Tectonophysics 490 (2010) 1–14

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North Marmara Trough architecture of basin infill, basement and faults, from PSDMreflection and OBS refraction seismics

Anne Bécel a,⁎,1, Mireille Laigle a, Béatrice de Voogd b, Alfred Hirn a, Tuncay Taymaz c,Seda Yolsal-Cevikbilen c, H. Shimamura d

a Sismologie, Institut de Physique du Globe de Paris, UMR 7154, Franceb Université de Pau et des Pays de l'Adour, CNRS UMR 5212, 64013 Pau cedex, Francec Seismological Laboratory, Dept Geophysical Engineering, Faculty of Mines, Istanbul Technical University, Maslak-34469, Istanbul, Turkeyd Institute of Seismology and Volcanology, Sapporo, Hokkaïdo, Japan

⁎ Corresponding author. Institut de Physique du GlobPlace Jussieu, Case 89, 4ème étage, T24-14, 75252 Paris ce27 47 81/+33 1 44 27 39 14; fax: +33 1 44 27 38 94.

E-mail address: becel@ipgp.jussieu.fr (A. Bécel).1 Now at: Institute of Earth Sciences ‘Jaume Almera’,

0040-1951/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tecto.2010.04.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 January 2009Received in revised form 8 April 2010Accepted 9 April 2010Available online 18 April 2010

Keywords:Seismic refractionSeismic reflectionNorth Anatolian faultSea of MarmaraPre-stack depth migrationActive faults imaging

The reflection and refraction seismic data collected during the SEISMARMARA Leg 1 survey in the Sea ofMarmara provide detailed imaging of sedimentary record and fault activity with deep penetration into itsbasement. First, a detailed analysis of pre-stack depth-migrated seismic lines crossing the Central Basinenable us to discuss the space and time relations of the large and smaller nested basins of the innerdepression, as well as the diversity of style and rate of activity of motion at the diverse basin border faults.Second, forward modeling of OBS refraction arrival times reveals the effect of compaction on thesedimentary pile whereas its layering imaged by MCS as seismic reflectors rather recorded the tectonicevolution. Another major result of the refraction modeling is the identification of the crystalline basement.The latter is imaged about 1 or 2 km deeper than the base of the layered sedimentary sequence imaged onthe coincident MCS profiles.This basement exhibits sharp topography across the Central High, the Kumburgaz Basin and the eastern tip ofthe Cinarcik–North Imrali Basin in an unexpected way with respect to the sea-bottom depressions. Wefurthermore imaged several large tilted basement blocks, which separate the deep basins as between theCinarcik and Imrali basins. Despite the varying width of the NMT and the sizes of the tilted blocks, wepropose that the imaged finite deformation results from a similar process of partitioning deformation overmore than one or even two faults across the NMT that may have changed activity with time and space.

e — UMR 7154, Sismologie, 4,dex 05, France. Tel.: +33 1 44

CSIC, Barcelona, Spain.

l rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The North Marmara Trough (Fig. 1) is the continuation of thenorthern strand of the North Anatolian Fault that produced the twodestructive earthquakes of Izmit (M 7.4) and Düzce (M 7.2) in 1999.The epicenters were located slightly eastwards of the Marmara Sea. Itis prone to future major earthquakes as it has experienced in the pastand is a present seismic gap between the site of the 1912 Ganosearthquake to its west and the time–space progression of largeearthquakes from the East over the last 50 years to its eastern edge(Hubert-Ferrari et al., 2000). It is also a key geodynamical region, areleasing bend between these Ganos and Izmit faults, of largetranstension between the North Anatolian strike-slip fault (Armijoet al. 1999), and the Aegean region of extension deformation.

Recently new data have been collected in the Sea of Marmaraincluding multi-beam bathymetry (Le Pichon et al., 2001; Armijo etal., 2002; Rangin et al., 2004), geodesy with campaign andcontinuous GPS (Straub et al., 1997; Reilinger et al., 1997; McCluskyet al. 2000; Flerit et al., 2003), offshore monitoring of the currentmicroseismicity (Sato et al., 2004), high resolution and shallowerpenetration seismic profiles (Smith et al., 1995; Aksu et al., 2000;Okay et al., 1999, 2000; Imren et al., 2001; Parke et al., 2002;Demirbag et al., 2003; Ates et al., 2003; Rangin et al., 2004), whichsampled only the shallow sedimentary cover. From these data,several recent tectonic models of the Marmara Sea evolution havebeen proposed.

Here we present new images in depth of the North Anatolian Faultsystem in theSea ofMarmara. Thedeeppenetration improved images ofthe reflectivity structuredown to thebasement allowing a discussiononthe basin formation and evolution. The second aimof this paper is to usethe wavefield recorded by OBS at variable offsets as refraction–reflection sounding and profiling. This complements the view on thevelocity–depth structure of the upper crust architecture and penetratesat depth into the basement below the syn-kinematic sediments.

Fig. 1. SEISMARMARA-Leg1 survey. Locationmapofprofiles shotbyN/ONadir and recordedbothby the 4.5 km longstreamer forMCSandOBS (black squares). Black lines indicate theMCSprofiles acquiredwith the 2900 cu. in. source, black dashed lines indicate those acquiredwith the strongest 8100 cu. in. Red thick lines with identification are the profiles discussed in thispaper. Theprofile SM40mentioned in this paper and shown inLaigle et al. (2008) is inblack thick line. Bathymetry fromsea-bottommulti-beamofN/O le SuroitMarmara cruise, 2000, andinterpreted faults (grey lines) are from Armijo et al., 2002.

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In 2001, the SEISMARMARA survey on the ship N/O Nadir wascarried out as the first deep seismic experiment, with crustal-scalereflection and Ocean-Bottom-Seismometer (OBS) refraction seismicswithin the North Marmara Trough (Laigle et al., 2008; Bécel et al.,2009). This narrow region reaching 1200 m water depth contrastswith the southern part of the Marmara Sea which is a broad andsedimented shelf with large buried basins, interrupted by tectonicallyuplifted islands. The North Marmara Trough is composed of threelarge bathymetric basins. These are from west to east the TekirdagBasin, the Central Basin and the Cinarcik Basin. The basins are sepa-rated by two NE-trending highs, theWestern and Central Highs. Northof the Central High and between the Cinarcik and Central basins thereis the smaller Kumburgaz Basin.

During the SEISMARMARA-Leg1 survey (Fig. 1), the data werecollected by using an air-gun source-array operated in single-bubblemode (Avedik et al., 1996) of 8100, 2900 or 2600 cu. in. and a 360-channel digital streamer of 4.5 km length. The receiver group intervalwas 12.5 m, with a common mid point (CMP) interval of 6.25 m. Shotintervals varied between 150 m, 50 m and 25 m. The differentacquisition parameters used allowed varied resolution and penetra-tion, which hence provided different fold coverages of 15, 45, and 90,respectively. A grid of 4main E–Wregional lines and over 20 transectswith about 10-km line spacing have been collected over the northernhalf of the Marmara Sea. In addition, the SEISMARMARA surveycomprised 37 3-components OBS (from ISV Hokkaïdo, Japan) arraywhich was deployed before the shots by the ship R/V SISMIK 1, at thecrossing points of the E–W profiles and N–S transects as a sparse gridthat covered the entire trough and its margins. These OBS recordedcontinuously the shots and the current seismicity, along with a set ofland seismometers spread around the Sea of Marmara.

2. Pre-stack depth-migrated images through the Central Basin,Kumburgaz Basin and Central High

All MCS reflection profiles have been processed to CMP brute stacktime-sections. Many have been reprocessed in diverse ways depend-ing on the research target. This paper focuses on transects whereseveral OBS have recorded the shots and for which reliable reflectiondepth sections could be derived either by PSDM or by simpler post-stack depth migration. This allows an attempt at approaching simul-taneously the interface architecture and layer velocities by jointlymodeling reflection and refraction data.

We will discuss how the observed data lead to a better under-standing of the deformation in the Sea of Marmara. The deformationhistory is recorded by the syntectonic sedimentation in subsiding

basins. The MCS data provide images of the stratigraphy of thesebasins required to decipher their tectonic evolution.

On three N–S transects which are marked in Fig. 1 (profiles SM 46and SM 47 through the center and the NE edge of the Central Basin,and profile SM 28 across the Kumburgaz Basin and Central High), anadvanced form of seismic processing, PSDM, has been performed. ThePSDM is the most accurate seismic processing method for imagingsupra-crustal structures in the context of strong lateral velocityvariations (Guo and Fagin, 2002), due to its ability to focus the imageand locate the real position of reflectors. Previous studies have shownthe successful use of this technique in case of continental extension atcrustal levels (Reston et al., 1996). This processing implies theprogressive building from top to bottom by iterative updating of acorresponding 2D interval velocity model. Its accuracy allows propermigration in the depth domain, taking into account the seismic wavepropagation and raypath bending.

2.1. Central Basin: north–south cross-section

2.1.1. Bathymetry of the area under investigationThe Central Basin (Fig. 1) is a box-shaped structural depression as

documented from earlier multi-beam coverage (Le Pichon et al.,2001). This Basin has been formed along the North Anatoliancontinental strike-slip fault that has developed a zone of transtension.These types of basins that form along strike-slip faults occur ondifferent scales and levels of complexity and have been first definedby Carey (1958) and Burchfiel and Stewart (1966).

The current view of the Central Basin origin is by pull-apartformation, although whether it is still active is another debate. In thispoint of view, the NE-SW striking border faults are considered as theside-stepped strike-slip segments whereas the orthogonal borderfaults are the normal faults along the side-step that have beenseparated from each other. If we consider the Central Basin as a pull-apart basin, this would be along the side-step direction of a non-overlapping stepover as described in the analogue model of McClayand Dooley (1995) (Fig. 1).

The MCS profile SM 46 (Fig. 2A) was recorded with a shot intervalof 25 m (airgun source was of 2600 cu. in.) giving a 90-fold-coveragewith a 50 m trace interval. Together with the length of 4.5 km of thestreamer, this high-fold coverage allows an efficient suppression ofthe sea-bottommultiple on the far offset traces. Previous surveys useda 1.2 km long streamer (Okay et al., 1999, 2000; Parke et al., 2002) andwere limited to depth penetration by the sea-bottom multiple. Thepre-stack depth migration has been performed on the CMP gathers ofthis north–south MCS profile SM 46.

Fig. 2. (A) Profile SM 46— north–nouth pre-stack depth-migrated section across the Central Basin, vertical exaggeration of 1.5. F1, F2 are the basin sidewalls, F3 and F4 are the intra-basin faults. F5 indicates a fault contact that does not crop out. Inverted green triangles refer to the fault scarps imaged at sea bottom (Armijo et al., 2002). (B) Inset respectivelyshows the location of the profile SM 46 on a position map and on a corresponding schematic view of a pull-apart basin. (C) Interpreted vertical section through an analog experimentrun with a 90° releasing side-step. Figure modified from McClay and Dooley, 1995. AA′ indicate the side-step.

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2.1.2. Geometry of stratification and fault architectureA seismic layering typical of sediment deposits is revealed on

the depth section of profile SM 46 (Fig. 2A) down to depths as largeas 6 km forming a deep sedimentary basin, thus indicating strongsubsidence between boundary faults. Intra-basin tectonics is alsorevealed and particularly well-expressed deeper than 2 km by largevertical throws of sedimentary reflectors. The sediment layering alsoshows significant variation of dips due to the faulting effect and drag-folds close to the faults. Hence, this profile shows the sedimentaryrecord of the normal component of the faulting activity. The horizonshave been tilted after deposition by repeated and continuing slip onthe faults, local layer dips reaching as high as 10°, and by rotation dueto the strike-slip component of the fault. The strike-slip faultingcomponent is however difficult to identify since its direction ofmotion is out of the plane of the reflection profile.

Four main faults (F1 to F4) (Fig. 2A) are clearly identified throughthis depth section down to 4–6 km depth. Their outcrop has beenrecently detected by multi-beam bathymetry from their scarps at thesea-bottom. F1 and F2 are basin border faults and F3 and F4 are cross-basin faults. The cross-basin fault zone cuts diagonally across the pull-apart basin (Fig. 2B) and orthogonally to this N–S profile. These faults,with a clear vertical slip component in the plane of section, may be

transtensional faults with both normal and strike-slip components,and where one of the components may dominate. A dip comprisedbetween 75 and 85° is measured for the F3 fault, which value is alower bound for the true dip if the profile were not a dip-line.

An important additional feature is imaged, which is not apparent onthe sea-floor: under the outcrop of fault F4 and with an opposite dip, afault contact F5 separates the continuous stratification dipping from F2from a volume of complex structure (Fig. 2A). Between F4 and F5, whichdoes not crop out, a strongly tectonized triangular area is seen,broadening downwards to 5 km depth. In the discussion about theevolution of the basin opening and its evolution with time, the questionof the nature of thematerial of this cone is important since it bears on theproportion of syn to pre-kinematic sediments above the 6 km depth ofthe sedimentary basin. Profile SM40 shown in Laigle et al. (2008) (Fig. 1)had a more favorable azimuth to distinguish the stratification in thistectonized triangle since this profile cut across the extensional borderfaults whereas the profile SM 46 is along the stepover. It revealssediments with similar stratification in this triangular zone as on thesides. This establishes, that this tectonized triangle is made of syn-kinematic material probably all the way down instead of being a hugeremnant of tectonized pre-kinematic material as could be a defaultoption interpretation from its lackof internal image reflectivity onSM46.

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2.1.3. Intra-basin faultsThe depth section of Profile SM 46 also reveals that the intra-basin

fault (F3) has been active since the early stage of the formation of thelarger basin by the border faults F1 and F2 (Fig. 2A). Indeed, whereasthe sea bottom exhibits only a small scarp, the normal throwcomponent of the intra-basin fault increases by up to 1 km in thedeepest, older layers. This indicates that deformation increased withdepth on this fault, which has thus been active throughout depositionof the basin sediments. This does not support the view that the intra-basin faults are recent (e.g., Le Pichon et al., 2001), suggesting that themain activity of the Central Basin focused with time to a small innerpull-apart basin with a side step of 5 km, or a single fault. The faultsbounding the inner basin have indeed been active throughout thedeposition of the syn-kinematic sediments, thus faults have co-existed at diverse scales from the onset of pull-apart activity. Thelarger system of the Central Basin hence appears to include multiplebasins since early times.

The fact that these crossing faults are not recent is however not indisagreement with a pull-apart formation for the Central Basin. In theanalogue models of McClay and Dooley (1995), a cross-basin faultappears in the early stage of the basin formation as synthetic R Riedelshears. Other analogue models illustrate this point for certainconditions of friction on the basal décollement introduced into themodel andwhich represents the control level of the basin formation inthe simulation of Rahe et al. (1998).

2.1.4. Prekinematic basement and the question of “control level” ofpull-apart basin

PSDM processing allows us to resolve the fine sedimentarylayering and its intersection by faults to a depth of at least 5–6 km(Fig. 2A). The syn-kinematic sedimentary sequence does reach thatdepth. The top of a pre-kinematic basement could thus be taken as itsbottom, here at a depth of about 5.5 km. This basement shows a slightnorthwards dip (between F2 and F1 faults) rather than forming acompletely flat-bottomed basin as in the case of a strictly symmetricalgraben.

At first sight comparing Fig. 2A of the Profile SM 46 with thesection of the analogue model of McClay and Dooley (1995)(Fig. 2C) along the stepover, the general arrangement showsstriking similarities with the inclination of stratification awayfrom the two basin edges, as if dragged at the faults that wouldlimit it and with the main subsidence in the center. This central partis indeed subsiding between intra basin faults. However the faultlocations in the analogue model section involve not only the syn-kinematic material but also the pre-kinematic material that issketched there in blue,black and white, as a stratified pile, above abase or control level (Fig. 2C). Since there is no evidence of thick

Fig. 3. Pre-stack depth-migrated section of the p

stratified sediment in the sidewalls of Profile SM 46, the whole5.5 km thickness of stratified material in the basin has to beconsidered as syn-kinematic. If there was a so-called control levelas built into the analogue model, the pre-kinematic material aboveit would, in the case of the Central Basin, be of very reducedimportance in volume, both horizontally and in depth with respectto the size of the side-step and the thickness of syn-kinematicsediments, or be located deeper than the volume imaged.

What PSDM allows us, compared to time-sections of regularseismic processing, is to have a true-scale depth-section image. Inparticular we may view it without vertical exaggeration, in Fig. 3.Although 6 km is an impressive thickness for a recent sedimentarybasin, the image also indeed reveals an aspect ratio of V/H of 1:3, i.e. arather flat aspect with respect to those commonly considered in pull-apart analogue modeling. Here both the side-step, which is 15 kmwide and also the increased basin length between normal-faultsidewalls is also of 15–20 km. This appears quite large with respect tothe 6 km thick syn-kinematic sediments when we compare this to thegeometries commonly considered in analogue modeling. The ana-logue models have instead narrowly spaced sidewalls with respect toimportant thickness of syn-kinematic sediments (Fig. 2C). Theanalogue models have an aspect ratio between the sediment fillthickness and the spacing between de sidewall of about 0.5–1.

At closer look, there are significant differences with the section ofMcClay and Dooley (1995) along the side-step, which is approxi-mately the orientation of Profile SM 46. In their analogue model, thematerial above the so-called control level is minced and thinned. Inthe seismic image, above the 6 km level of the bottom of the deepbasin, there are layered sediments syn-kinematically tectonized withthe basin subsidence, whereas the material under the margins isdifferent, non-stratified and probably basement. Hence we do notdistinguish a so-called pre-kinematic layer which would have beendeformed during the pull-apart evolution. It may have been thinnedor disrupted by material withdrawal, or the mechanism of basinformation may be different.

In Laigle et al. (2008), an alternative interpretation was proposed,depending on characteristics of the deeper structures at crustal scalenot imaged here by MCS. The Central Basin was considered as agraben, which is subsiding because crustal material at depth beneaththe top of the basement would have been removed, possibly carriedsideways using a detachment. A thinning of the crust under the deepbasin with withdrawal of crustal material between the top of thebasement and the top of the lower crust has been further investigatedand documented by wide-angle reflection and refraction seismic data(Bécel et al., 2009). This crustal thinning could occur by pure-shearstretching or by simple-shear tectonics of layer omission. The lattercase could involve a deep detachment, from under which material is

rofile SM 46 with no vertical exaggeration.

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excised on top of the lower crust, as being the level that controls thebasin structural evolution (Fig. 2A).

In any case the very strong depression of the basement beneaththe basin revealed here has a major implication for the size of thelargest earthquakes in the Sea of Marmara segment of the NorthAnatolian Fault. The largest size for continental earthquakes is whenthey rupture thewhole thickness of the seismogenic layer of the crust,which is generally viewed as the brittle continental basement. Thevery large depth of over 6 km to the top of the continental basementunder the deep basins decreases by this amount the thickness of theseismogenic layer and thus the vertical dimension of the fault-planeavailable for the nucleation of the larger earthquakes commonlyassumed as 15–20 km, which is reduced by 30–50%.

2.2. Depth section across the north eastern corner of the Central Basin

2.2.1. Stratified sediments and faultsOn the PSDM depth section of Profile SM 47 across the eastern

corner of the Central Basin (Figs. 1 and 4 inset), the “box-shaped”basin is still 5 km deep but becomes asymmetrical. This is inagreement with the analogue modeling of pull-apart basins by Raheet al. (1998), which shows an asymmetry at basin tips.

This profile reveals strong along-strike variations in the centralbasin morphology. Stratified sediments are well resolved in depthdown to 4.2 km depth. Beneath the deepest part of the sea-bottom,thefirst 1.5 km is imagedwithwell-expressed layers. At greater depth,the image becomes less sharp; however, the same pattern remainsdetectable.

This profile is cutting across the NE rim, which in a pull-apartmodel has a dominant normal-faulting component (fault F1′) and theSE rim with a major strike-slip component (fault F4′) (Fig. 4).Slumping is observed on the northern slope of the basin and theextensional faulting zone as it has been also observed by Parke et al.(2002) in this area. There is a chaotic domain due to the slumping andthe changes of fault activity, which is separated from gently, stratified

Fig. 4. Pre-stack depth migration of the profile SM 47 across the NE corner of the Central Basyellow lines, inverted green triangles and F1″, F1′, F3′, and F4′ indicate the fault imaged at thepositive flower structure. The white transparent mask is to indicate the vessel course changelocation of the profile SM 47 and a corresponding schematic view of a pull-apart basin. Bas

sediment deposits. The transition between these two domains is clearat sea-bottom and becomes less clear in depth.

The basin-bounding fault corresponding to fault F1 seen on profileSM 46 is called here F1′ since it is here the extensional border fault ofthe basin. Large throws in the upper sedimentary layers are observedandmeasurable for the uppermost one. There is another fault, south ofF1′, which does not crop out and appears to be sealed (at 2.0 kmdepth) by the shallower, highly reflective layers. To the southeast ofthe region of the corresponding fault to F3′, thewall of themost recentlayers is faulted and crops out in a positive flower structure. Thedeeper layers of this upper reflective zone are less affected by up-throws on their southern tips.

The Profile SM 47 is not a dip-line with respect to the faultorientations mapped at sea bottom in the northeastern part of theCentral Basin. The dip of the recent normal ‘faults’measured is only anapparent dip, which gives a lower bound of the fault dip, which is hereabout 70°. This fault on the rim seems still active with a significantthickness of very recent sediments on the footwall of the scarp clearlyimaged at sea bottom in the PSDM section.

On the southeastern rim of this basin, we observe a damaged zonewith short reflectors, which can be associated with the faulted zone ofthe main strike-slip NAF, with a reverse component. The positiveflower structure is not well-resolved here since the faulted zone isclose to the vertical which is adverse for the seismic reflectionmethod.

2.2.2. Evolution of the fault and the basinThe depth section reveals a structure that suggests that there have

been three episodeswith time in the fault activity.Whatwe see here is aclear fanning northwest of F3′ of the deeper, older sediments indicatingtheir tilting as they were deposited, whereas there is a tendencyupwards for more recent deposits to have subsided at the same rate onboth rims of the basin. At present, with the positive flower structurereworkingearlier sediments at the SWedge of theCentral Basin, a rathertranspressive style at the basin edge is underlined here as it is observedin the analogue modeling (Rahe et al., 1998).

in. Vertical exaggeration of 1.5. The fault imaged on this depth section are indicated bysea bottom (from Armijo et al., 2002). UF: unnoticed fault. Black box is to underline theand hence when the streamer was not completely straight. Inset respectively shows theement observed to the south-east is in brownish thick line.

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From sea-bottom morphology, Armijo et al. (2002) interpretedcompressive structures (reverse faults F3′ on Fig. 4) on the northeastern edge of the central basin. Le Pichon et al. (2001) and Imren etal. (2001) proposed some shortening to occur at the eastern extremityof the Central Basin, along the western boundary of the Central Highwhere they interpreted two reverse faults from their sea-bottommapping.

In the footwall of the extensional sidewall fault F1′, the basementreflector dips continuously southwards from the northern part of theCentral Basin where it almost crops out, down to 2.5 km, where it isabruptly interrupted by normal faulting south of where F1′ crops out.Its downthrown part is not resolved beneath 3.5 km, being lost underthick sediments. It seems to be further downthrown by deeper faultslocated a few kilometers to the SE from F1′ and which do not crop out.

2.2.3. First Imaging of an unnoticed ‘major’ fault, still activeThe depth-migrated section of line 47 (Fig. 4) reveals a hitherto

unnoticed fault hence labeled ‘UF’. This fault seems not to have beenidentified at sea-bottom by bathymetric mapping, even with multi-beam. However it seems still active to the present days as shown bythe very small scarp detected on the seismic line, with the 6.25 mhorizontal resolution between the CMP points and a relatively broadband seismic signal and 4 ms sampling, i.e. about 3 to 6 m of verticalresolution of sea-bottom depth.

This fault cuts into the basement. The throw is rather small for theshallow sediments of the first kilometer below the sea level (Fig. 4)and is suddenly 750 m for the basement well imaged on the southernpart of the profile. If this fault has not been reported from the regional

Fig. 5. Stack time section of the profile SM 3 across the Central Basin. Red lines indicate the horefraction modeling. The blue lines indicate the final velocity depth model restretched to tw

image at sea-bottom, it may be because it has only a small throwbecause of its strike-slip nature.

The location and nature of this fault suggest its possibleinterpretation as the strike-slip fault that could have controlled theformer opening of the wide Central Basin as a pull-apart. The fault dipin the plane of section, which is a lower bound on its true dip, is about70° on Profile SM 47. This fault, which is still active, is clearly of crustalscale since it brings face to face basement blocks of differenttopographies imaged as an apparent large throw.

2.3. A refraction velocity view of the Central Basin

A profile SM 3 (Fig. 1) shot with the largest source to be recordedby OBS is close to profile SM 46 discussed before which was recordedwith a tight shot spacing to allow PSDM processing, but with thesmaller source. The corresponding MCS profile SM 3 recorded withthe large shot interval is displayed as a stacked time-section in Fig. 5.

In support of the following discussion, supplementary data(Figs. A_1A and A_1B) contains figures of the observations (record-sections on OBS) and the modeling details with the fit of the preferredmodel, as well as extensive captions.

OBS-line 3 is a north–south line across the deep Central Basin. Onthis line, three instruments (OBS 26, 10, and 21) are located beneaththe shot lines. Two other instruments (OBS 33 and 37) further south,which are in-line with the others, have been considered in order tohave a better control on the basement velocity and topography.

The Central Basin is the largest basin in the Sea of Marmara, so thatboth the OBS and the shot line are in the deep basin (except OBS 37that is outside) whereas in general shot lines cross sea-bottom and

rizons digitized, then converted to depth to build the starting velocity model for the OBSo-way travel times. Inset shows the location map of the profile SM 3.

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basement topography on other profiles. The case here is mostfavorably close to the sampling of the velocity–depth structure in ahorizontally layered basin rather than dominated by structuralheterogeneity and variations along the ray-paths.

2.3.1. Building of the starting modelThe initial model for refraction modeling is built from the

digitizing of main horizons on the MCS reflection stack time-section.These are then transformed into a 2D depth section of correspondingboundaries between layers in which the interval velocity of MCSprocessing is taken as constant velocity in each layer for the depthtransformation of the horizons digitized on the stack time section. Thesediment pile was divided in 5 layers following the main reflectorswith interval velocities coming from the detailed rms velocity analysisof profile SM 40 (Fig. 1). Under the sediment pile imaged, a pre-kinematic basement velocity of 4.50 km/s and a further refractor with6 km/s for the crystalline basement were introduced from evidenceon other profiles (e.g., Profile SM 5 discussed in the next section). Theobserved first-arrival on the OBS record-sections do not show clearevidence of the multipathing expected for head-waves along severalinterfaces corresponding to strong velocity steps in this initial model.Trial and error model adjustment with the procedure of Zelt andSmith (1992), Zelt (1999) led to introducing velocity–depth gradientin layers and smaller contrast at their boundaries (inset Fig. 6). This ismore realistic for such a deep basin where sediment compaction isnecessarily occurring. The gradients have been chosen so that theaverage velocity of each layer remains close to the initial constantvelocity used for depth transformation of the horizons.

2.3.2. Modeling and tuning of the refraction modelAnother result of the forward modeling in order to fit the

calculated travel times to the observed ones for the refracted waveswithin the sedimentary layers, we had to alter the 2D topography of

Fig. 6. Final interface and velocity model of the OBS profile SM 3 from refraction modeling (vat 10 km offset in the final velocity model.

the sedimentary layer boundaries. We expected that the modelingwould change the dips of the iso-velocity lines of the initial 2Dinterface depth model, because this having been depth-transformedfrom the MCS unmigrated time-section, which gives us a lower boundof the true dip. But instead, the modeling led to the contrary, the fitbeing achieved by decreasing the dip of the isovelocity lines fromthose in the initial model, resulting in the 2D refraction velocity–depth model in Fig. 6.

This difference is illustrated in Fig. 5, which compares on top of theMCS reflector image, the initial model which interfaces had beendigitized following reflectors (red-colored isovelocity contours) to thefinal refraction velocity–depth 2-Dmodel transformed to two-waytimes restretched to time (blue isovelocity contours). The major resultis that isovelocity lines for the sedimentary infill have a smoothertopography within the inner part than the reflectors, thus cuttingthrough them. Tectonic disruption and tilting can bemeasured from thereflectors that are markers of a level of deposition at a same age.Refraction isolines are more isobaric, recording the effect of pressureand compaction on thebulk properties like density and velocity throughthe layers of the sediments during their burial.

3. A broad zone of active subsidence with deep tiltedbasement blocks

3.1. Kumburgaz Basin and Central High, OBS profile SM 5

3.1.1. Reflection basement topography from deep penetrationMCS profileProfile SM 5 of 27 km length is a north–south profile orthogonal to

the known structures of the Marmara Trough. The OBS-line iscomposed of 3 OBS, which are coincident with the MCS line and theOBS 25, further North (Figs. 1 and 8B).

The coincident MCS line (Fig. 7B) has been shot with the largestsource, which could only be shot every 150 m, giving a 15-fold stack.

ertical exaggeration of 3). The inset in the right bottom corner shows the velocity model

Fig. 7. (A) Starting velocity model in depth of OBS profile SM 5. (B) Stacked time section of the profile SM 5 with orange lines for the main horizons that have been digitized andconverted to depth to build the starting velocity model (A). The white transparent mask is to indicate the vessel course change and hence when the streamer was not completelyaligned. (C) Final velocity depth model from OBS refraction modeling, vertical exaggeration of 3. (D) 1-D final velocity model in depth extracted at the 10 km-offset in the finalvelocity model. (E) Final velocity depth model restretched to two-way travel times (green lines) superimposed to the stack time section.

8 A. Bécel et al. / Tectonophysics 490 (2010) 1–14

In addition to standard CMP stack processing, pre-stack FK filtering,deconvolution, and post-stack time-migration enhanced the resolu-tion of the shallow part and the clarity of the deep image. Thereflective pre-kinematic basement exhibits a strong topography withseveral highs and lows. From 1.3 s at the northern end under thenorthern rim of the Kumburgaz Basin it dips southward to almost 4 sover a distance of 10 km (CMP 52850), then rises steeply back to 2 swithin a 3 km distance (CMP 53500), documenting a steeply dippingbasement at the northern edge of the Central High. This shape isrepeated a second time, with a southward downslope to the south ofthe Central High and the edge of a subsequent basement high towhere this profile crosses the E–W profile on the southern rim of theNorth Marmara Trough. There are thus places with 3 s thickness ofsediments. This is surprisingly large, only moderately less than in theCentral Basin where the sea-bottom is deeper. The profile crosses the

region where a single active fault has been reported by all researchersfor the NAF as having its surface expression as a strike-slip segmentrunning along the Kumburgaz Basin between the NW edge of theCinarcik Basin and the middle of the Central. A strike-slip fault,expected to be near-vertical, can of course not directly be imaged byvertical reflection seismics.What is shown on the section in Fig. 7B is avertical zone, 2 km wide between CMP 52500 and 52800 with noreflectors or only very short reflection or diffraction segments thatseparates differing crustal reflectivity on either side of the fault. Thedisturbed zone can be interpreted as the fault shadow areacorresponding to the path of the vertical North Anatolian strike-slipfault zone.

Under the central Kumburgaz Basin the basement depression islocally not far from being as deep as under the Central Basin but is of amuch smaller size (meaning not clear). On this transect on the other

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hand there is a second element contributing to accommodateextension. Indeed, the southern slope of the Central High is markedat the sea-bottom by short local north dipping normal-fault segments,then by the scarp that forms the southern rim of the North MarmaraTrough. This scarp has veered from its E–W strike north of MarmaraIsland and along the Central Basin, and taken the ESE strike towardsthe southern escarpment of the North Imrali Basin (Fig. 1). What theseismic image shows is that although there is not a deep marine basinalong this rim, it is there that the maximum thickness of sedimentshas been deposited throughout the whole history and wheresubsidence has probably been the largest. The proximity to landsediment sources may prevent a deep-water basin to mark in thebathymetry. Although individual active faults are not well imaged onthe post-stack depth-migrated section (Fig. 8) and thus cannot betraced, there is continuing localized subsidence.

3.1.2. Refraction contribution: the basin infill velocity structure is distinctfrom its reflectivity structure

In support of the following discussion, supplementary data(Fig. A_2A and A_2B) contains figures of the observations and themodeling details with the fit of the preferred model, as well as ex-tensive captions.

3.1.2.1. Starting velocity depth model from the MCS line. Like in the caseof line SM 3 in the Central Basin discussed earlier, a 2D velocity–depthstructure for the sedimentary upper crustal part is firstly derived fromthe coincident multichannel seismic profiles and then adjusted byrefractionmodeling. In order to account for the strong 2D topography,principal reflectors have been digitized on the MCS sections and

Fig. 8. (A) Post-stack depth-migrated section with the velocity depth model from the OBS52500, the 2 vertical white dotted lines indicate the faulted zone. Anothermajor fault, whichOBS profile SM 5.

similarly depth-transformed with almost the same velocities as pro-file SM 3 (Fig. 7A).

3.1.2.2. Refraction contributions. The base of the reflector pile seen ontheMCS profile and converted to depth, has been tested for whether itis the interface that refracts the energy that is seen with an apparentvelocity of 6 km/s in first arrivals at the far offsets on the OBS section.The observations forced us to introduce an additional interface, acrystalline basement refractor located significantly deeper, by 1–2 km, than the deepest clear MCS reflector taken as the pre-kinematicbasement. Reversed observations then constrain its velocity to about5.7 km/s, as expected for a buried crystalline basement. This interfacedoes not appear as a reflector on the stacked time section. Thecrystalline basement is essentially unreflective on the MCS profile.One cause may be that the largest impedance contrast is at the base ofthe syn-kinematic sediments deposited in the Plio-Quaternary on arapidly subsiding pre-kinematic basement. This is likely Mesozoicplatform carbonates possibly affected by metamorphism, such as themarble as appears on Marmara Island. This sharp interface can maskreflections at deeper levels of lesser impedance contrast, since thevelocity step may not be large from metamorphic carbonates toigneous or metamorphic basement rocks. Another cause may be theusual diffractive rough topography at the small scale of the crystallinebasement, which may be regarded as a former topographical surfacecorrugated by exposure.

Trial and error modeling led to introducing gradients into eachsedimentary layer (Fig. 7D) to give a better fit of the sedimentary partbetween the observed and computed times. The preferred 2Dvelocity–depth model resulting from this modeling (Fig. 7C) hasbeen transformed to two-way reflection times and superimposed on

refraction modeling superimposed. Vertical exaggeration of 1.5. In the region of CMPcuts into theMesozoic basement is indicated near the CM 55700. (B) Positionmap of the

10 A. Bécel et al. / Tectonophysics 490 (2010) 1–14

the MCS section in Fig. 7E. In Fig. 8A, alternatively, the 2D velocity–depth refraction model has been superimposed on a depth-trans-formed version of the post-stack time-migrated MCS. In theKumburgaz Basin and its northern flank there is an indication thatlocally isolinesmay indicate low velocities to greater depth than in theinitial model, which had been constructed with velocity contoursfollowing reflections. This may mark the location of the active strike-slip segment and indicate that in this accumulating depression thematerial is mobilized and fractured by the active strike-slip deforma-tion, maintaining a localized low bulk velocity to larger depth throughthe reflective pile.

The architecture of reflectors and interfaces shows strongheterogeneity from which we can discuss the tectonic evolution.This north–south cross-section can be interpreted in terms of tiltedblocks bounded by normal faults, which have a dip of about 50°. Theycould have formed earlier in a broad extensional domain that isactively subsiding at present, with localization of the strike-slipdeformation. The tilted block crests are located just south of theKumburgaz depression and beneath OBS 32, where the NMT basin-bounding normal fault crops out. We note that the first crest is notexactly where it could have been expected, where the sea bottomforms a local high around OBS 07. The real basement crest position isin fact further north and the local high of the Central High is in factwhere the sediments are thicker on top of the footwall-tilted block. Afaulted zone associated with the presumed presently active strike-slip single segment of the North Anatolian strike-slip fault is imaged.The imaging of the deep structure, including the basement topog-raphy documents that the present strike-slip at the surface is here inthe hangingwall of a north-dipping normal fault, which is structuredby a pre-existing basement structure. This may allow a partitionbetween the normal and strike-slip components, from depth tosurface.

3.2. Image across the single active strike-slip fault NW of Cinarcik, OBSprofile SM 28

3.2.1. Subsurface continuation of the single strike-slip fault surface traceof the NAF

A three times tighter shot interval of 50 m and 45-fold stack andthe higher frequency source (2900 cu. in.) with respect to theprevious section, have been used for a second transect of the NAF inthe Kumburgaz Basin in order to give a complementary image. ThisProfile SM 28 is about 10 km to the East with respect to the line SM 5and strikes more NW-SE (Fig. 9A). An adapted processing includingdeconvolution, multiple attenuation, and pre-stack depth migrationhave been performed. This profile also cuts through the outcrop ofthe NAF at the sea bottom mapped by Armijo et al. (2002) and LePichon et al. (2001) from themulti-beam survey. This fault is crossedjust to the west of where it has been recognized to have displacedtwo parts previously joined of a submarine hill by 5 km of right-lateral shear.

In the Kumburgaz Basin the PSDM images clearly a generalnorthward fanning of recent sediment layers at least down to 3 kmdepth (Fig. 9A around CMP 13500 and Fig. 9B). This indicates a normalcomponent of motion in extension controlled by a fault that crops outat the northern edge of the Basin. This is indeed where the activestrike-slip is reported. It is the only active fault mapped to connect theCinarcik and Central Basins. Near the northern rim and towards thesurface, the high resolution of the PSDM depth section allows us toidentify the presence of fine-scale complexity (Fig. 9B). Whencompared with the post-stack time-migrated section, the improve-ment of the PSDM image brings out fine structure from what waspreviously appearing as noise, in particular the small scale heteroge-neity of the upper kilometer where material appears minced into fewhundred meters slivers.

Although a single fault has been reported frommapping of the sea-beam bathymetry, the finer resolution of the sea-bottom topographyobtained locally by seismics with respect to mapping from discretelines show that the sea-bottom is affected by very small topographicvariations over the 1 kilometer wide zone at the sea-bottom, centeredon the sea-beam mapped single fault. The PSDM section reveals thatthis sea-bottom topography can be continued to depth as faults tracedthrough the first kilometer where they are branching on a reflectordipping steeply southwards, from 1.8 to 3 km depth (Fig. 9B). Thismay be interpreted as the master fault within the bedrock which mayundergo oblique strike-slip that causes a strike-slip motion with anextensional component in the medium above, that splays in the upperlayers into a flower structure.

The negative flower structure together with the general sedimentfanning and southward dip of the deeper fault consistently documentsa normal component of motion on the master fault. In thisinterpretation the strike-slip fault would indeed have been seismi-cally imaged. This is because of its normal component since in the twoupper kilometers it has indeed both an oblique geometry and anoblique slip. The North Anatolian strike-slip fault is here slightly southdipping from the vertical.

An important new finding is that at its SE end, this profileevidences a strongly subsided basin, where it cuts the westerncorner of the Imrali Basin, south to the Central High. Sedimentsappear to reach at least 5 km thickness. The less consolidated recentsediments have even a thickness almost of the same order than inthe Kumburgaz Basin.

3.2.2. Contribution of refraction observationsIn support of the following discussion, supplementary data

(A_3A, A_3B, A_3C) contains figures of the observations and themodeling details with fit of the preferred model, as well as extensivecaptions.

The penetration is here less than for Profile SM 5 because of theweaker source. A basement that would be the base to the stratifiedsediments cannot here be followed continuously along the wholeprofile. Pieces of strong reflectors are however seen at 3 km depthunder the Kumburgaz Basin, and 4–5 km under the western corner ofthe Cinarcik Basin.

We built a starting model from the pre-stack depth-migratedsectionwith velocity gradients slightly higher than those taken for therefraction modeling of profile SM 3 and SM 5. To take into account thenormal and strike-slip component of the fault on the northern rim ofthe Kumburgaz Basin, a throw in the sedimentary layer and in thebasement has been introduced. This did not fit the observations andwas to be discarded. Indeed, the faulted zone is marked only by achange of amplitude in the wavefield recorded by the OBS. The smallheterogeneity of the supra-crustal structure revealed by the pre-stackdepth migration is not marked by a change of arrival time in thewavefield recorded by the OBS. Interfaces are smooth on the velocityprofile from refraction modeling (Fig. 10). We also had to add, as inother cases, a basement refractor at greater depth than the deepestMCS reflector.

The major result of this modeling illustrated in Fig. 10 andSupplementary data A_3A,B and C is that the part of the crystallinebasement, well sampled by direct and reverse rays from shotsrecorded on the five OBS with high signal to noise ratio, reveals asudden deepening of 1.5 km, with southwards dip in the plane ofsection right beneath the northern margin of the Kumburgaz Basin.This occurs 2 km further North than the fault zone imaged by thePSDM reflection section beneath the NAF fault trace at the sea bottom.The basement, flat at 4.5 km depth beneath Kumburgaz, starts to dipagain towards the southwestern corner of Cinarcik Basin where itreaches 5.5 km depth. From the modeling on the two southernmostOBS, the basement seems to rise up again to 4.5 km depth beneathOBS 30, as also identified on the MCS profile SM 28.

Fig. 9. (A) Pre-stack depth-migrated section of the profile SM 28 across the Kumburgaz Basin and the Central High with the velocity field obtained with the depth focusing analysismethod (vertical exaggeration of 1.5). Inverted green triangle indicates the fault scarp imaged at the sea bottom by Armijo et al. (2002). The white transparent mask is to indicate thevessel course change and hence when the streamer was not completely straight. Inset shows the location map of the profile SM 28. Black box indicates enlarged section in (B). (B)Comparison between the pre-stack depth-migrated section (on the left) and post-stack time-migrated section (on the right) near the northern rim of the Kumburgaz Basin: thePSDM allows to resolve the fine-scale heterogeneity and reveals here a negative flower structure. Inverted green triangle indicates the fault scarp imaged at sea bottom by Armijo etal. (2002).

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4. Conclusion and discussion

4.1. Reflection and refraction modeling: methodology, processes ofsedimentation and basement

The resolution of the reflectivity structure is achieved for thewhole thickness of the sedimentary domain inclusive of the deepestbasins down to the basement. This leads to a better understanding ofthe deformation in the Sea of Marmara as it has been recorded bythe syn-kinematic sediments in these subsiding basins. OBS recordsections are used to complement the view on the velocity–depth

structure of the infill, and to penetrate at depth into the basement,below the syn-kinematic sedimentary sequence.

We focused here on transects, which have several OBS and wherereliable depth sections have been derived from PSDM or by post-stackdepth migration and in an attempt to approach simultaneouslyinterface architecture and layer velocities by jointly modellingreflection and refraction data. Such a study of the shallow structuralheterogeneity resulting from past evolution is also necessary forlinking it with the presently active tectonics such as expressed in themajor earthquakes. These are considered to occur in the seismogeniclayer of the brittle upper crust commonly assumed to extend from the

Fig. 10. Final interface and velocity model of the profile SM 28, vertical exaggeration of 3. Inset shows the 1-D velocity model in depth extracted at the 18 km offset in the final model.

12 A. Bécel et al. / Tectonophysics 490 (2010) 1–14

surface to 15 km depth and more specifically here from stressmodeling in this region (King et al., 2001; Muller and Aydin, 2005)and from the deeper end of the 1999 Izmit earthquake obtained fromrupture modeling and the aftershock cut-off depth (Cakir et al., 2003).It is of importance to image the top of the seismogenic layer which isthe top of the basement and that is shown here to lie over 6 km deepat places. The seismogenic layer thickness appears then reducedbecause of the deeper position of its top.

The sampling of a several kilometers thick sedimentary sequencedown to its basement, by coincident normal incidence reflection ofMCS and variable offset wide-angle reflection–refraction on severalOBS provides one case of a complementary approach. The MCSprofiles of four N–S transects have been used to build the a priorirefraction model which consists of continuous interfaces separatinglayers with different velocities. The joint approach takes into accountthe sharp and strong variation of the shallow structure which is onlyprovided by the MCS, and unresolved by the OBS. Secondly thisapproach allows constraining the velocity–depth function or layeringin the upper crust, since the sedimentary infill gives few measurablefirst arrivals velocities on the OBS because of the large water depthand the low velocities of the sediments.

In the course of developing the joint interpretation as presentedbelow, one of the important constraints from the refraction modellinghas been that it adds vertical gradients in those layers to the simple 2Dconstant–velocity layer model derived from MCS. This supports aninterpretation that compaction plays amajor role in defining sedimentvelocity. Furthermoremodelling also leads to changing the 2D shape ofthe velocity isolines by smoothing them with respect to thetopography of the reflectors seen on the coincident MCS. This alsosupports compaction as controlling the velocity structure.

The velocity in the sedimentary basin infill varies from 1.5 to1.8 km/s at the sea-bottom to about 4 km/s on top of the deepest clearreflector under the pile of stratified sediments that is interpreted asthe basin's pre-kinematic basement. The average vertical velocitygradient and the mechanism of basin infilling make likely that thislarge velocity–depth variation may be the result of the mere com-

paction of material sequenced by glacio-eustatic cycles (Laigle et al.,2008). Compaction curves depend on lithology and other materialproperties. For a broad range of parameters, the porosity decreasesfrom the critical value of 40% at the limit from the suspension state tothe consolidated state (Nur et al., 1998) by compaction with in-creasing burial depth to less than 10–5% at 4–5 km (Baldwin andButler, 1985). The velocity–depth variation measured here throughthe basin infill appears in the range of the porosity-dependent, hencethrough compaction, depth-dependent, P-velocities for common sedi-ments (e.g. Lee, 2002).

Reflectors can be thin lithological inclusions of high localimpedance contrast embedded in a medium of smoothly varyingvelocity with depth. This may be understood as the effect ofcompaction on deposits of monotonous structure and lithology,with short, sharp and thin inclusions, at changes in the glacio-eustaticcycle (Laigle et al., 2008). MCS, resolving these reflectors that aremarkers of the sea-bottom attitude when they deposited is thusfundamental to understanding evolution, because the present 2Dimage of reflectors in depth provides a record of tectonic deformationsince that time.

The joint reflection–refraction analysis illustrates another aspect ofstructure under the sediments. There is clearly multipathing inpropagation beneath the bottom of the stratified sediments, withclearly a faster, deeper branch than the interface wave, head-wave orrefraction corresponding to the latest normal-incidence reflector. Thelatter represents the pre-kinematic basement that is the topographicsurface before the basin formation, and the other, presumably thecrystalline basement, that may not be a good continuous horizonvisible in vertical reflection data.

4.2. Architecture of basin infill and basement

The region studied is linking the large deep basins of Cinarcik andthe Central Basin that have been discussed in Laigle et al. (2008). Itencompasses the eastern tip of the latter, the Kumburgaz Basin,Central High and the western tip of the Cinarcik–North Imrali basins.

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As major results: the MCS reflection data allow resolving details ofpresent active tectonics in the upper sedimentary part whereasrefraction seismics allows identifying and imaging the basementtopography. The topography of the basement is shown not to bemirrored in the present sea-bottom bathymetry in the transitionregion, which is not the case under the Central Basin. This points to anearly extensional regime, with subsidence that is still active but mayseem less so, as its elements are buried. The faster sedimentation nearthe southern rim of the Trough may prevent a negative bathymetricexpression. The more recently and presently active features at sea-bottom only correspond to parts of this inherited deep structuralheterogeneity, which may have partly guided it.

The PSDM profile SM 46 is a north–south profile across the CentralBasin. In the view of the basin as a pull-apart, the profile is thusapproximately along the side-step and close to the section of McClayand Dooley (1995) through their analogue model of a non-over-lapping stepover. The most striking feature is that the sedimentarylayering of the syn-kinematic infill is imaged to a true depth as large as6 km.

At first sight, the depth section has striking similarities with thesection of the analogue model but major differences appear at fineranalysis. The most important one is the rather flat aspect for thestructure with respect to those commonly considered in pull-apartanalogue modelling, with here a sedimentary basin that although asdeep as 6 km is much broader, over 16 km. The other main differenceis the pre-kinematic material and the control level beneath built intothe analogue models of the pull-apart, which is not distinguishedunder the Central Basin. An option would then be to consider also agraben character for the Central Basin beneath which crustal materialbelow the top of the basement would have been suppressed assuggested already in Laigle et al. (2008), and supported by thecorresponding shallowing of the Moho (Bécel et al., 2009).

The pre-stack depth-migrated profile SM 47 cuts across the north-eastern edge of the Central Basin. On this profile the sedimentary infillbecomes asymmetrical but is still 5 km deep. This profile revealsstrong along-strike variations in themorphology. The basin appears tohave evolved in time and in space with changes of depocenters.Different natures of faulting are imaged on the two rims. Thesouthwestern one appears as a positive flower structure, whichmarks the local transpressive nature of the fault, imaged at sea bottom,whereas the northeastern rim has a normal-faulting component.

Fig. 11. 3D interpretative schema of the basement topography between Cinarcik an

A hitherto unnoticed fault at sea bottom can be imaged with itstrue geometry at depth and is revealed cutting into the basement. Thisfault is still active as shown by the small throw imaged at sea bottom.If this fault has not been reported from the sea-bottom image, it maybe because of the small throw due to its possible strike-slip nature.

The N–S profile SM 5 cuts across the Kumburgaz Basin, the CentralHigh and the southern rim of the NMT, where this veers from being E–W in the west, towards SE as the North Imrali fault, south of thecorresponding basin. It exhibits strong basement topography withtwo lows and a high in-between. The northern low corresponds to theKumburgaz Basin with a basement low locally not far from being asdeep as under the Central Basin. The second one, along the southernrim of the NMT where this widens to the East, also has an impressivethickness of sediments although it does not show as deep a marinebasin at sea-bottom. This important basin is in the hangingwall of thesouthern rim of the NMT. The basement high in-between has a shapethat suggests its interpretation as a basement-tilted block bounded bynormal faults. The refraction modelling confirmed the sharp topog-raphy of the basement, and that a crystalline basement exists belowthe deepest reflector interpreted as the pre-kinematic basement.

Sampling the Kumburgaz Basin 10 km to the east with respect tothe profile SM 5, the profile SM 28, with its high-fold coverage and itshigh frequency MCS, gives us a fine resolution of a more complicatedgeometry than the single surface trace identified as the NAF strike-slipfault. The PSDM allows us to identify the fine-scale heterogeneity. Anegative flower structure flanks to the north a zone with generalnorthward sediment fanning. A southward dip of a the deeper faultthat limits the basement high corresponding to the northern rim of theNMT documents a normal component of motion on the master faultwhere only the strike-slip nature is reported at sea-bottom outcrop.The surface trace attributed to the active strike-slip North AnatolianFault would lie here on a south-dipping normal fault in the basement.On the OBS sections of this profile, the small-scale heterogeneity is notrecorded, for instance, the faulted zone is not marked in the wavegeometry but in the amplitude only, attenuating the waves.

Another new finding is that on this SE striking profile, at its south-eastern end, a recently strongly subsiding basin is underlined. Thisprofile also revealed a basement deeply subsided to 6 kmdepthwherethe profile cuts the western prolongation of the south western cornerof the Cinarcik Basin proper, in the footwall of its active extensionalborder fault. The basin is in the hangingwall of the southern rim of the

d Central Basins from the joint interpretation of the profiles SM 28 and SM 5.

14 A. Bécel et al. / Tectonophysics 490 (2010) 1–14

NMTwhere this veers in, or is cut by, the SE striking North Imrali Fault.This southern rim is thus highlighted as an important fault of its ownfor the finite extension accrued in addition to those inside the troughthat shape its morphology such as the three deep marine basins.

In the present area of study which links the Central Basin to theCinarcik and North Imrali Basins, a single active, strike-slip fault isreported at sea-bottom, defined by its trace from the northwesterncorner of the Cinarcik Basin to the eastern corner of the Central Basin.However, seismic cross-sections to the basement reveal complexitiesburied at depth in this area with pieces of tilted basement blockswhich separate deep sedimentary basins, that cannot be inferred fromsea-bottom morphology (Fig. 11).

This zone represents the transition between the eastern third ofthe NMT with two broad sedimentary basins, the Cinarcik and theNorth Imrali Basin separated by a large tilted block and the westernhalf of the NMT that contains only one broad basin the Central Basinbetween its rims, with a minor tilted block between their southernwalls. In this transition zone, two narrow basement deeps arerevealed, one forming the Kumburgaz Basin to the North and asecond deep sedimentary basin on the tilted top of a piece of tiltedbasement block which delineates them in the hanging wall of thesouthern rim of the NMT where this broadens from its western toeastern half. The width of the NMT, and sizes of the tilted blocks itcontains and basins they control vary along the NMT. Nevertheless asimilar process prevails, of partitioning deformation over more thanone or even two faults across the NMT that may have changed activitywith time at places.

Acknowledgements

The SEISMARMARA seismic experiment was operated as a jointintegrated project between Turkish and French scientists, researchinstitutions and universities, technical facilities and funding agencies,coordinated by TUBITAK in Turkey and INSU-CNRS in France. N/ONadir and the seismic source and streamer operated by IFREMER/GENAVIR were allocated in the frame of French national facilities. TheOBS provided and operated by ISV Hokkaïdo, Japan, as well as the landstations of the INSU refraction pool were funded by ACI CATNAT of theFrench Ministry of Education and Research, with additional supportand personnel of these institutions and of Turkish ones coordinated byTUBITAK. R/V Sismik 1 of MTA was operated for TUBITAK.

MCS processing has been done with the software Géovecteur®(CGG) and ProMax®. The PSDM was carried out with the Sirius®Software at IFM-GEOMAR, under a TMR grant of the European Union.Wewould like to thank D. Klaeschen for help with the pre-stack depthmigration processing. We thank E. Flueh, J. McBride and other journalreviewers who helped to correct significantly the original version ofthe paper. This is a IPGP contribution number 2586.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.tecto.2010.04.004.

References

Aksu, A.E., Calon, T.J., Hiscott, R.N., Yasar, D., 2000. Anatomy of the North Anatolian faultzone in the Marmara Sea, western Turkey: extensional basins above a continentaltransform. GSA Today 10, 3–7.

Armijo, R., Meyer, B., Hubert, A., Barka, A., 1999. Westward propagation of the NorthAnatolian fault into the northern Aegean: timing and kinematics. Geology 27,267–270.

Armijo, R., Meyer, B., Navarro, S., King, G., Barka, A., 2002. Asymmetric slip partitioningin the Sea of Marmara pull-apart: a clue to propagation processes of the NorthAnatolian Fault. Terra Nova 14, 80–86.

Ates, A., Kayiran, T., Sincer, I., 2003. Structural interpretation of theMarmara region, NWTurkey, from aeromagnetic, seismic and gravity data. Tectonophysics 367, 41–99.

Avedik, F., Hirn, A., Renard, V., Nicolich, R., Olivet, J.L., Sachpazi,M., 1996. “Single-bubble”marine source offers new perspectives for lithospheric exploration. Tectonophysics267, 57–71.

Baldwin, B., Butler, C.O., 1985. Compaction curves. AAPG Bull. 69, 622–626.Bécel, A., Laigle, M., de Voogd, B., Hirn, A., Taymaz, T., Galvé, A., Shimamura, H., Murai, Y.,

Lépine, J.-C., Sapin, M., Ozalaybey, S., 2009. Moho, crustal architecture and deepdeformation under the North Marmara Trough from the SEISMARMARA Leg-1offshore-onshore reflection–refraction survey. Tectonophysics 467, 1–21.

Burchfiel, B., Stewart, J., 1966. ‘Pull-apart’ origin of the central segment of Death Valley,California. Geol. Soc. Am. Bull. 77, 439–442.

Carey, S., 1958. The Tectonic Approach to Continental Drift, Continental Drift — ASymposium. University of Tasmania, Hobart, pp. 177–363.

Cakir, Z., de Chabalier, J.B., Armijo, R.,Meyer, B., Barka, B., Peltzer, G., 2003. Coseismic andearly post-seismic slip associated with the 1999 Izmit Earthquake (Turkey), fromSAR interferometry and tectonic field observations. Geophys. J. Int. 155, 93–110.

Demirbag, E., Rangin, C., Le Pichon, X., Sengor, A.M.C., 2003. Investigation of thetectonics of the Main Marmara Fault by means of deep-towed seismic data.Tectonophysics 361, 1–19.

Flerit, F., Armijo, R., King, G.C.P., Meyer, B., Barka, A., 2003. Slip partitioning in the SeaofMarmara pull-apart determined fromGPS velocity vectors. Geophys. J. Int. 154, 1–7.

Guo, N., Fagin, S., 2002. Becoming effective velocity-model builders and depth imagers,part 1— the basics of prestack depth migration. Lead. Edge 21, 1205–1209.

Hubert-Ferrari, A., Barka, A., Jacques, E., Nalbant, S., Meyer, B., Armijo, R., Tapponier, P.,King, G.C.P., 2000. Seismic hazard in the Marmara Sea following the 17 August 1999Izmit earthquake. Nature 404, 269–272.

Imren, C., Le Pichon, X., Rangin, C., Demirbag, E., Ecevitoglu, B., Görür, N., 2001. TheNorth Anatolian Fault within the Sea of Marmara: a new interpretation based onmulti-channel seismic andmulti-beam bathymetry data. Earth Planet. Sci. Lett. 186,143–158.

King, G.C.P., Hubert-Ferrari, A., Süleyman, S., Meyer, B., Armijo, R., Bowman, D., 2001.Coulomb interactions of the 17 August 1999 Izmit Turkey Earthquake. Earth PlanetSci. 333, 557–569.

Laigle, M., Bécel, A., de Voogd, B., Hirn, A., Taymaz, T., Ozalaybey, S., the Members of theSEISMARMARA Leg1, 2008. A first deep seismic survey in the Sea of Marmara:whole crust and deep basins. Earth Planet. Sci. Lett. 270, 168–179.

Le Pichon, X., Sengor, A.M.C., Demirbag, E., Rangin, C., Imren, C., Armijo, R., Gorur, N.,Cagatay, N., de Lepinay, B.M., Meyer, B., Saatchilar, R., Tok, B., 2001. The active mainMarmara fault. Earth Planet Sci. Lett. 192, 595–616.

Lee, M.W., 2002. Modified Biot–Gassmann theory for calculating elastic velocities forunconsolidated and consolidated sediments. Mar. Geophys. Res. 23, 403–412.

McClay, K., Dooley, T., 1995. Analoguemodels of pull-apart basins. Geology 23, 711–714.McClusky, S., Balassanian, S., Barka, A., Demir, C., Ergintav, S., Georgiev, I., Gurkan, O.,

Hamburger, M., Hurst, K., Kahle, H., Kastens, K., Kekelidze, G., King, R., Kotzev, V.,Lenk, O., Mahmoud, S., Mishin, A., Nadariya, M., Ouzounis, A., Paradissis, D., Peter, Y.,Prilepin, M., Reilinger, R., Sanli, I., Seeger, H., Tealeb, A., Toksöz, N., Veis, G., 2000.Global positioning system constraints on the plate kinematics and dynamics in theeastern Mediterranean and Caucasus. J. Geophys. Res. 105, 5695–5719.

Muller, J.R., Aydin, A., 2005. Using mechanical modelling to constrain fault geometriesproposed for the northern Marmara Sea. J. Geophys. Res. 110, B03407.

Nur, A., Mavko, G., Dvorkin, J., Galmudi, D., 1998. Critical porosity: a key to relatingphysical properties to porosity in rocks. Lead. Edge 17, 357–362.

Okay, A.I., Demirbag, E., Kurt, H., Okay, N.A., Kuscu, I., 1999. An active deep marinestrike-slip basin along the North Anatolian fault in Turkey. Tectonics 18, 129–147.

Okay, A.I., Kaslilar-Ozcan, A., Imren, C., Boztepe-Guney, A., Demirbag, E., Kuscu, I., 2000.Active faults and evolving strike-slip basins in the Marmara Sea, northwest Turkey:a multichannel seismic reflection study. Tectonophys 321, 189–218.

Parke, J.R., White, R.S., McKenzie, D., Minshull, T.A., Bull, J., Kuscu, I., Görür, N., Sengör, C.,2002. Interaction between faulting and sedimentation in the Sea of Marmara,western Turkey. J. Geophys. Res. 107 (B11), 2286.

Rahe, B., Ferrill, D.A., Morris, A.P., 1998. Physical analog modeling of pull-apart basinevolution. Tectonophysics 285, 21–40.

Rangin, C., Le Pichon, X., Demirbag, E., Imren, C., 2004. Strain localization in the Sea ofMarmara: propagation of the North Anatolian Fault in a now inactive pull-apart.Tectonics 23, TC 2014.

Reilinger, R.E., McClusky, S.C., Oral, M.B., King, R.W., Toksoz, M.N., Barka, A.A., Kinik, I.,Lenk, O., Sanli, I., 1997. Global Positioning System measurements of present-daycrustal movements in the Arabia–Africa–Eurasia plate collision zone. J. Geophys.Res. 102, 9983–9999.

Reston, T.J., Krawczyk, C.M., Klaeschen, D., 1996. The S reflecor west of Galicia: evidencefrom prestack depthmigration for detachment faulting during continental breakup.J. Geophys. Res. 101, 8075–8091.

Sato, S., Kasahara, J., Taymaz, T., Ito, M., Kamimura, A., Hayakawa, T., Tan, O., 2004. Astudy of microearthquake seismicity and focal mechanisms within the Sea ofMarmara (NW Turkey) using ocean bottom seismometers (OBSs). Tectonophysics391 (1–4), 303–314 (Active Faulting and Crustal Deformation in the EasternMediterranean Region).

Smith, A.D., Taymaz, T., Oktay, F., Tüce, H., Alpar, Basaran, H., Jackson, J.A., Kara, S.,Simsek, M., 1995. High-resolution seismic profiling in the Sea of Marmara(northwest Turkey): Late Quaternary sedimentation and sea-level changes. Geol.Soc. Am. Bull. 107, 923–936.

Straub, C., Kahle, H.G., Schindler, C., 1997. GPS and geologic estimates of the tectonicactivity in the Marmara Sea region, NW Anatolia. J. Geophys. Res 102, 587–601.

Zelt, C.A., 1999. Modelling strategies and model assessment for wide-angle seismictraveltime data. Geophys. J. Int 139, 183–204.

Zelt, C.A., Smith, R.B., 1992. Seismic traveltime inversion for 2-D crustal velocity structure.Geophys. J. Int. 108, 16–34.