Geo-Marine Letters (1998) 18 : 81—89 ( Springer-Verlag 1998

J. Mascle · E. Chaumillon

An overview of Mediterranean Ridge collisional accretionary complexas deduced from multichannel seismic data

Abstract The Mediterranean Ridge (eastern Mediterra-nean) is a large accretionary complex that results from theAfrica—Europe—Aegean plates convergence. Multichannelseismic data, combined with previous results showed thatthe ridge comprises distinct major structural domainsfacing different forelands: (1) An outer domain is boundedto the south by the ridge toe. Underneath the Ionian andLevantine outer Ridge, Messinian evaporites act as a ma-jor decollement level. (2) An axial, or crestal, ridge domainwith mud diapiric and mud volcano activity is bounded tothe north by backthrust. (3) A less tectonized inner Ridgedomain, possibly a series of former forearc basins, abutsthe Hellenic Trench. The ridge displays strong along-strike variations. These variations can be interpreted asconsequences of an ongoing collision against the Libyancontinental promontory.

Introduction

In the eastern Mediterranean, the Mediterranean Ridge(MR) (Fig. 1) forms a huge accretionary complex createdby the subduction of African lithosphere beneath Europeand in particular beneath the Aegean microplate (Le Pi-chon et al. 1982; Ryan et al. 1982). Intense shallow defor-mation and the presence, at depth, of Messinian evaporiticsequences have, however, strongly hindered the study ofits internal structure. In this paper we briefly present anddiscuss a recent set of multichannel seismic reflection data(Mascle et al. 1994) that illustrate the variable structuralstyles that characterize the MR both in cross section andalong its strike.

Today, the MR forms a 1300-km-long and 150- to300-km-wide curved feature extending over much of the

J. Mascle ( ) ) E. Chaumillon1Geosciences Azur, Observatoire Oceanologique de Villefranche,B.P. 48, 06235 Villefranche-sur-Mer Cedex, France

Present address:1University of La Roc Relle, Earth Sciences Department

deep basin from the Ionian to the Levantine seas (Fig. 1).Northward, the MR is bounded by the Hellenic Trenchsystem; southward, it faces three different forelands(Chaumillon and Mascle 1995): the oceanic Ionian abyssalplain, the Libyan continental margin promontory, and theHerodotus abyssal plain. The latter believed to be under-lain by a remnant of Mesozoic oceanic crust (De Voogdet al. 1992).

On the average the MR is expressed by gentle south-dipping slopes (1°—2°) and exhibits intense shallowdeformation, well imagined on GLORIA surveys (Beldersonet al. 1978; Kenyon et al. 1982; Stride et al. 1977), onnear-bottom side-scan sonar data (Kastens et al. 1992;Limonov et al. 1994), and on swath bathymetry (LePichon et al. 1979, 1982; Foucher et al. 1993). This mor-phology is interpreted as caused by compressive deforma-tions, gravitational gliding, clay diapirism, and/or mudvolcanism. These mud volcanoes yielded brecciated shaleclasts (Camerlenghi et al. 1992), suggesting that a potentialdecollement exists at depth, potentially within Early Cre-taceous shales (Ryan et al. 1982). According to Kastens(1991), sedimentological and detailed morphological datasuggest an increase of the ridge growth rate during andafter Messinian times; they also tend to indicate that thetectonics seen along the external domain are consistentwith a shallow decollement level located near the top ofthe Messinian evaporite-rich layers (Kastens et al. 1992).

New data

Among the 3600 km of multichannel continuous seismic(MCS) reflection profiles recently recorded (1993) acrossthe eastern Mediterranean (Fig. 1) (Mascle et al. 1994),four lines (PM 03, 06, 19, and 30), provide complete crosssections across the MR, from its foreland to its forearcdomains, respectively. These lines have formed the basis ofrecent interpretation and discussion of the structural char-acteristics of the MR, which can be divided along its strike

Fig. 1 Bathymetric map of central eastern Mediterranean (fromUNESCO; contour interval 200 m). Multichannel seismic reflectionlines from the Prismed survey (Mascle et al. 1994) are indicated.Lines PM 03, 06, 19, and 30 are shown on inset of Fig. 3

into three distinct domains: a western Ionian branch,a Central Libyan domain, and an eastern Levantinebranch (Chaumillon et al. 1996; Chaumillon and Mascle,1997).

An important result from the MCS data set concernsthe structural variations seen on cross section along theridge. Following Truffert et al. (1993), we distinguish,based on the deformation style, outer, axial (or crestal),and inner MR domains. A second important result, de-rived from these data, concerns the identification of a shal-low decollement level, well identified beneath both theIonian and Levantine outer MR. The good lateral conti-nuity observed between characteristic reflectors of lowerMessinian evaporites (B reflector of Finetti 1976) and theseismic sequences imaged near the base of the deformedwedge, are well illustrated on MCS data (Fig. 2) (Mascleet al. 1994; Chaumillon 1995; Chaumillon et al. 1996).

Moreover, we feel that the deformational styles and thevery gentle average slopes detected over most of the outerMR provide good evidence for low internal and basalfriction within the accreted complex (Chaumillon et al.1996). Both seismic and morphological observations arethus in good agreement with the mechanical properties ofthe evaporite layers, acting as a decollement level withinan accreted wedge (Woidt 1978).

Main structural domains of the Mediterranean ridge

Figure 3 illustrates four interpreted cross sections of theentire MR derived from MCS lines PM 03, 06 19, and 30.These tracings allow the discussion of the structural char-acteristics of the MR in cross section and along-strike.

Outer MR

From west to east, the contact between the MR and itssouthern foreland shows strong lateral variations, clearly

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Fig. 2 Section and interpretation of migrated line PM 03 across the contact between the Sirte (Ionian) abyssal plain and the present-day MRfront of deformation

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expressed by different structural styles. Facing the Ionianabyssal plain, the contact is underlain by subdued andnarrow anticlines (a few km wide) and wider synclines(Fig. 2) (Chaumillon et al. 1996).

Northeast of this contact, the Plio-Quaternary cover isaffected by small wavelength folds and associated reversefaults, indicating active shortening processes. This de-formed recent cover rests on an almost reflection-free

Fig. 4 Example of migrated section (line PM 06) across the contactbetween the Libyan continental margin and the MR front of defor-mation; this area is relatively narrow ((10 km) and is affected bysoutward-verging thrust sheets

seismic sequence that progressively thickens northeast-ward and is bounded, at depth, by a subhorizontal, dis-continuous and strong reflector resting on a well-layeredand subhorizontal seismic sequence. This last sequencecan be correlated laterally with reflectors attributed topre-Messinian units recorded beneath the Ionian andSirte abyssal plain (Finetti 1976; Mascle et al. 1994).Acoustic similarities between the abyssal plain evaporiticlayers and the base of the MR deformed wedge are con-sidered, together with good lateral continuity and vel-ocities deduced from pre-stack depth migration, as strongarguments for the southwestern-most 70 km of the ridge

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(Fig. 3) being mainly made up of thick accreted Messinianevaporitic sediments (Chaumillon and Mascle 1995;Chaumillon et al. 1996). Moreover, we believe that10°—20° northward-dipping reflectors, detected within thiswedge, indicate thrust zones that have produced thicken-ing of the sedimentary pile entering the accreted complex(Mascle et al. 1994).

Towards Libya, the outer MR is characterized chieflyby the lack of typical evaporite seismic facies, steepertopographic slopes, and evidence of gravity gliding. Onseismic data, imbricated thrust sheets can be detectedclose to the deformation front (Fig. 4). In our interpreta-tion, such a distinctive structural pattern is a direct conse-quence of changes in Messinian facies and thickness(Chaumillon et al. 1996). Eastward, the outer MR showsan obvious increase in fold wavelength and fault spacing(Fig. 3) that clearly correlates with the drastic thickeningof the Plio-Quaternary cover, itself directly related to theNile deep-sea fan (Chaumillon et al. 1996).

MR axial and inner domains

The MR axial domain, from where the occurrences of muddiapirs and volcanoes have been briefly reported (Cita

Fig. 5 Section of line PM 19 across the axial MR, showing theToronto mud volcano area

et al. 1981; Ryan et al. 1982; Camerlenghi et al. 1992;Limonov et al. 1994; Robertson et al. 1996), may bedivided into several subregions, even if the transitionbetween the outer and axial MR appears almost relativelysimilar everywhere. This transition is marked by sharpslope breaks, increased shallow deformation, progressiveincrease of shale diapirs and mud volcanoes, and thepresence, at depth, of discontinuous and strong reflectors.North of Cirenaica, the axial MR is characterized asa wide ('100 km), and almost flat area (Fig. 3). This areahas an average water depth of 2000 m, but rises locally upto 1300 m (near 22°55@E and 33°50@N; Fig. 1).

On interpreted cross sections (Fig. 3), the western andeastern axial MR show contrasting morphologies andshallow structures. For example, southwest of Crete,line PM 06 (Fig. 3) illustrates the presence of two almostflat regions, bounded northwestward by steep (10°)slopes with associated internal southward dipping reflec-tors. We interpret these features as surface expressions ofmajor structural boundaries, such as backthrust or trans-pressive fault zones between the axial and inner MRdomains.

In contrast lines PM 17 and PM 19 (Fig. 3) exhibitrather different morphostructures, locally affected bydoming (Fig. 5), which we correlate with a mud diapiricfield (Toronto volcano) recently surveyed and drilled(Limonov et al. 1994; Robertson et al. 1996).

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In general, we consider the structural characters of theaxial MR as indicative of increased shortening.

Discussion

In cross section, the Ionian, Libyan, and Levantine MRbranches show an overall comparable S—N succession ofstructural units. Each branch includes an outer zone,a crestal domain, and an inner zone, which marks thetransition with the Hellenic Trenches.

As indicated above, the transition between the axialand inner MR domains seems to correlate with majorbackthrust zones (Lallemant et al. 1994; Chaumillon1995). This, combined with the detection at depth of highseismic velocities, has been used as an argument for defin-ing the inner domain as a potential continental backstop(Truffert et al. 1993; Lallemant et al. 1994). The MCSresults briefly discussed here cannot help to demonstratethat this area lies above a stretched continental crust;however, they support evidence of backthrusting and ofgravitational gliding towards the northern borderingtrench areas, which clearly substantiate recent tectonicactivity.

Within the northern bordering Matapan and Plinytrenches, the seismic data illustrate a few images ofwrench-related deformations (Pedersen 1995) that indi-cate probable active strike—slip motion, as also supportedby swath bathymetry data (Huchon et al. 1982). This,together with the important shallow seismicity, makes itdifficult for us to interpret the trench domains as a seriesof forearc basins or as a simple area of continental back-stop.

Despite similar structural characteristics, major differ-ences exist between the MR western, central, and easternbranches. For example, the MR outer domain showsobvious variations from W to E, such as the average widthof regional topographic slopes, the fold wavelength, thedip and fault spacing, and the presence of thrusts. Thesecharacteristics have been related to differences in the na-ture and in the thickness of incoming sediments (presenceor absence of Messinian evaporites), to variations in re-gional kinematics (frontal versus oblique convergence),and to the progress of ongoing collision with the Africanpassive margin, chiefly off Libya (Chaumillon and Mascle1995).

The MR thrust-bounded crestal domain also displaysimportant lateral variations. Southwest of Peloponnesus,it is a narrow ridge (30 km wide), progressively wideningsoutheast of Crete (where it reaches 100 km). Off Libya,the axial MR, where particularly abundant mud diapiricand volcanic fields have been described, corresponds to anelevated and practically flat wide region extending fromthe base of the Libyan margin to the Poseidon Trench(SW of Crete).

Along its Ionian branch, the inner MR domain includesquite thick and almost undeformed, sedimentary basins,while south of Crete, the crestal MR appears directly in

thrust contact over the Cretan margin. SE of Crete, theinner MR is structurally much more complex. There, webelieve that the Strabo and Pliny wrench-fault systemshave disconnected and block-faulted a wide area (theStrabo seamounts) that was once part of a less-disruptedinner MR domain, similar to the Ionian one.

Finally the northern bordering Hellenic Trenches them-selves are divided into two main features: (1) The deepMatapan and Pliny trenches running at the base of theAegean continental margin, in which MCS data andswath bathymetry illustrate strike—slip activities affectingthe present-day sedimentary infilling (Huchon et al. 1982;Pedersen 1995); and (2) the shallower Poseidon andStrabo troughs, which are best interpreted as backthrust,or nappe, fronts; implying a general overthrust of the MRsedimentary pile over the southern Cretan margin and theStrabo Seamounts basement, respectively.

Conclusions

Integration of recent MCS data with previous seismicprofiles (Finetti 1976), swath bathymetry (Le Pichon et al.1979; Foucher et al. 1993), side-scan sonar and sampling(e.g., Limonov et al. 1994) allows the construction ofa schematic structural sketch of the entire MR (Fig. 6).This sketch illustrates that compression chiefly occursalong the outer and crestal MR, while transpressive defor-mations seem to chiefly characterize the inner MR and theHellenic Trench area.

We assume here that active ongoing collision processesbetween the MR and the Libyan promontory representsthe best explanation to account for the along-strike MRstructural variability. Recent present-day velocity fieldmodels indicate a minimum velocity within the centralAegean and lateral accelerations towards the Ionian andLevantine basins with respect to Europe (Le Pichon et al.1995). We consider this result to be in good agreementwith our geodynamic interpretation. Our seismic data,however, tend to favor a deformation partitioning at thelevel of the Hellenic Trench domain rather than within theinner MR.

Moreover, we infer that precollisional processes be-tween the central MR and the African margin may havealready begun sometime prior to Messinian times, some6 Ma ago. We believe that such an hypothesis may betterexplain most morphostructural characteristics of the MR.Today, the high-standing central domain (between south-ern Crete and Libya) corresponds to a relatively flat area,without good seismic evidence of thick Messinian evap-oritic cover. We thus consider that this region was alreadyan elevated rise during Messinian times and would havedisconnected two deeper basins (the Ionian and Levan-tine), where thick evaporites, now partly incorporated intothe outer MR, accumulated. Within the MR, the availabil-ity of thick, and mechanically weak, sediments would haveled to an upward migration of the main decollement levelfrom early Cretaceous shales to Messinian evaporites,

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Fig. 6 Structural sketch of the MR constructed using Prismed MCSdata and OGS seismic lines (from Finetti 1976). Swath bathymetryand side-scan sonar data have also been used to interpolate the mainstructural lineaments. The MR appears to be made of three maindomains bounded by major thrust zones. A major backthrust marksthe boundary between the axial and inner MR. The Hellenictrenches denote either a series of chiefly strike—slip active lineaments(Matapan and Pliny trenches) or backthrust or nappe fronts(Poseidon and Strabo trenches)

and thus induced a faster outward growth of the MRdeformation front.

Finally it should be noted that the MR average topo-graphic slopes (Fig. 3) do not agree with the standardconvex-shaped cross section of typical accretionaryprisms. We believe that the topographic slope increases,observed at the boundary between the outer and axial MRdomains (Ionian and Levantine), directly relate to anincrease in basal friction (Davis 1983). If correct, theboundary between the outer and crestal MR area mayindicate a change in basal friction. Knowing that shalediapirs, mud volcanoes, and active fluid vents are chieflydocumented from within the axial MR, we are thus in-clined to believe that these manifestations are surfaceexpressions of an active deep-seated detachment level,potentially located within Aptian shales (Ryan et al. 1982).If valid, this may be a good indication in favor of two mainsuccessive decollement levels acting at depth within theMR, one near the base of the Messinian evaporites andthe second within early Cretaceous shales.

Finally our hypotheses imply that the present-dayMR results from at least two distinct accretionary stages:

(1) a pre-Messinian stage, during which an early accretedtectono sedimentary complex, now located beneath theaxial and inner MR, developed due to an early Cretaceousshale decolement; and (2) a Messinian to post-Messinianstage, chiefly facilitated by thick Messinian evaporites, hasresulted in the construction, and the important southwardgrowth, of the two wide and subdued MR Ionian andLevantine outer domains.

Acknowledgments The MCS data were collected in 1993 during thePrismed cruise on board the R/» Nadir. We thank IFREMER,GENAVIR, and CNRS-INSU for technical and financial support.

An expanded version of this paper was presented during theUNESCO-IOC-ESF 4th meeting on ‘‘Sedimentary basins of theMediterranean Sea’’ held in Moscow in early 1996 as part of theUNESCO-ESF ‘‘Training-through research’’ program. We thankthe convenors for the invitation. Contribution number 221 ofU.M.R. GEO-SCIENCES AZUR.

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