteristic of much of theMediterranean Ridge. On the migrated sectionthis topography corresponds to folding of thePlio^Quaternary section, with a fold wavelengthof about 1^2 km and a fold amplitude of about100 ms (that is 75 m). The slope of the wedge isunusually small, being about 1.2‡ at the deforma-tion front and £attening to the northeast. Overthe ¢rst 55 km of the wedge (Fig. 4), de¢ned byReston et al. (2002) as the Frontal Slope of thewedge, the average slope is 0.9‡. For a Coulombwedge, such low slope angles are characteristic ofextremely low e¡ective basal friction, which (asdiscussed below) might be expected for highpore £uid pressures and weak material at the de-tachment level.Short-wavelength folding appears to have af-

fected the entire Plio^Quaternary section (Unit5) and the top of the evaporites : the M re£ectionparallels the sea£oor (Figs. 3 and 4). On the basisof the fold wavelength, and the degree of thicken-ing required, Kastens et al. (1992) deduced thatthe basal detachment lies at the top of the salt atthe base of the upper evaporites (Fig. 2). We ob-serve, however, that the entire Messinian sectionthickens towards the northeast and thus suggestthat the basal detachment may lie near the base ofthe Messinian section, as also inferred by Chau-millon and Mascle (1997). We propose that,although there is probably local decoupling atthe top of the salt, this does not represent themain basal detachment, nor mark a site of majorhorizontal movement. Instead, we follow Chau-millon and Mascle (1997) and suggest that thetop of the salt (base of the Upper Evaporites,Fig. 2) instead marks a change in deformationalstyle from ductile deformation and £ow within thesalt and lower evaporites to tight folding andbuckling above. This would result from a changein the competence of the rocks within the Messi-

Fig. 3. Detail of the deformation front on Pro¢le IMERSE-5A, showing possible backthrust and short-wavelength folding (cob-blestone topography). Note that the detachment occurs at the base of the evaporites and that the slope dips little more than 1‡.M=base Plio^Quaternary; B=base Messinian; K= top Mesozoic carbonates; O= top oceanic crust.

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nian, with the topmost Messinian (probably anhy-drite and gypsum) being more competent than theunderlying salt.Beneath the Plio^Quaternary, the Messinian

section (Unit 4) starts to thicken behind the de-formation front (shotpoint (SP) 6400, Fig. 4), andcontinues to do so for at least 50 km. In contrast,the underlying Pre-Messinian section (Units 3 and2), if anything, thins on the time section to thenortheast (Fig. 4). For this reason, we infer thatthe basal detachment (the boundary between ac-creting and subducting sediments) is at the base ofthe Messinian near the deformation front andserves as a detachment between distributed defor-mation above and the underthrust section below.Little direct thrusting is imaged within the Mes-

sinian (Fig. 4) as deformation may instead beductile, but a possible backthrust with minor dis-placement is associated with the onset of deforma-tion (Fig. 3). This structure has an apparent dipto the southwest, and appears to detach at thebase of the Messinian at SP 6600, 10 km south-west of the deformation front. This supports thenotion that the base of the evaporites marks themain detachment near the deformation front. Thepresence of backthrusts at a deformation front isuncommon, but has been explained by extremelylow e¡ective basal friction at the detachment else-where (MacKay, 1995), related perhaps in thiscase to elevated pore pressure (see below).

2.2.2. Pro¢le 6The Messinian (Unit 4) beneath the Sirte Abys-

sal Plain is up to 600 ms (over 1 km) thick onPro¢le 6 (Fig. 5). A marked unconformity withinthe Messinian is imaged, and the top of the Mes-sinian is marked by a hummocky topography.Beneath the Messinian, pre-Messinian Tertiary(+Mesozoic?) clastics (Unit 3) are about 1 s thick,and are underlain by up to 1.8 s of probableMesozoic carbonates (Unit 2). Top basement ap-

pears to lie at about 8.8 s two-way travel-time(TWT).The deformation front at SP 460 (Fig. 5) is

again characterised by the onset of rough ‘cobble-stone’ topography, again consisting of folds ofamplitude up to 100 m and wavelength of 1^2 km. However, the ¢rst 13 km of the wedge(between SPs 460 and 720) is steeper here thanon Pro¢le 5A, dipping on average at 2.3‡ to thesouthwest. Beyond SP 720 however, the wedgeslope is reduced to less than 1‡. The steeper aver-age slope of the sea£oor means that the time im-age presented in Fig. 5 is markedly distorted bythe variable sea£oor topography. However, afterconverting to depth, the base of the evaporitesand deeper re£ections lie at about the same depthat the right of the section as at the left. Compar-ison of the thickness of the incoming strata withthose at the right of the section shows that thick-ening has taken place largely within the Messinian(Unit 4) as on Pro¢le 5A, again indicating thatthe basal detachment may lie at the base of theevaporites.The change in morphology of the wedge at SP

720 may be related to the subduction of a mor-phological feature: beneath SP 720, the top of abasement high can be identi¢ed on the time-mi-grated pro¢le (Fig. 5). Re£ections from the £anksof this structure are very weak to the northeast,but to the southwest can be traced at least to 8 sTWT and possibly deeper. The basement high onPro¢le 6 appears to be buried beneath the top ofUnit 3, that is beneath the basal detachment, butthree factors may help explain the in£uence itappears to have had on accretion:(1) stratigraphic thinning of Unit 3 on top of

seamount could gives it di¡erent properties (e.g.less £uid?), resulting in a change in the e¡ectivebasal friction at the top of this unit (base of theevaporites) ;(2) draping (combined with di¡erential compac-

Fig. 4. Pro¢le IM94-5A across the frontal slope of the wedge (Sirte Transect). Passing to the northeast of the deformation front,the Messinian (bounded by re£ections M and B) thickens, whereas the Pre-Messinian strata thin. We infer from this that thebasal detachment lies at the base of the evaporites. Folding of the Plio^Quaternary and of the top of the evaporites evidencesshortening in the ¢rst 55 km of the wedge. The average frontal slope over this range is 0.9‡ to the southwest; the basal detach-ment dips to the northeast at 1.1‡. These ¢gures imply a £uid pressure greater than 98% of lithostatic beneath the evaporites.

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Fig. 5. Pro¢le 6 at the deformation front (Sirte Abyssal Plain). Here the frontal slope is relatively steep but £attens to less than1‡ above a small basement high imaged in the seismic. This high probably forms part of the African plate and is ploughing itsway through the accretionary wedge. The basal detachment is not clearly imaged, but may cut down from the base of the evap-orites to the southwest of the seamount to the Mesozoic section or even to top basement at the seamount itself.

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tion ^ see point 1 above) of Unit 3 over the base-ment high would mean that this was also a highduring the Messinian. This would mean that theseamount would probably have existed as a topo-graphic feature at the level of the detachment;(3) more simply, it is unlikely that Pro¢le 6

passes over the peak of the seamount. This wouldmean that any estimate of the height of the sea-mount from this one pro¢le would be an under-estimate: the seamount may indeed have pro-truded above the top of Unit 3.It is probable that a combination of all three is

important, but without more detailed work (e.g.three-dimensional seismic re£ection) it is not pos-sible to estimate the relative importance of each.To the northeast of this buried basement high,

the Mesozoic carbonate section (Unit 2) can beidenti¢ed as a layered sequence between about 6.6and 7.6 s TWT (SPs 900^1050). Although thelayers appear disrupted at SP 1025, there is noevidence for major shortening within these strata,

implying that the main basal detachment lieshigher in the section. From correlation with otherpro¢les in the region (correlation with Pro¢les 18and through 18 with 5A), we suggest that weakdiscontinuous re£ections 1.2^0.8 s above the topof the carbonates may mark the basal detach-ment, again corresponding to the base of theevaporites.

2.2.3. The Bannock Region (Pro¢les 16 and 18)Seamount subduction appears to have in£u-

enced the tectonics of the deformation front inthe vicinity of the Bannock structure (Fig. 6).This is a complex topographic high surroundedby a moat of basins (Bacino Bannock, Camerlen-ghi and McCoy, 1990). The high protrudes abovethe level of the surrounding wedge, the basins arepartially ¢lled with saturated brines formed by thedissolution of the Messinian evaporites. It is be-lieved that the Bannock structure, which also cor-responds to a 30-mGal free air gravity high,

Fig. 6. Sea£oor morphology (illuminated from 15‡E) derived from detailed swath bathymetry in the region of the Bannock struc-ture (von Huene et al., 1996). The trail of the Bannock seamount can clearly be identi¢ed, as can a possible second buried base-ment high and a possible overspill channel coming from the lip of the Bannock basin. The Bannock structure itself consists of amoat of basins surrounding a complex of central highs (Camerlenghi and McCoy, 1990). TWTs to the £anks of the subductingseamount are marked by circles.

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formed by the subduction of a seamount attachedto the African plate (von Huene et al., 1997).A detailed description of the morphology of the

Bannock structure itself is given by Camerlenghiand McCoy (1990) and a discussion of the originof the Bannock structure and associated morpho-logical features is given by von Huene et al., 1997and need not be repeated here. Certain key as-pects of the Bannock structure are, however, ofrelevance for the theme of this paper, namely thestyle of frontal accretion along the western Med-iterranean Ridge.The Bannock structure is crossed by two seis-

mic pro¢les: 18 and 16 (Fig. 6). Line 18 runsNW^SE, approximately parallel to the deforma-tion front and perpendicular to the convergencedirection. As such it cuts across Pro¢les 3, 5A, 6and 16, enabling correlation of re£ections andseismic units between these pro¢les. The base ofthe evaporites (Re£ection B), thought to mark thebasal detachment at least on Pro¢le 5A, is clearlyimaged on the northwesterly portion of line 18(Fig. 7) as are the underlying Pre-Messinian strata(Units 3 and 2) still attached to the African Plate.Moving to the southeast, the Pre-Messinian strataand the base of the evaporites (identi¢able as anegative-polarity re£ection) appear to onlap anorthwest-dipping re£ection that can be tracedup towards the central high of the Bannock struc-ture. On the other side of the high a matchingsoutheast-dipping re£ection can be traced todepth. Similar re£ections, in this case dippingsouthwest and northeast, can be identi¢ed on Pro-¢le 16 (Fig. 8): again they project up from depthtowards the central high of the Bannock structure(see also Fig. 6). We interpret these dipping re-£ections as the £anks of a seamount attached tothe African plate (the dipping re£ections on Pro-¢le 18 clearly root at the level of the oceaniccrust).As discussed in detail by von Huene et al.,

1997, the Bannock structure has been formed bythe subduction of a seamount, which by breach-ing the entire accretionary wedge has allowed thedissolution of the evaporites by seawater fromabove (due to the removal of the Plio^Quater-nary) and perhaps also the escape of £uids fromwithin and beneath the Messinian (allowed as theevaporitic cap has been breached). The trail thepassage of the seamount has left in the accretion-ary wedge can be seen in the bathymetric data asa disturbed region of ridges and troughs trendingc. 50^60‡E (Fig. 6): the orientation of this trail isconsistent with a local convergence direction be-tween Africa and the Mediterranean Ridge that isparallel to pro¢les 5A, 6 and 16 (Le Pichon et al.,1995). A re-entrant in the front of the wedge (Fig.6) appears to be associated with a second shorterdisrupted zone (von Huene et al., 1997). A secondsmaller topographic high may have entered thesystem here and is currently buried c. 30 kmSSW of the Bannock structure and about 18 kmNE behind the deformation front.As shown by the bathymetric and seismic data,

the subduction of the Bannock seamount has in-£uenced the morphology and style of accretion. Incontrast to the c. 1‡ dip of the frontal slope onPro¢le 5A, on Pro¢le 16 (Fig. 8), the top of thewedge for the ¢rst 14^15 km dips at about 2.5‡(SPs 3750^3440) after which it £attens to less than1‡. The deformation front (SP 3750) is againmarked by a sharp onset of cobblestone topogra-phy (amplitude and wavelengths as above), but noclear detachment can be identi¢ed. This may re-£ect the absence of a single sharp detachment, ormight represent imaging problems beneath therough sea£oor of the front of the wedge. The Me-sozoic carbonates interpreted beneath the abyssalplain are also poorly imaged beneath the wedge.We suggest that these di¡erences between Pro-

¢le 5A and Pro¢le 16 may represent di¡erences inthe mechanics of accretion resulting from the pas-

Fig. 7. Pro¢le IM94-18 (time-migrated). To the northwest, the basal detachment is marked by a strong re£ection at the base ofthe Messinian (B). Further southeast (SP 1000) this abuts a northwest-dipping re£ection. The underlying Pre-Messinian stratalikewise end abruptly at this structure (SPs 700^1000), which is interpreted as the £ank of a subducted seamount. The re£ectioncan be traced to the Bannock Structure (see Fig. 6). We suggest that the seamount has provided an outlet for the passage of £u-ids and hence for the dewatering of the Pre-Messinian section.

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sage of the seamount into the accretionary wedge.First, by providing an outlet for £uids trappedbeneath the Messinian, the seamount may havereduced £uid pressure at depth and hence in-creased the e¡ective friction at the base of theevaporites, leading to a di¡erent style of frontalaccretion. This is developed more below in 3. Dis-cussion. Second, much of the section between theseamount and the deformation front is likely torepresent material that has £owed around the sea-mount to ¢ll the subduction wake (Fig. 6). Assuch, this material is likely to be strongly dis-rupted and probably partly dewatered, again re-ducing the e¡ectiveness of the base of the evapor-ites as a major detachment level. If on this pro¢lethe base of the Messinian does not mark the basaldetachment, we might expect a quite di¡erentstyle of deformation near the front of the wedge,as implied by the steeper frontal slope along Pro-¢le 16 compared to Pro¢le 5A. Unfortunately, theseismic data do not further constrain the depth atwhich the detachment may lie on the former pro-¢le.

2.3. Between the Sirte and Messina Abyssal Plains

Pro¢les 3 and 4 cross the deformation frontbetween the Sirte and Messina Abyssal Plains.Here, the abyssal plain is very narrow and colli-sion between the African continental margin andthe Mediterranean Ridge may be incipient. As theimage is clearer on Pro¢le 3 (Fig. 9), we will limitthe discussion to that pro¢le.To the west of the deformation front on Pro¢le

3, the base of the Messinian is again marked by anegative-polarity re£ection (B) occurring about400 ms beneath the M re£ection (e.g. SP 2950).However, the thickness of the evaporites varieswith the topography of the underlying units,and appears to pinch out at SP 3220 and to beonly about 100 ms thick at SP 3040 (Fig. 9). B

can, however, be followed east beneath the defor-mation front as a discontinuous re£ection (poorimaging perhaps) to SP 2500 where it cuts downby c. 500 ms to a sub-horizontal, more continuousre£ection. Above this, the section thickens to theeast, below it it thins in the same direction; wethus infer that the base of the Messinian (the Bre£ection) corresponds to the basal detachment(D, Fig. 9) as on Pro¢le 5A and that the frontalportion of the wedge consists dominantly of Mes-sinian evaporites.The depth of the basal detachment at the east

end of the section is con¢rmed by correlationswith pro¢les 18 and 5A, which also identify theunderlying Units 2 and 3 there (Fig. 9). The topof Unit 2, the K re£ection, dips gently to the westfrom the eastern end of the time section as far asSP 2400, where it appears to steepen down to 7.7s at SP 2900. Farther west K appears to undulatewith a wavelength of c. 10 km as it rises up to 7.2s at the west end of the line: as discussed below,this might represent, at least in part, velocity pull-up beneath overlying high velocity blocks.The travel-time between the basal detachment

(D, corresponding to B, the base of the evapo-rites) and the top of Unit 2 (K) increases slightlyto the west as far as SP 2450 (Fig. 9). Over the 20km between here and the deformation front at SP2850, the B and K re£ections diverge by c. 500ms, perhaps indicating that Unit 3 thickens sub-stantially over this interval. However, from SP2450 a weak re£ection, sub-parallel to the K re-£ection, can be traced down to the west from thelevel of B, thus forming the lower bound of awedge-shaped region beneath the B re£ection. IfUnit 3 does thicken to the west from SP 2450, itwould appear that it is only the very upper part ofUnit 3 that does thicken in this direction. Weprefer to consider the wedge-shaped region be-tween this re£ection (broken line in Fig. 9) andB as a separate unit (Unit 3b), as it is not clear if

Fig. 8. Pro¢le IM94-16 (time-migrated), showing the section from the deformation front to the Bannock structure. Here there isno clear detachment developed at the base of the Messinian evaporites. Instead, several landward-dipping thrust faults are im-aged, and appear to cut down into the Pre-Messinian section. Furthermore, the frontal slope is far steeper here than elsewhere inthe study area. We suggest that the release of pore pressure in the Pre-Messinian sediments by the escape of £uids up the £anksof the Bannock structure has led to a rather di¡erent style of accretion here.

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it is part of the same stratigraphic succession asthe underlying rocks or if it is allochthonous andwas emplaced at the top of Unit 3 at the start ofthe Messinian. In particular, it might represent alarge thrust sheet (although its geometry wouldimply east-directed thrusting which may be un-likely), or a large olistostrome (our preferred in-terpretation). The emplacement of such a largeolistostrome, probably from the African marginjust to the west, might be related to the draw-down of sea level during the Early Messinianand consequent changes in the stability of themargins; recent work has suggested that thedraw-down and erosion of the margins of theMediterranean may have preceded the depositionof evaporites in the deep basin (Clauzon et al.,1996).In front of the wedge, the top of Unit 3b ex-

hibits considerable topography, which is only par-tially mirrored by the sea£oor re£ections and ap-pears to have been largely smoothed out duringthe Messinian: the evaporites ¢ll small sub-basinsbut appear absent on the local highs (e.g. SP3220, Fig. 9). The topography we thus date asPre- or Early Messinian, it is probably an originalfeature of Unit 3b (the proposed olistostrome)formed during its emplacement. The lack of evap-orites on the highest portions of the unit impliesthat either the evaporites have since been removedby mass-wasting or dissolution, or no evaporiteswere deposited on the crest of this feature.The carbonates of Unit 2 appear on the time

section (Fig. 9) to be folded and generally dis-rupted beneath and to the west of the deforma-tion front (SPs 2650^3050). This may at least inpart represent pull-up (velocity e¡ects) caused bylocal velocity variations (such as large high-veloc-ity blocks of basement) within the olistostrome.Some of the apparent structure of the carbonatesmay be real and represent original depositional

variations or an earlier phase of deformation,but until the velocity structure in this region canbe determined in more detail, the matter remainsinconclusive.A weak, apparently east-dipping re£ection (in

depth at about 20‡) can be traced (Fig. 9) fromthe local high at the top of the olistostrome (SP3220) across the entire thickness of Unit 3b to c.6.3 s beneath SP 3080, which is the level of thetop of Unit 3 in our preferred interpretation. Itmay continue deeper, cutting down into Unit 3towards K, although the obliquity of the pro¢lemeans that migration does not correctly reposi-tion re£ections.We interpret this east-dipping structure as a

fault of some kind, but as it projects up towardsthe Plio^Quaternary c. 18 km to the west of theonset of folding in the Plio^Quaternary (the de-formation front, Fig. 9), we consider it unlikely tobe a major thrust fault. In fact, it appears to beassociated with a normal o¡set of the top of Unit3b (a second such o¡set occurs slightly to the eastat SP 3000), but dips at rather low an angle(c. 20‡) for a normal fault. We suggest that it isa fault that developed during the emplacement ofthe olistostrome, perhaps bounding a large high-velocity block (see above): where the re£ectionintersects the top of Unit 3, the re£ection fromthe latter appears to change polarity, consistentwith the lateral termination of a high-velocityblock within Unit 3b. Interestingly, there issome indication of a local bathymetric high asso-ciated with this fault : although the re£ections ap-pear poorly imaged and may be at least partlyfrom out of the plane of the section, a structurec. 2^3 km across may occur on the sea£oor (Fig.9). We speculate that this might be a mud volcanoproduced by £uid escape from Unit 3 (judging bythe depth to which this fault appears to penetrate)up the east-dipping fault at a place where the

Fig. 9. Pro¢le IM94-3 (migrated with water velocity) across the deformation front between the Sirte and Messina Abyssal Plains.The base of the evaporites (grey line) appears to mark the basal detachment, as above this level the section thickens (tectonically)to the west and below it thins (stratigraphically+compaction e¡ects) to the west. Top of Mesozoic carbonates (K) and top base-ment (O) both show considerable relief, interpreted as pre-convergence structure. A wedge-shaped unit (3b, base marked by astriped line) beneath the abyssal plain and the very front of the wedge is interpreted as a Pre-Messinian olistostrome that is beingsubducted. The location of the plate boundary detachment at the front of the wedge is somewhat unclear.

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evaporitic cap appears to be absent. Fluid expul-sion in front of the wedge toe is consistent withthe results of modelling of Westbrook et al. (per-sonal communication).The accretionary tectonics in general appear

very similar to those of Pro¢le 5A with the basaldetachment occurring at the base of the evapor-ites : above this the section thickens towards theeast, whereas beneath it Units 3 and 2 appear tothin (compaction and stratigraphy?) towards theeast. Over most of the pro¢le the wedge dips atless than 1‡ to the west, but the slope steepens tomore than 2‡ for the frontal 5 km (SPs 2750^2850). (Note, however, that these dips are notparallel to the convergence direction, nor perpen-dicular to the deformation front, and so can onlybe discussed in relative terms.) The steepening ofthe very front of the wedge may be related to thepartial subduction of the olistostrome, or mayre£ect changes in the mechanics of the detach-ment. For instance as the evaporites thin, dewa-tering in the vicinity of the deformation frontwould likely reduce overpressure and result inthe steepening of the frontal slope. We furthernote that near the deformation front itself, thebasal detachment cannot be clearly identi¢ed,and may lie just within the top of Unit 3b (theproposed olistostrome), the top of which appearsdisrupted (SPs 2500^2750), perhaps by thrustingabove the detachment (Fig. 9), although we can-not rule out the pull-up e¡ects of the overlyingrough sea£oor and M re£ector topography.

2.4. Messina Abyssal Plain(Pro¢les 0A, 0B and 1)

Three pro¢les (0A, 0B and 1) cross the defor-mation front in the Messina Abyssal Plain. Allthree show that the deformation front is marked

by a gradual onset (SPs 300^350 on Pro¢le 1, Fig.10) of folding in the Plio^Quaternary sequenceand the formation of ‘cobblestone’ topography.The folds nearest the deformation front have awavelength of around 2 km and an amplitude ofc. 100 m. The wavelength is longer than on theSirte Transect, possibly re£ecting the greaterthickness of the Plio^Quaternary section here(up to 300 ms on the Messina Abyssal Plain, thin-ning to c. 200 ms by SP 500, compared to a fairlyconstant 200 ms on the Sirte Abyssal Plain andfrontal slope on Pro¢le 5A, Figs. 3 and 4). Thefrontal slope is particularly gently dipping, beingabout 0.7‡ over the ¢rst 25 km, and £atteningthereafter, requiring that the e¡ective basal fric-tion be extremely low, probably indicating high£uid pressure at depth.The base of the Messinian is marked by a pro-

nounced negative polarity re£ection (B): the Mes-sinian is approximately 200 ms (c. 400 m) in theabyssal plain. Beneath the Messinian, Unit 3 ap-pears as a poorly re£ective zone just over 1 sthick. This is underlain by a series of stronglow-frequency re£ections, interpreted as comingfrom the carbonates of Unit 2. Again, the stra-tigraphy appears similar to that from the Sirtetransect, except that the Messinian here is farthinner.The top of the Messinian appears folded even

out in the Messina Abyssal Plain. This foldingmust predate the deposition of the Plio^Quater-nary sequence, and may indicate that the locus ofdeformation has migrated arcward during thePlio^Quaternary. Folding at base Messinian levelis far less pronounced than above, and movingunder the deformation front as far as SP 490,the B re£ection marks a boundary between east-ward-thickening sediments above and apparentlyundeformed strata (Units 3 and 2) beneath. Here

Fig. 10. Pro¢le IM94-1 near the deformation front. The Messinian evaporites are about 200 ms (400 m) thick at the left of thesection, but appear to thicken and to be folded beneath the abyssal plain before reaching the current deformation front. We inferthat the deformation front migrated arcward during the Pliocene. Beneath the frontal portion of the wedge, the Plio^Quaternaryis locally strongly folded and the Messinian appears to thicken arcward whereas the Pre-Messinian if anything thins, supportingthe idea that the detachment may lie at the base of the evaporites. However, several more deeply rooting structures appear todisrupt the base of the evaporites. These may be related to variations in the basement topography, although it is not clear if theyare cause or e¡ect. We suggest that the basal detachment cuts down to a Pre-Messinian level within the ¢rst 20 km of thewedge.

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it appears therefore that the base of the Messinianmarks a detachment level.At SP 490 however, the base of the Messinian is

o¡set (east side up) by an east-dipping thrustfault. This corresponds to a change in the defor-mation of the top Messinian and the Plio^Quater-nary: east of SP 490, the folding (best developedin the M re£ection) is far less pronounced than tothe west. East of SP 550, folding becomes againmore pronounced. Here the base Messinian ap-pears o¡set down to the east: between SPs 490and 550 there appears to be a ‘pop-up’ structure.We interpret this pattern as representing a com-plex mixed mode of deformation: where the fold-ing of the Plio^Quaternary is tight, it accom-modated ductile shortening of the Messinianevaporites; where folding is less intense, the evap-orites have themselves been faulted by a deepercutting east-dipping thrust system. However, thegeneral £atness of the wedge implies that the porepressure has been maintained at depth, so thatlittle £uid has escaped: the o¡set is less than thethickness of the evaporites.East of SP 550, the base of the Messinian rap-

idly becomes obscured. Although the pattern ofthe deeper strata is somewhat confused, it appearsthat the carbonate sequence is disrupted by fault-ing, consistent with an interpretation that thebasal detachment has cut down to deeper levels.although the image is not clear enough to deter-mine to which stratigraphic level.

3. Discussion ^ wedge mechanics

3.1. E¡ect of evaporite stratigraphy and thickness

Both the depositional thickness and type ofevaporites are strongly in£uenced by the deposi-tional environment within the evaporite basin.The style of accretion near the deformation frontvaries along strike, and appears to be dependenton the thickness of the evaporite sequence, andhence on their stratigraphy. In the Sirte AbyssalPlain, the evaporites are over 1 km thick: therethe base of the Messinian acts as the basal detach-ment for perhaps the ¢rst 50 km of the wedge,except where disrupted by the Bannock structure.

In the Messina Abyssal Plain (Fig. 10), the evap-orites are only a few hundred metres thick: onPro¢le 1 the base of the Messinian appears toonly locally act as a detachment, and deformationrapidly involves deeper strata. Here the apparentslope angle is the lowest of any of the cases con-sidered: this low angle is particularly surprising aswedge taper should increase with convergence ob-liquity (Platt, 1993, equation 75). We suspect thatone reason for the low slope angle is the deposi-tion of deep-water Plio^Quaternary sediments inthe Abyssal Plain: as noted above, Unit 5 thick-ens westward o¡ the Ridge’s frontal slope andsmooths out M re£ection topography. In the in-tervening region (Pro¢les 3 and 4), the deforma-tional style appears similar to that along Pro¢le5A (e.g. Fig. 9), at least beyond the very frontalportion of the wedge where an olistostrome ap-pears to have entered the system and the evapo-rites are very thin.

3.2. The mechanics of the detachment

As discussed above, the basal detachment, atleast for Pro¢les 5 and 3, appears to lie at thebase of the evaporites, and on Pro¢le 1 to at leastpartially follow this boundary but locally to bedeeper. This seems at ¢rst somewhat unexpected,as evaporites are known to be relatively weak andcommonly to form detachment horizons; the lowslope angle of the wedge has been proposed byKastens et al. (1992) to be a result of a de¤colle-ment in a weak salt layer. In this section, we dis-cuss under what conditions the clastics directlybeneath the evaporites may be weaker than theevaporites themselves and why the western Med-iterranean Ridge accretionary complex is so un-usually £at. We start by using the Coulombwedge model of Davis et al. (1983) to constrainlikely £uid pressures at the base of the evaporites,and hence the basal shear stress. We con¢rm theseresults for the plastic wedge model of Chapple(1978), which might perhaps be more appropriatefor the evaporite-dominated MediterraneanRidge. We then discuss the relative strength ofa basal detachment beneath the evaporites andone within the evaporites and suggest that, givenhigh £uid pressures beneath the evaporites, the

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basal detachment is likely to lie beneath the Mes-sinian.

3.3. Accretionary wedges and Coulomb theory

Generally, an accretionary wedge can be con-sidered as a ‘critical’ Coulomb wedge (Davis etal., 1983), the geometry of which (de¢ned by thesurface slope K and basal slope L) is related pri-marily to the strength of the material, the stressdirection and the shear strength of the basal de-tachment. For the submarine case the taper (K+L)in radians is given by Davis et al. (1983) as:

K þ L ¼ W bð13V bÞ þ L ð13bw=b Þð13bw=b Þ þ Kð13V Þ ð1Þ

where Wb is the coe⁄cient of basal friction, b isthe sediment wedge density, bw that of water, andK is a dimensionless quantity (usually 2^4) depen-dent on the orientation of the principal compres-sive stress c1 relative to the fault plane. Near thetoe of the wedge, c1 is approximately horizontaland K is constant, but as the wedge thickens c1

becomes inclined. The £uid pressure ratio V isde¢ned as:

V ¼ Pf3bwghc z3bwgh

ð2Þ

where Pf is the £uid pressure, h the wedge thick-ness, and cz is the vertical normal stress. Vb is the£uid pressure ratio at the basal detachment itself(Vb = 1 is lithostatic pressure, Vb = 0 is hydrostaticpressure).Eq. 1 shows that the angle of a Coulomb wedge

will shallow if the basal friction coe⁄cient is low,or if the £uid pressure at the base is high (bothresult in a low e¡ective basal friction). Althoughthe rheology of evaporites is probably not Cou-lomb, but rather visco-plastic (see below), theCoulomb wedge model does provide some insightinto the structure of the Mediterranean Ridge.The extremely low slope angle (K) may be an in-dication that £uid pressures at the detachment areclose to lithostatic. Because the evaporites act as aseal, such high pore pressures may be expectedwhere the detachment lies within or at the base

of the Messinian evaporites, but are less likely todevelop if the detachment was at the top of theevaporites.Of the pro¢les discussed in this paper, only

Pro¢le 5A parallels the local convergence direc-tion (shown by the trail of the Bannock structureand in agreement with geodetic results from LePichon et al., 1995) and is una¡ected by localanomalies such as seamounts. We thus apply theCoulomb wedge analysis to this pro¢le to deter-mine the degree of £uid overpressure at the basaldetachment. On this pro¢le, the average slope an-gle over the frontal 50 km of the wedge (up to SP5400) is 0.9‡. On depth sections, the average slopeof the basal detachment over this range is alsoextremely low (about 1.1‡), although its exact val-ue is less important when £uid pressure is high(Dahlen, 1984). Assuming reasonable values forbasal friction (0.6^0.85) and for K (2^4, see Dah-len, 1984), it is possible to deduce the £uid pres-sure ratio at the detachment. We ¢nd that Vb isgreater than 0.98, and substituting back into Eq. 2that £uid pressure is c. 99% of lithostatic.We can use the expression relating basal shear

stress db to V to calculate shear stress at variouspoints along the detachment (after Davis et al.,1983):

d b ¼ ðb3bwÞghK þ ð13V ÞKbghðK þ L Þ ð3Þ

Beneath the frontal slope of pro¢le 5A, the de-tachment cuts down from about 1.3 km (SP 6380)to about 3.0 km (SP 5400) beneath the sea£oor,implying that db increases from about 0.3 MPa atthe toe of the wedge to about 0.7 MPa at thedetachment beneath SP 5400.

3.4. A plastic wedge?

It might be argued that evaporites (especiallysalt) are plastic (e.g. Spiers et al., 1990) and sothat the Mediterranean Ridge would be better ap-proximated by a plastic rather than a Coulombwedge. If so, the above analysis of £uid overpres-sure and wedge mechanics might be £awed. Theproperties of a plastic wedge were analysed byChapple (1978) and subsequently generalised byPlatt (1990, 1993). Platt (1990) showed that for

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a submarine wedge, the frontal slope K and thedetachment slope L (both in radians) can be re-lated for low angles according to the formula:

d b ¼ ðb3bwÞghK þ 2c cðK þ L Þ ð4Þ

where h is the depth to the detachment, cc is theyield stress of the material in the wedge and db isthe yield stress of the detachment. Consideringagain the shallowest and deepest portions of thedetachment along Pro¢le 5A (near the deforma-tion front and at SP 5400), and taking K=1‡ andL=1.1‡, we can solve for db : depending on theexact value of cc (discussed below), db is probablybetween 0.5 and 1 MPa. From this and Eq. 3 wecan work out the amount of overpressure needed(Table 1) to reduce the basal yield stress db (as-suming frictional behaviour for the basal detach-ment beneath the evaporites) to such an amount,as after Davis et al. (1983):

13V b ¼d b

W bghcos Lð5Þ

For a plastic wedge model, we also ¢nd (Table1) that the basal detachment must be somewhatweaker than the material in the wedge to explainthe low slope angle. For reasonable values of cc(as discussed below for a largely evaporite wedge,these may be about 5 MPa or less), we ¢nd that dbincreases from about 0.6 MPa near the deforma-tion front to about 1.0 MPa by SP 5400. Thesevalues for db are comparable to the results ob-tained for a Coulomb model. They are far lowerthan lithostatic and for reasonable values of W

(0.75 used) require that V be about 0.97^0.99.

This implies that in the case of the MediterraneanRidge, £uid pressures exceed 98% of lithostatic,much the same as for a Coulomb wedge.

3.5. Comparison with the yield strength of salt

Our interpretation from the seismic data thatthe basal detachment developed at the base ofthe evaporites, rather than within them as previ-ously suggested (e.g. Kastens et al., 1992) mayseem surprising given the generally perceivedweakness of salt. In the above section we havederived values for the basal shear stress db atthe detachment for both Coulomb and plasticwedge models, assuming that the detachment isin the overpressured clastics beneath the imperme-able evaporites. The low taper of the Ridge ine¡ect requires (Eq. 4) that the basal shear stressis considerably smaller than the yield stress withinthe wedge, implying that the evaporites must beconsiderably stronger than the basal detachment.In this section we ¢rst use the known rheology ofhalite to estimate the yield stress within the wedgeand thus constrain the yield stress and pressurecoe⁄cient at the basal detachment (Table 1). Wethen consider the possibility of a detachmentwithin the lower evaporites and show that giventhe strength of the wedge this remains less likelythan one within the underlying overpressuredclastics. We concentrate our analysis on the toeof the wedge where the basal detachment ¢rstdevelops.Even at higher temperatures than likely for

the deep detachment, dry salt undergoing shearor compression has a yield stress of about 5 or

Table 1Values of basal yield stress db and of pressure coe⁄cient V for di¡erent locations (at the deformation front and at SP 5400 onPro¢le 5A, Fig. 4) and for di¡erent values of cc, the yield stress within the wedge

Internal yield stress Deformation front SP 5400

cc = 0.5 db = 0.29; V=0.985 db = 0.66; V=0.99cc = 1 db = 0.32; V=0.98 db = 0.70; V=0.985cc = 2 db = 0.40; V=0.98 db = 0.775; V=0.98ccc = 5 ddb = 0.62; VV= 0.97 ddb = 0.99; VV= 0.98cc = 10 db = 0.98; V=0.95 db = 1.36; V=0.97cc = 20 db = 1.72; V=0.91 db = 2.09; V=0.95cc = 100 db = 7.6; V=0.6 db = 7.95; V=0.83

For realistic estimates of the strength of the wedge (cc estimated at 5 MPa, see text), the required low basal shear stress(db = 0.6^1 MPa) requires extremely high £uid pressures (Vv 0.97).

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6 MPa (Franssen and Spiers, 1990) at a widerange of di¡erent strain rates. A value of 5 MPafor the internal yield stress of the wedge impliesthat the shear stress at the basal detachment isless than 1 MPa (Table 1), requiring £uid pressurewithin 2% of lithostatic (Eq. 2). The yield stress ofwet salt is considerably lower (Spiers et al., 1990)due to the possibility of deformation by pressuresolution, but is strain rate- and temperature-de-pendent. Thus to assess the possible in£uence ofwet salt on wedge rheology and evolution, weneed to ¢rst constrain the temperature and strainrate near the wedge toe where the basal detach-ment develops.There is no information on the temperature at

the level of the basal detachment beneath thewestern Mediterranean Ridge. However, the basaldetachment of the Barbados prism has beendrilled and studied in some detail (e.g. Shipley etal., 1995). Here, although deeper beneath thewedge it may reach 70‡C or more (Fisher andHounslow, 1990), the temperature at the basaldetachment near the deformation front is about35^40‡C. This elevated temperature probably rep-resents the transient passage of £uids along thede¤collement: transient £uid £ow rates there mayreach 1037 m/s, although the steady-state £owpredicted by numerical modelling of wedge dew-atering is c. 1039 m/s (Screaton et al., 1990). Thisis an order of magnitude greater than £ow rates insteady-state models (less than 10310 m/s) obtainedby Westbrook et al. (personal communication) forrealistic models for the Mediterranean Ridge.Although no information on transient £uid £owrates exists for the study area, we suggest thatthese are also likely to be considerably lowerthan for the Barbados wedge. Given also thehigh thermal conductivity of the evaporites andthe low heat production from the overridden Ju-rassic oceanic crust (probable), it seems likely thattemperatures beneath the basal detachment arelower than 35‡C near the deformation front ofthe Ridge. Temperatures near the deformationfront within the wedge itself are likely to be lowerand may average c. 20‡C.The strain rate within the wedge near the de-

formation front (where the basal detachment de-velops) can be estimated from the rate at which

the wedge has developed if certain simplifying as-sumptions (e.g. constant thickness of incomingevaporitic section) are made. The cross-sectionalarea of the front 50 km of the wedge (comprisingdominantly Messinian evaporites and hence theregion of interest here) along Pro¢le 5A is 95km2. From this the amount of shortening in thefrontal 50 km of the wedge since the Messiniancan be estimated to be about 45 km. This is aminimum estimate as it ignores the possibility ofevaporite loss through dissolution: at current con-vergence rates (c. 30 mm/yr) between Africa andthe Aegean region the accretion of a 1-km-thickevaporite layer should have generated a wedge ofcross-sectional area c. 160 km2. By consideringthe growth of the Messinian frontal portion ofthe wedge as a self-similar process (e.g. West-brook, 1994), it is possible to deduce the growthrate of the wedge: in the case of the Messinianwedge that dominates the frontal portion of theMediterranean Ridge, the low porosity of the in-coming evaporites (halite, anhydrite) allows thesimplifying assumption that there has been nocompaction due to porosity loss. Strain rate justbeneath the toe of the wedge where the basal de-tachment develops is naturally higher than furtherback in the wedge and can be calculated fromincremental natural strain. Near the toe of thewedge this is c. 0.3 for an interval of 50 000 years,giving a strain rate of just over 10314 s31.At this strain rate, wet salt deforms by pressure

solution with a yield stress of c. 0.7 MPa at 20‡C,which as discussed above is a reasonable averagetemperature within the wedge (Spiers et al., 1990).This is our minimum estimate of the internal yieldstrength of the evaporites within the frontal por-tion of the wedge as it assumes the weakest pos-sible rheology (wet salt) at a minimum estimateof the strain rate (no loss of material from thewedge through dissolution), and would requirean even lower yield stress at the basal detachment(c. 0.3 MPa and £uid pressures within 1% oflithostatic ^ see Table 1). If the temperature with-in the frontal portion of the wedge was as high as50‡C (which we consider too high) the yield stresswould be c. 0.35 MPa, requiring still lower basalshear stress. However, it seems unlikely that a wethalite rheology would be appropriate for the Mes-

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sinian evaporites as a whole for two reasons.First, these contain large proportions of otherminerals, such as anhydrite which are somewhatstronger. Second, evaporites (particularly halite)have a very low permeability and so are unlikelyto be generally ‘wet’. We thus conclude that theyield strength within the wedge is probably some-what higher than these values and may approach5 MPa.The above analysis has considered the rheology

of the whole thickness of the wedge for the strainrates appropriate near the deformation front andhas not considered the possibility that a thin layerof halite in contact with water-saturated clasticsor other sediments (and hence wet) might form adetachment within the lower evaporites (Chaumil-lon et al., 1996). For a narrow detachment zone(100 m thick) between the subducting AfricanPlate and the shortening wedge, the strain rateis close to 10311 s31. At this strain rate, wet salthas a yield stress of about 6 MPa (at 50‡C), anddeformation is by cross-slip-controlled creep(Spiers et al., 1990). Thickening the detachmentto perhaps 500 m could reduce the strain ratesomewhat to about 10312 s31, for which the yieldstress at 50‡C is about 4.5 MPa (Spiers et al.,1990), although it is less likely that the thickerzone would be dominantly halite and also lesslikely that it would be wet. A thinner detachmentwould in contrast require higher deformationrates and hence higher yield stresses; lower, per-haps more realistic temperatures would also re-quire higher yield stresses. The basal yield stressof a wet halite de¤collement is thus considerablygreater than for very overpressured clastics (Table1). We thus conclude that it is also unlikely that ade¤collement within a halite layer, even if this waswet, would form the main detachment.The above discussion illustrates that overpres-

sured clastics (pore pressure within 1 or 2% oflithostatic over the ¢rst 50 km of the wedge) be-neath the evaporites are, at temperatures andstrain rates appropriate for the MediterraneanRidge, likely to be considerably weaker than theoverlying evaporites. The overpressured clasticswould have yield stresses of 0.3^0.7 MPa in aCoulomb model and 0.6^1.0 MPa in a plasticmodel for the overlying wedge, whereas the evap-

orites will have yield stresses for realistic modelsin excess of 4 MPa. This analysis supports seismicevidence that the basal detachment lies at the baseof the evaporites. We suggest that as long as thepore pressure is within a few per cent of litho-static, the basal detachment is likely to form be-neath the evaporites.

3.6. The e¡ect of dewatering

A remaining question is what is likely to hap-pen if for some reason the pore pressure directlybeneath the evaporites were to drop beneath a fewper cent of lithostatic. This might occur near sitesof £uid escape, such as for instance the Bannockstructure where the evaporites have been breachedor where the evaporites are locally too thin toprovide an e¡ective seal. In this case, the strengthof the zone at the base of the evaporites wouldincrease: if this exceeded the shear strength eitherhigher or lower in the section, we might expect anew detachment to form where the shear strengthwas lowest, for instance within impermeableshales where an evaporitic seal is not required tomaintain high pore pressures.Escape of £uids up the £anks of the subducting

Bannock seamount may have had two e¡ects :¢rst, they appear to have removed some of themore soluble (and coincidentally weaker) unitswithin the evaporites such as halite, and second,they may have reduced the pore pressure beneaththe evaporites enough to a¡ect the accretionarytectonics. We note that near the current deforma-tion front, the mean slope angle on Pro¢le 16 is2.5‡. Assuming the same L as on Pro¢le 5A (wedo not image the detachment clearly on pro¢le16), we can deduce that in a Coulomb modelbasal shear stress may be about 0.76 MPa and V

0.96. The basal shear stress near the deformationfront for Pro¢le 16 is thus more than double thatfor Pro¢le 5A where dewatering has not occurred.In a plastic model, again assuming that L is as forPro¢le 5A, we ¢nd that for Pro¢le 16 (assumingcc = 5 MPa), the basal shear stress may be about1.2 MPa (again about double its value on Pro¢le5A), and V about 0.94, requiring signi¢cant loss of£uid pressure behind the Bannock structure.If the shear strength of the clastics at the base

MARGO 3070 9-7-02


of the evaporites increases to values greater thanthat of either deeper levels or weak levels withinthe evaporites, the main detachment may move toa di¡erent level. For instance, it is possible that inthe region of the Bannock structure, dewateringhas strengthened the top of Unit 3, so that de-tachments develop within the evaporites and/or atdepth. Where already shortened and thickenedwithin the wedge, the salt within the evaporitesmay have acted as a local detachment, promotingthe £ow of the upper evaporites and the Plio^Quaternary around the seamount. However,nearer the current deformation front, salt thick-nesses may be too low and a deeper detachmentmay form, perhaps in shales near the top of thecarbonates (Unit 2). The result would be a com-plex mixed mode of deformation behind the Ban-nock structure.

4. Conclusions

Whether the Mediterranean Ridge accretionarywedge is modelled as a Coulomb or plastic wedge,the location of the basal detachment at the baseof the evaporites on Pro¢le 5A is due to £uidoverpressuring beneath low-permeability Messi-nian evaporites. The low slope angle of the wedgeis consistent with an extremely low basal friction(i.e. coupling across the detachment), which canonly be explained by £uid pressures within 2% oflithostatic at the level of the detachment. The re-quired basal shear stress is less than the strengthof the evaporites under appropriate conditions oftemperature and strain rate.The accretionary tectonics within the IMERSE

data set vary depending on conditions of the sub-ducting section. On Pro¢les 6 and 16, the subduc-tion of basement highs has a¡ected the accretion-ary style, perhaps by providing an outlet for £uidsand hence allowing £uid pressure to drop. OnPro¢le 1 (Messina Abyssal Plain transect), theevaporites are far thinner and may have beenless e¡ective in preventing the escape of £uids.Here it seems that the basal detachment may besomewhat deeper than the base of the Messinian.In the region between the Messina and SirteAbyssal Plains, it seems that the proximity of

the African margin has a¡ected the accretionarytectonics of the edge as a large olistostrome hasentered the system and caused a local steepeningof the frontal slope. Here, the local absence ofevaporites in the foreland may have allowed theescape of £uids from the wedge through this olis-tostrome to the surface.

Acknowledgements

This work was supported by the EuropeanUnion MAST Programme under ContractMAS2-CT93-0062 and by the Deutsche For-schungsgemeinschaft under Grant Hu 470/4. Thedata acquisition was helped by the skill and pro-fessionalism of the crews of the OGS Exploraunder Captain Bonetti and the RV Meteor underCaptain Mu«ller. Contributions of our partners inthe IMERSE project helped develop the conceptspresented here, as did comments by Marc-Andre¤Gutscher. Reviewers John Platt and Eli Silverraised some interesting questions which helpedtighten the manuscript considerably. Pro¢le 1was originally processed by colleagues at ENS,Paris ; Pro¢le 3 by colleagues at DEP-EKY,Athens; all pro¢les presented here were processedor reprocessed at GEOMAR.

References

Camerlenghi, A., McCoy, F., 1990. Physiography and struc-ture of Bacino Bannock (Eastern Mediterranean). Geo-Mar.Lett. 10, 23^30.

Chapple, W., 1978. Mechanics of thin-skinned fold-and-thrustbelts. Geol. Soc. Am. Bull. 89, 1189^1198.

Chaumillon, E., Mascle, J., 1997. From foreland to forearcdomains: new multichannel seismic re£ection survey of theMediterranean Ridge Accretionary Complex (Eastern Med-iterranean). Mar. Geol. 138, 237^259.

Chaumillon, E., Mascle, J., Ho¡mann, H.-J., 1996. Deforma-tion of the western Mediterranean Ridge: Importance ofMessinian evaporitic formations. Tectonophysics 263, 163^190.

Clauzon, G., Suc, J.-P., Gautier, F., Berger, A., Loutre, M.F.,1996. Alternate interpretation of the Messinian Salinity Cri-sis : Controversy resolved? Geology 24, 363^366.

Dahlen, F.A., 1984. Noncohesive critical Coulomb wedges: Anexact solution. J. Geophys. Res. 89, 10125^10133.

Davis, D., Suppe, J., Dahlen, F.A., 1983. Mechanics of fold

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and thrust belts and accretionary wedges. J. Geophys. Res.88, 1153^1172.

De Voogd, B., Tru¡ert, C., Chamot-Rooke, N., Huchon, P.,Lallemant, S., Le Pichon, X., 1992. Two-ship seismic sound-ings in the basins of the Eastern Mediterranean Sea (Pasi-phae cruise). Geophys. J. Int. 109, 536^552.

Finetti, I., 1976. Mediterranean ridge: A young submergedchain associated with the Hellenic Arc. Boll. Geophys.Teor. Appl. 14, 31^65.

Fisher, A.T., Hounslow, M., 1990. Transient £uid £owthrough the toe of the Barbados Accretionary Complex:Constraints from Ocean Drilling Program Leg 110 heat£ow studies and simple models. J. Geophys. Res. 95,8845^8858.

Franssen, R., Spiers, C., 1990. Deformation of polycrystallinesalt in compression and shear at 250^350‡C. Geol. Soc.London Spec. Publ. 54, 201^214.

Kastens, K., Breen, N.A., Cita, M.B., 1992. Progressive defor-mation of an evaporite-bearing accretionary complex: SeaMarc1, sea beam and Piston-core observations from theMediterranean Ridge. Mar. Geophys. Res. 14, 249^298.

Le Meur, D., 1994. Etudes Sismiques de la Ride Me¤diterra-ne¤ene. DEA Thesis, ENS Paris VI^Paris XI, 52 pp.

Le Pichon, X., Chamot-Rooke, N., Lallemant, S., 1995. Geo-detic determination of the kinematics of central Greece withrespect to Europe: Implications for Eastern Mediterraneantectonics. J. Geophys. Res. 100, 12675^12690.

MacKay, M., 1995. Structural variation and landward ver-gence at the toe of the Oregon accretionary prism. Tectonics14, 1309^1320.

Moore, J.C., Vrolijk, P., 1992. Fluids in accretionary prisms.Rev. Geophys. 30, 113^115.

Platt, J., 1990. Thrust mechanics in highly overpressured accre-tionary wedges. J. Geophys. Res. 95, 9025^9034.

Platt, J., 1993. Mechanics of oblique convergence. J. Geophys.Res. 98, 16239^16256.

Polonia, A., Camerlenghi, A., Davey, F., Storti, F., 2002. Ac-cretionary processes, structural style and syncontractionalsedimentation in the Eastern Mediterranean Sea. Mar.Geol., this volume.

Reston, T.J., Fruehn, J., von Huene, R., IMERSE WorkingGroup, 2002. The structure and evolution of the WesternMediterranean Ridge. Mar. Geol. S0025-3227(02)00174-3

Ryan, W.B.F., Hsu, K. et al., 1973. Init. Rep. DSDP. 13.Ryan, W.B.F., Kastens, K.A., Cita, M.B., 1982. Geologicalevidence concerning compressional tectonics in the EastMediterranean. Tectonophysics 86, 213^242.

Screaton, E.J., Wuthrich, D.R., Dreiss, S.J., 1990. Permeabil-ities £uid pressures and £ow rates in the Barbados RidgeComplex. J. Geophys. Res. 95, 8997^9007.

Shipley, T.H., Ogawa, Y., Blum, P. et al., 1995. Proc. ODPInit. Rep. 156.

Spiers, C., Schutjens, P., Brzesowsky, R., Peach, C., Liezen-berg, J., Zwart, H., 1990. Experimental determination ofconstitutive parameters governing creep of rocksalt by pres-sure solution. Geol. Soc. London Spec. Publ. 54, 215^228.

Tru¡ert, C., Chamot-Rooke, N., Lallemant, S., De Voogd, B.,Huchon, P., Le Pichon, X., 1993. The crust of the WestMediterranean Ridge from deep seismic data and gravitymodelling. Geophys. J. Int. 114, 360^372.

von Huene, R., Reston, T., Kukowski, N., Dehghani, A., Fa-bel, E., IMERSE Working Group, 1997. A transverse scarfrom a subducting seamount beneath the MediterraneanRidge. Tectonophysics 271, 249^261.

Westbrook, G.K., 1994. Growth of accretionary wedges o¡Vancouver Island and Oregon. In: Westbrook, G.K., Car-son, B., Musgrave, R.J. et al. (Eds.), Proc. ODP Init. Rep.146 (Part 1), 381^388.

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