Y Thomas
email: thomasy@ifremer.fr

B Marsset

G K K Westbrook

Céline Grall

L Géli

P Henry

G Çifçi

A Rochat

H Saritas

G Cifçi

Contribution of high

Keywords: 3D seismic imaging, High Resolution, Sea of Marmara

INTRODUCTION

Major marine research fields in the environment, palaeoclimatology, geological hazard or mineral resources need High Resolution (HR) data to observe short timeperiod phenomena (e.g. [START_REF] Mahieux | Comparison between highresolution seismic and sequence stratigraphic approaches applied to the upper Jurassic deposits of the Dover Strait area (Northern France)[END_REF][START_REF] Berné | The Impact of Quaternary Global Changes on Strata Formation: Exploration of the Shelf Edge in the Northwest Mediterranean Sea[END_REF] or small objects [START_REF] Vardy | Decimetreresolution 3D seismic volume in shallow water: A case study in small-object detection[END_REF] and to understand and quantify sedimentary processes at the metric scale. Conventional seismic surveys carried out for oil exploration purposes may reach 25 m lateral resolution and 6 m vertical resolution. Using a tailored seismic acquisition configuration and appropriate processing, lateral resolution from HR seismic surveys ranges from 4 to 20 m and vertical resolution from 1 to 5 m, whereas Very High Resolution (VHR) surveys aim at decimetre resolution (table 1).

The optimal lateral resolution for a given seismic layout may only be reached using a 3D seismic survey [START_REF] Cartwright | 3D seismic technology: the geological 'Hubble[END_REF]. Reflections from out-of-the-plane, which are considered to be noise in 2D processing, become signal in 3D processing [START_REF] French | Two-dimensional and three-dimensional migration of modelexperiment reflection profiles[END_REF]. 3D migration processing will then ensure optimal resolution by collapsing the radius of the first Fresnel zone [START_REF] Sheriff | Nomogram for Fresnel-zone calculation[END_REF]. The limit of the resulting horizontal resolution is a function of the dominant wavelength, the depth of the reflection point and the survey aperture width [START_REF] Chen | Resolution limits of migrated images[END_REF].

3D imaging has brought unmatched definition of geological structures, thus increasing the quantitative characterisation of sediment properties with the combination of seismic data and in situ measurements. Initiated several decades ago by the oil industry, marine 3D seismic acquisition has today become an acquisition standard.

Following the current trend to achieve higher resolution, the oil industry still makes substantial investments in 3D marine seismic research and development [START_REF] Long | Seismic frequency bandwidth constraints in deepwater survey locations[END_REF][START_REF] Carlson | Increased resolution and penetration from a towed dual-sensor streamer[END_REF][START_REF] Soubaras | Variable-depth streamer -a broadband marine solution[END_REF]. 3D HR acquisition follows the same rules as 3D exploration seismic; however improving the final resolution requires higher resolution sources and more closely spaced receivers. The price for higher resolution is therefore lower penetration and smaller surveyed areas limited to several tens km².

The objective of this contribution is to highlight the recent effort made by the scientific community to develop 3D HR and VHR facilities tailored to their research fields. The first part recalls the fundamentals of the 3D technique as applied to a HR seismic survey and recently developed academic 3D seismic acquisition systems for high-resolution imaging are examined. The second part illustrates the benefits of 3D versus 2D through a HR seismic data set acquired on the Western High in the Marmara Sea as part of an international research effort to assess the seismic hazard in the area. This illustration focuses on structures and stratigraphic features related to the activity of the North Anatolian Fault.

3D HIGH AND VERY HIGH RESOLUTION SEISMIC ACQUISITION

The aim of a 3D acquisition system is to obtain a volume of amplitude response, free from artefacts, with the optimal lateral resolution allowed by the frequency content of the seismic source and by the seismic layout. Valid considerations for any 3D survey should therefore include:

1) adequate spatial sampling to prevent spatial aliasing [START_REF] Sheriff | Encyclopedic dictionary of applied geophysics: Society of Exploration Geophysicists[END_REF], 2) adequate accuracy of the source and receiver positioning consistent with the expected lateral resolution [START_REF] Archer | Spatially and temporally correlated navigation errors: how do they manifest themselves in seismic data?[END_REF], 3) ability to obtain a uniform fold to avoid artefacts generated by an uneven survey footprint [START_REF] Gesbert | From acquisition footprints to true amplitude[END_REF], 4) adequate length of the streamers which should be long enough to enable the determination of seismic velocities (Dix, 1955), 5) homogeneity of the seismic data throughout the survey in terms of signal to noise ratio and frequency content.

Most of these considerations bring critical constraints to HR and VHR 3D surveys. Spatial sampling to prevent aliasing requires close streamer spacing, which in turn makes it difficult to deploy long streamers (Fig. 1). Positioning accuracy becomes paramount in the dynamic marine environment to attain the desired high resolution.

Tidal variations have to be corrected for in HR surveys, while centimetre motion of source and receivers have to be compensated for in VHR applications [START_REF] Wardell | 3D Pre-processing techniques for marine VHR seismic data[END_REF][START_REF] Vardy | Decimetreresolution 3D seismic volume in shallow water: A case study in small-object detection[END_REF]. Changes over time of the water column velocity have also to be monitored [START_REF] Mackay | Water-column changes pose seismic challenges[END_REF]. Source and receivers have to be towed at a shallow water depth to preserve the high frequency content of seismic signal, making HR and VHR 3D surveys highly sensitive to the sea conditions. 3D academic seismic layouts (table 2) range from almost industrial multi-streamer configuration [START_REF] Mutter | Seismic Imaging in Three Dimensions on the East Pacific Rise[END_REF] down to VHR systems of decimetre resolution [START_REF] Gutowski | Three-dimensional high-resolution acoustic imaging of the subseabed[END_REF][START_REF] Vardy | Decimetreresolution 3D seismic volume in shallow water: A case study in small-object detection[END_REF]. The scope of scientific applications is broad: imaging of the crustal mid-ocean ridge [START_REF] Mutter | Seismic Imaging in Three Dimensions on the East Pacific Rise[END_REF], fluid flow through subseabed sediments and gas hydrates [START_REF] Plaza-Faverola | Repeated fluid expulsion through subseabed chimneys offshore Norway in response to glacial cycles[END_REF][START_REF] Perez-Garcia | Linked halokinesis and mud volcanism at the Mercator mud volcano, Gulf of Cadiz[END_REF], active deformation zones [START_REF] Scheidhauer | Development of a system for 3D highresolution seismic reflection profiling on lakes[END_REF][START_REF] Crutchley | Insights into active deformation in the Gulf of Cadiz from new 3D seismic and high-resolution bathymetry data[END_REF], shallow-water engineering [START_REF] Vardy | Decimetreresolution 3D seismic volume in shallow water: A case study in small-object detection[END_REF][START_REF] Vardy | The geological 'Hubble': A reappraisal for shallow water[END_REF], and archaeological studies [START_REF] Plets | The use of a high-resolution 3D Chirp sub-bottom profiler for the reconstruction of the shallow water archaeological site of the Grace Dieu (1439), River Hamble, UK[END_REF][START_REF] Müller | Ultra-high-resolution marine 2D-3D seismic investigation of the Liman Near Surface Geophysics -Marine special issue 20 Tepe/Karantina Island archaeological site (Urla/Turkey)[END_REF]. Whereas most of these systems were developed in the framework of research programs, the P-cable project (University of Tromsø, National Oceanography Centre and IFM GEOMAR; [START_REF] Planke | Ny metode for innsamling av høyoppløselig 3D seismikk[END_REF] has reached an almost industrial level, with numerous significant HR applications [START_REF] Petersen | High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Artic sediment drift[END_REF][START_REF] Bangs | The mechanics of intermittent methane venting at South Hydrate Ridge inferred from 4D seismic surveying[END_REF][START_REF] Crutchley | Insights into active deformation in the Gulf of Cadiz from new 3D seismic and high-resolution bathymetry data[END_REF][START_REF] Perez-Garcia | Linked halokinesis and mud volcanism at the Mercator mud volcano, Gulf of Cadiz[END_REF][START_REF] Plaza-Faverola | Repeated fluid expulsion through subseabed chimneys offshore Norway in response to glacial cycles[END_REF].

Since 1997, the French Marine Research Institute (Ifremer) has been involved in marine 3D VHR seismic imaging to study the fine-scale geometry of recent deposits [START_REF] Marsset | Very High Resolution marine 3D seismic method for detailed site investigation[END_REF][START_REF] Marsset | Very High Resolution 3D seismic: a new imaging tool for sub-bottom profiling[END_REF][START_REF] Marsset | Analysis of Holocene sedimentary features on the Adriatic shelf from 3D very high-resolution seismic data (Triad survey)[END_REF]. A 3D HR system was later developed to carry out larger scale surveys [START_REF] Thomas | High Resolution marine 3D seismic: A new surveying tool for the Scientific Community[END_REF][START_REF] Meunier | New tools for Ocean Survey: The Challenge of High Resolution[END_REF].

Recent applications have included environmental studies, geological hazards [START_REF] Nouzé | First results of a geophysical survey on the northern flank of the Storegga slides (Norway)[END_REF], sedimentary processes and fluid flows [START_REF] Hustoft | High-resolution 3D-seismic data indicate focused fluid migration pathways above polygonal fault systems of the mid-Norwegian margin[END_REF]. Target sizes are several tens of km² with theoretical resolution limits of 12 m horizontally and 3.5 m vertically (table 2). The following 3DHR seismic example, acquired in the Sea of Marmara, is presented to illustrate and to evaluate the advantages of 3D HR versus 2D HR seismic to study tectonic features and fluid flow in the vicinity of the active North Anatolian Fault.

3D HR MARMARA SURVEY

Geological Context

The North Anatolian Fault (NAF) is the major plate boundary between Anatolia and Eurasia [START_REF] Şengör | Strike-slip faulting and related basin formation in zones of tectonic escape: Turkey as a case study[END_REF][START_REF] Armijo | Westward propagation of the North Anatolian fault into the northern Aegean: Timing and kinematics[END_REF][START_REF] Pichon | The active Main Marmara Fault[END_REF]. The northernmost branch of the NAF runs through the Sea of Marmara [START_REF] Parke | Active faults in the Sea of Marmara, western Turkey, imaged by seismic reflection profiles[END_REF][START_REF] Grall | Heat flow in the sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault[END_REF] where basins and highs are associated with the present-day fault network (Fig. 2). The Mw 7.4 İzmit earthquake devastated part of north-western Turkey in 1999 [START_REF] Barka | The 17 August 1999 İzmit earthquake[END_REF], and raised great concern about the probability of another earthquake in the Istanbul region populated by more than 12 million inhabitants. As a result, extensive research efforts have been made to improve the understanding of the seismo-tectonics in this region, including several scientific cruises since 2000 [START_REF] Henry | the Marnaut Scientific Party[END_REF][START_REF] Laigle | A First deep seismic survey in the Sea of Marmara: Deep basins and whole crust architecture and evolution[END_REF][START_REF] Shillington | Gas, slumps and faulting in the Marmara Sea: new results from TAMAM highresolution seismic reflection data[END_REF].

The occurrence of gas hydrates and the composition of the fluids actively being emitted on the Western High, (Fig. 2) opened new perspectives on the processes that are active close to the NAF [START_REF] Géli | Gas emissions and active tectonics within the submerged section of the North Anatolian Fault zone in the Sea of Marmara[END_REF][START_REF] Bourry | Free gas and gas hydrates from the Sea of Marmara, Turkey: Chemical and structural characterization[END_REF][START_REF] Zitter | Cold seeps along the main Marmara Fault in the Sea of Marmara (Turkey)[END_REF][START_REF] Tryon | Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara: A diversity of sources and processes[END_REF]. This feature was therefore selected for the deployment of a 3D high-resolution seismic survey to investigate the sub-seabed distribution of fluid conduits and the stratigraphic / structural framework. The location of the 3D survey was defined according to the results of the previous 2D HR survey carried out in this area as part of the Turkish-American Marmara Multichannel research project [START_REF] Shillington | Gas, slumps and faulting in the Marmara Sea: new results from TAMAM highresolution seismic reflection data[END_REF].

Methodology

The 3DHR seismic source consists of small volume air guns (mini-GI gun, 110 Hz dominant frequency) able to produce a repetitive signal but with the reduction of the bubble oscillations common in traditional air guns. Two source arrays, 12.5 m apart, are fired alternately in order to halve the cross-line sampling interval for a given number of streamers (Fig. 3; table 2, HR3D system). Two streamers are deployed 25 m apart using two eight-meter long rigid bars fixed to the vessel's frame. Each streamer hosts 48 channels, with a 6.25 m group interval. This seismic layout prevents spatial aliasing of dipping events up to 40° in the in-line direction and 20° in the cross-line direction (Fig. 1).

Positioning of sources and receivers is determined using the DGPS position and gyrocompass of the vessel, and magnetic compasses from 3 depth controllers along each streamer. Given the accuracy of these sensors and the short length of the streamers (400 meters), receiver positions are calculated within an accuracy of 2 m at the head of the streamers, to 4 m at the tail. Source positions are measured to an accuracy of 1 metre. This precision in positioning is consistent with the expected resolution; positioning accuracy falls between /4 and /3 (where  is the dominant wavelength) which is acceptable to preserve the main frequency content of the seismic data. A major drawback is that this approach does not allow operating longer streamers, as accuracy will become poorer.

To obtain a uniform fold (around 20) using a bin size of 6.25 x 6.25 m, acquisition lines are sailed 25 m apart (Fig. 3). Data editing and updating of the fold map are performed at the end of each line to assess the homogeneity of the data and to adjust the acquisition program to acquire additional in-fill lines to cover gaps in the fold map.

Data set and processing

The 3D HR survey of the MARMESONET cruise covered an area of 3 x 11 km² and was centred on and parallel to the main North Anatolian Fault where it cuts the Western High (Fig. 2). The water depth ranges from 500 m near the top of the Western High to 800 m in the NAF axial valley (Fig. 2). The east-west orientation of seismic lines was dictated by the presence of the maritime corridor linking the Dardanelle to the Bosphorus straits. The 3D HR survey was completed in 9 days of acquisition. It is worth noting that a surface current led to a slight drift of the streamers relative to the track (< 5°) and thus to an overlap of the CMP lines for track lines 25 m apart. As a result of this streamer drift, very few in-fill lines were required to fill fold gaps. A total of 135 lines were recorded (20 million traces), including 5 re-shoot lines, and 7 in-fill lines.

Prior to processing, the seismic data were edited for frequency content and signal-to-noise ratio. The immersion of the streamers was monitored using the data from the depth controllers but also from the position of the notch frequency in the seismic signal. This screening identified faulty seismic traces, and 1.6 % of the total number of traces were thus rejected for various reasons, such as: out-of-range streamer immersion, electronic failure, high noise from nearby ships, bad sea conditions, or source breakdown.

The shot gathers were then processed, including corrections for source delay variations and band-pass filtering (20-250 Hz). After positioning processing and binning using the real positions of sources and receivers, constant water velocity Normal Move Out was applied followed by 3D stacking and a two-pass constant velocity Stolt migration (Fig. 4).

The imaging processing requires the knowledge of the velocity distribution within the sediments to better focus the seismic data. As source-receiver offsets of the 3D seismic layout are too short with respect to the water depth, attempts to constrain the velocity field using 2D long offset HR seismic data were carried out. This was achieved by acquiring 2D HR seismic data during the PirMarmara cruise of the Turkish R/V K. Piri Reis (DEU, Izmir, 2010) using a 1500 m long streamer and a 6.25 metres receiver group interval. This 2D data set allowed the velocity of the upper sedimentary cover to be assessed with a relatively low mean interval velocity of 1650 m/s. A threelayer velocity model, 1515 m/s for the water column, 1650 m/s for the upper sedimentary cover, and a constant gradient of 500 m/s increasing for the underlying sediments was then employed to perform 3D stacking prior to constant velocity twopass Stolt migration (Fig. 4).

Results and discussion

Resolution

The sea-bottom 3D seismic amplitude map demonstrates the ability of 3D data to properly focus the seismic signals both in shape and in amplitude. The chaotic stack amplitude data (Fig. 5A), related to diffractions, are not properly focused in 2D (2D inline migration, Fig. 5B). Two-pass 2D migration collapses properly the diffractions on the sea-bottom, as emphasised by the seismic amplitude map (Fig. 5C) and the seismic bathymetry (Fig. 5D).

Comparisons between micro-bathymetry acquired with an AUV and the seismic bathymetry provided the opportunity to evaluate the resolution of the seismic data. The AUV was operated at an altitude over the seafloor of 70 meters; its 200 kHz multibeam provides a lateral resolution of 2 meters and a depth resolution of one centimeter:

-

The first area of comparison presents small circular depressions from the AUV map (Fig. 6A; see Fig. 5D for location) that are not visible on the seismic bathymetry (Fig. 6B). These structures have a diameter of around 25 meters; their maximal depressions range between 25 to 50 centimeters. The depth sampling of the seismic survey around 0.4 m (for time sampling of 0.5 ms and velocity of 1515 m/s) does not allow these depressions to be imaged.

Near Surface Geophysics -Marine special issue 10 -

The second area of comparison was selected on the mounds closed to the NAF (see Fig. 5D for location). Most of the structures shown by the AUV bathymetry (Fig. 6C) are well imaged by the 3D seismic HR data (Fig. 6D). Structures on the crest of the mounds, as close as 25 meters and with only 1 meter of local variation in depth, are distinguished well in both data sets. Consequently, the lateral resolution of the 3D HR seismic is shown to be between the 12 m theoretical resolution limit and the 25 m resolution obtained for these features.

Seismic sections

The results of 2D migration of a stacked section shows that 2D seismic data do not allow proper imaging in comparison with the results of 3D migration (Stolt two-pass 2D migration) of the same section (Fig. 7). Out-of-plane reflections that cannot be focused are obvious after 2D processing, whereas the 3D migration shows faults, amplitude anomalies, and stratigraphy in great detail.

The depth penetration of the HR seismic data from the Western High is limited by strong impedance contrasts, strong deformation and steeply dipping horizons. The resulting penetration is strongly variable: from no penetration on the NW outcrop down to 600 ms TWT in the SW corner of the survey, and primarily depends on the thickness of the gently deformed upper sedimentary cover and on the degree of deformation of the deeper sediments (Fig. 8).

The imaging of the deeper structures with relatively high dips could be improved by using more advanced imaging methods such as 3D DMO correction, 3D finite difference migration or 3D Kirchhoff pre-stack migration. However, this requires a wellconstrained 3D velocity model, and despite the good quality of the 2D long offset data set, reliable seismic velocities in the deeper structures could not be obtained, due to strong out-of-plane reflected and diffracted acoustic energy. This limitation in imaging complex 3D structures with velocity variation is common to any 3D seismic layout operating with too short an offset with respect to the target depth.

The illustrations from the Marmara data set are, therefore, limited to the upper sedimentary sequence where 3D HR imaging is of excellent quality.

Geological insights

The sea-bottom amplitude is relatively uniform except on the northern side of the NAF where two localised areas of high amplitude are associated with the outcrops of consolidated material, which are also well expressed as positive features in the seismic bathymetry (Fig. 5C-5D). The structure on the NW border of the survey is related to folds that bring to the surface consolidated sedimentary sequences older than those covering most of surveyed area. The second high amplitude area is located close to the NAF (Fig. 5C, in the middle of the survey). It is related to the presence of authigenic carbonate deposits and gas hydrates reported at this site by [START_REF] Tryon | Pore fluid chemistry of the North Anatolian Fault Zone in the Sea of Marmara: A diversity of sources and processes[END_REF]. The seismic bathymetry also highlights the different networks of faults on the Western High along the NAF (Fig. 5D).

Most of the upper coherent sedimentary sequence north of the NAF, which reaches up to 400 ms TWT thickness shows thinning, on-lapping and, locally, erosive unconformities (Figs. 7B and8). South of the NAF, the coherent sequence of sedimentary layers is less than 150 ms TWT thick, where it lies above the crest of an anticline that runs parallel to the NAF (Fig. 5D), but thickens southwards (Fig. 8). This, together with the onlapping nature of many of the layers, indicates syntectonic deposition on the flanks of this growing anticline.

Evidence of submarine slides is widespread, with numerous episodes of slide occurrence. The broken strata at the top of a submarine slide are depicted clearly in the 3D processed image of Figure 7B. The anticline south of the NAF, also exhibits abundant evidence of sliding from both its flanks, some recent and some buried under a thin cover of sediment.

The strong shallow reflector H1, which is prominent across nearly all of the 3D survey, has a high amplitude and negative polarity (opposite to that of the seabed), which varies little in relation to the reflector's position or dip (Figs. 7B and8). This is likely due to a layer of a different lithology and high porosity resulting in lower acoustic impedance than the overlying sedimentary strata. Although this negative polarity may also be locally enhanced by the presence of gas, the uniformity of the phase and of the amplitude of the reflector, and its correlation with the detachment surfaces of the slides (Fig. 7B) support an interpretation of a layer of high porosity supported by overpressure as the cause of the negative polarity reflector.

Normal faults with a NW-SE strike pervade the eastern part of the 3D box (Fig. 5D). Examples of these normal faults are presented in Figures 8 and9. Many of the faults outcrop at the seabed, where they can be followed in the bathymetry (Fig. 5D).

More than half of the faults have their upper termination at or beneath reflector H1. The faults sole out in horizons between 150 and 200 ms beneath the seabed (Fig. 8).

It has been established from previous observations and measurements at the seabed that water and gas are being expelled into the Sea along the main Marmara fault zone [START_REF] Zitter | Cold seeps along the main Marmara Fault in the Sea of Marmara (Turkey)[END_REF]. The 3D HR seismic volume provides evidence of past fluid movement and present occurrence of gas beneath the seabed. Gas is widespread in the area and can be recognised from its effect on reflectors, locally increasing amplitude of negative polarity where the gas occupies traps, typically in small anticlines (Fig. 8). Although the observation is not obvious on seismic sections, local reflection brightening caused by gas can be seen on seismic amplitude maps along horizons extracted from the 3D volume. Horizon H2 (75 to 100 ms bsf, Fig. 8), which otherwise has a relatively weak amplitude, displays strong brightening caused by gas distributed along the NW-SE trends of the faults, as well as by gas occupying the crests of anticlines (Figs. 8 and 10).

CONCLUSIONS

Technological advances over recent years have allowed the development of efficient 3D marine seismic systems dedicated to High and Very High Resolution seismic surveys. The quality of the achieved 3D seismic imaging allows fine-scale stratigraphic and tectonic features to be recognised and analysed. This is crucial to assess complex interplaying of geological processes at high temporal resolution.

The seismic survey carried out in the Marmara Sea in 2009 illustrates the benefit of 3DHR seismic imaging of a complex geological structures, such as the Western High, cut by the North Anatolian Fault. One of the topics arising from more than ten years of intensive surveys in the area has been the temporal and spatial connection between fluid and seismic activity, especially in terms of earthquake monitoring. The preliminary analysis of the 3DHR data set has showed that understanding of the area derived from 2D seismic sections was badly constrained and often misleading, because of the poor imaging of three dimensional structures. The 3DHR survey, however, allowed the following features to be identified in the upper sedimentary sequence:

-the interplay between tectonic processes and stratigraphy, such as syntectonic deposition of sedimentary units and widespread submarine sliding caused by differential tectonic uplift of the seabed;

-the widespread occurrence of gas, trapped in the crests of anticlines and adjacent to faults through which gas is migrating and following pathways to the seabed through permeable strata and faults.

Further interpretation of this data set, in progress, will provide keys to unravel the sequence of tectonic events that brought about the existence of the NAF in its present situation and to understand the processes involved in the deformation associated with the activity of the NAF. Understanding observations of fluid expulsion from the seabed in relation to the deformational processes will be used to evaluate the potential of monitoring fluid movement in the assessment of seismic hazard in the Sea of Marmara. Definition of the seismic resolution ranges as stated in the article; theoretical lateral resolution limit is given as the main wavelength of the seismic signal () and theoretical vertical resolution limit as the quarter of . et al., 1999;[START_REF] Grall | Heat flow in the sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault[END_REF]. TB: Tekirdağ Basin, WH:

TABLES

Western High, CeB: Central Basin, CH: Central High, CiB: Çınarcık Basin. Bathymetry on the Western High: location of the 3D survey (blue box 3x11 km²); white dashed line:

the main dextral Marmara Fault track on the seabed; red star: site location where gas hydrates were found [START_REF] Bourry | Free gas and gas hydrates from the Sea of Marmara, Turkey: Chemical and structural characterization[END_REF]. The high amplitude black horizon is the H1 horizon (negative polarity).

FIGURE 10. Seismic amplitude along the horizon H2 south of the NAF (for location see Fig. 5D). The amplitude varies greatly in response to the presence or absence of gas,

showing brightening of 15 dB or more where gas is present: in the crests of anticlines and where H2 is cut by faults (Fig. 8); black dashed line: the main Marmara Fault track on the seabed.

FIGURE 1 .

 1 FIGURE 1. Maximum incidence angle without spatial aliasing function of frequency for

FIGURE 2 .

 2 FIGURE 2. Top Inset: map of the Sea of Marmara and the main fault segments of the

FIGURE 3 .

 3 FIGURE 3. 3D High Resolution seismic layout (IFREMER); dual streamers 25 m apart

FIGURE 4 .FIGURE 6 .

 46 FIGURE 4. Processing sequence applied to the 3D HR Marmara seismic data set; the

FIGURE 7 .

 7 FIGURE 7. Comparison between 2D and 3D imaging of a seismic section (for location

FIGURE 8 .

 8 FIGURE 8. Arbitrary section (top figure: location of the time slice at 880 ms, Fig. 9, and

FIGURE 9 .

 9 FIGURE 9. Time slice at 880 ms from the 3D volume of post-stack 2x2D migrated data

Table 1 . Seismic resolution

 1

	Resolution	Dominant frequency range	Lateral resolution	Vertical resolution
	"Exploration"	3075 Hz	> 20 m	> 5 m
	High Resolution	80375 Hz	19 -4 m	4.5 -1 m
	Very High Resolution	> 375 Hz	< 4 m	< 1 m

Table 2 . 3D academic marine acquisition devices and their main characteristics

 2

	Device	Source	Receiver	Resolution	Target
	3D CHIRP
	(NOC)

ACKNOWLEDGEMENTS

We wish to acknowledge Captain and crew of the R/V Le Suroît, and Genavir seismic team for their support in achieving 3D survey through the Marmara maritime traffic. The support of the Turkish coast guards of the Sea of Marmara was also very valuable. This support at sea and the setting of the final location of the 3D survey using previous Turkish seismic data were achieved thanks to the Seislab team of the Dokuz Eylül University (Izmir). The Marmara-DM cruise was supported by ESONET NoE, Network of Excellence, coordinated by IFREMER and co funded by EU as part of FP6 (2009). We also thank the reviewers for their comments, which have helped to improve the manuscript.


A M Celâl Şengör

Céline Grall

Caner Imren

Xavier Le Pichon

Naci Görür

Pierre Henry

Hayrullah Karabulut

Muzaffer Siyako

Caner İmren

J Tuzo

The geometry of the North Anatolian transform fault in the Sea of Marmara and its temporal evolution: implications for the development of intracontinental transform faults

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

In 1965, J. Tuzo Wilson defined a new kind of fault that could not be accounted for by Anderson's theory of faulting in a continuous medium [START_REF] Anderson | The dynamics of faulting and dyke formation with applications to Britain[END_REF][START_REF] Wilson | A new class of faults and their bearing on continental drift[END_REF] paper was also the founding paper of plate tectonics). Wilson pointed out that the new class of faults served to connect two plate boundaries (the word 'plate' is Wilson's) with one another by transforming the motion along them into another kind of motion (Fig. 1). He therefore called them 'transform faults' [START_REF] Wilson | A new class of faults and their bearing on continental drift[END_REF]. Because plate boundaries are so cleanly delineated in the oceans, there is seldom any doubt as to the nature of faulting in them, but the scattering of deformation in the continents into areas of complex strain, in places as wide as 3000 km (as in Asia now), makes it difficult to decide to what kind of fault class some of the large strike-slip faults moving in such regions should be assigned.

In the case of the North Anatolian Fault [START_REF] Ketin | Über die tektonisch-mechanischen Folgerungen aus den großen Anatolischen Erdbeben des letzten Dezenniums[END_REF][START_REF] Ketin | Über die nordanatolische Horizontalverschiebung[END_REF][START_REF] Ketin | San Andreas ve Kuzey Anadolu Fayları arasında bir karşılaştırma[END_REF][START_REF] Pavoni | Die Nordanatolische Horizontalverschiebung[END_REF][START_REF] Mckenzie | Active tectonics of the Mediterranean region[END_REF][START_REF] Şengör | The North Anatolian transform fault: its age, offset and tectonic significance[END_REF][START_REF] Barka | The North Anatolian Fault zone[END_REF][START_REF] Barka | Slip distribution along the North Anatolian Fault associated with the large earthquakes of the period 1939 to[END_REF][START_REF] Şengör | The North Anatolian Fault: A new look[END_REF], it is clear that it connects the East Anatolian zone of convergence [START_REF] Şengör | The post-collisional tectonics of the Turkish-Iranian Plateau and a comparison with Tibet[END_REF][START_REF] Dewey | Shortening of continental lithosphere: the neotectonics of Eastern Anatolia -a young collision zone[END_REF][START_REF] Şengör | East Anatolian high plateau as a mantle-supported, north-south shortened domal structure[END_REF]2008) with the Hellenic Trench [START_REF] Dewey | Aegean and surrounding regions: Complex multiplate and continuum tectonics in a convergent zone[END_REF][START_REF] Mckenzie | The relationship between strain rates, crustal thickening, palaeomagnetism, finite strain and fault moments within a deforming zone[END_REF][START_REF] Le Pichon | Implications des nouvelles mesures de géodésie spatiale en Grèce et en Turquie sur l'extrusion latérale de l'Anatolie et de l'Egée[END_REF][START_REF] Şengör | İstanbul Boğazı niçin Boğaziçi'nde açılmıştır[END_REF]) and as such qualifies as a transform fault [START_REF] Şengör | The North Anatolian transform fault: its age, offset and tectonic significance[END_REF][START_REF] Le Pichon | The Miocene-to-Present Kinematic Evolution of the Eastern Mediterranean and Middle East and Its Implications for Dynamics[END_REF]. It consists of a numerous family of faults going from the well-defined Karlıova Basin in the East (Şengör, 1979; Şaroğlu, 1985; Şengör et al., 1985) to the more diffuse transtensional region in the Northern Aegean (Fig. 2). Throughout its length, its various branches have been mapped in the field (see especially [START_REF] Herece | Kuzey Anadolu Fayı (KAF) Atlası/Atlas of North Anatolian Fault (NAF): Maden Tetkik ve Arama Genel Müdürlüğü, Özel Yayınlar Serisi[END_REF], and it is generally agreed that the broad region of deformation, a veritable keirogen 1 , called the North Anatolian Shear Zone (Fig. 3; [START_REF] Şengör | The North Anatolian Fault: A new look[END_REF]Le Pichon et al., in press), in which the fault now moves, came into existence sometime during the later medial Miocene between the Serravallian and the Tortonian (see especially [START_REF] Şengör | The North Anatolian Fault: A new look[END_REF]Görür and Elbek, submitted; between the Sarmatian and the early Pannonian in the Paratethyan scale and between MN7/8 and MN10 in the European mammal stages; much of the deposits along the North Anatolian Shear Zone have been dated using these two latter standards). What so far has been unclear is how the broad keirogen became localised into the narrow, single-strand (double strand west of Yeniçağa near Bolu: Fig. 2) main displacement zone(s), constituting the North Anatolian Fault. To study the manner of that localisation, the Sea of Marmara appears to be an ideal location, because a large number of both multi-channel seismic and chirp profiles, shallow gravity cores (down to 30 m from the sea floor, some of which are now dated and enabled assessments to be made of the sedimentological events recognized in them: [START_REF] Beck | Late Quaternary co-seismic sedimentation in the Sea of Marmara's deep basins[END_REF]Eriş et al., 2012 , Çağatay et al., in press), a number of deeper hydrocarbon exploration wells, recent seismic studies including the water column itself, plus a complete, detailed bathymetric coverage are available (for references prior to 2005, see Şengör et al., 2005; 1 A keirogen (from ancient Greek κείρω = shear) is abroad strike-slip shear zone in which more than one major strike-slip fault and associated structures take up the shear and it is the strike-slip counterpart of the shortening orogens and extending taphrogens. It was first defined in Şengör and Natal'in, 1996, p. 639, endnote 8) [START_REF] Armijo | Submarine fault scarps in the Sea of Marmara pullapart (North Anatolian Fault): implications for seismic hazard in Istanbul[END_REF][START_REF] Carton | Études Tectoniques en Mediterranée Orientale par Analyse de Données de Sismique Réflexion: Mer de Marmara (Bassin de Çınarcık) et Marge du Liban: École Doctorale des Sciences de la Terre[END_REF][START_REF] Gazioğlu | Morphological features of major submarine landslides of Marmara Sea using multibeam data[END_REF][START_REF] Cormier | North Anatolian Fault in the Gulf of Izmit (Turkey): rapid vertical motion in response to minor bends of a nonvertical continental transform[END_REF]Seeber et al., 2006;[START_REF] Carton | Seismic imaging of the threedimensional architecture of the Çinarcik Basin along the North Anatolian Fault[END_REF][START_REF] Dolu | Quaternary evolution of the Gulf of Izmit (NW Turkey): A sedimentary basin under control of the North Anatolian Fault Zone[END_REF][START_REF] Géli | Gas emissions and active tectonics within the submerged section of the North Anatolian Fault zone in the Sea of Marmara[END_REF][START_REF] Laigle | A first deep seismic survey of the Sea of Marmara: Deep basins and whole crust architecture and evolution[END_REF]Bécel, 2009;[START_REF] Kuşçu | Cross-basin faulting and extinction of pull-apart basins in the Sea of Marmara, NW Turkey[END_REF][START_REF] Bécel | North Marmara Trough architecture of basin infill, basement and faults, from PSDM reflection and OBS refraction seismics[END_REF][START_REF] Özeren | Mathematical modeling of a potential tsunami associated with a late glacial submarine landslide in the Sea of Marmara[END_REF][START_REF] Şengör | Broad shear zones and narrow strike-slip faults in orogens and their role in forming the orogenic architecture: The North Anatolian Fault as an active example[END_REF][START_REF] Gasperini | Cold seeps, active faults and the earthquake cycle along the North Anatolian Fault system in the Sea of Marmara (NW Turkey)[END_REF][START_REF] Shillington | Evidence for widespread creep on the flanks of the Sea of Marmara transform basin from marine geophysical data[END_REF][START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF][START_REF] Zitter | Distribution, morphology and triggers of submarine mass wasting in the Sea of Marmara[END_REF]Le Pichon et al., in press).

Moreover, the Marmara Sea and its surroundings are frequently visited by earthquakes of devastating size with tragic consequences and the long historical records kept since at least the second half of the first millennium BCE (Fig. 4) give us an unparalleled knowledge of their repeat times and natures. That yet another imminent large earthquake is now threatening İstanbul, one of the megalopolises of the world and a cultural treasure house of mankind (it is the oldest continuously inhabited human dwelling in the world, since ∼250,000 BCE, when the cave dwellers discovered its advantages as a location: see [START_REF] Howell | Culture and biology at a Crossroads: The Middle Pleistocene Record of Yarımburgaz Cave (Thrace, Turkey)[END_REF] has focused the attention of geologists, geographers and geophysicists on this Sea after the disastrous 1999 earthquakes in İzmit and Düzce in northwestern Turkey (see Şengör et al., 2005). The fact that the Marmara area has been a part alternately of the Tethyan and Paratethyan realms since the beginning of the Miocene has given it a stratigraphy of a resolution higher than the normal Tethyan regions, because of the complete and abrupt change of biotas during every transition (so far used only for the youngest sequences and for the frame of the Sea, except where well information has been released by the petroleum industry). All of these factors make the Marmara region an ideal site to study the evolution of a strikeslip system below it.

We have critically reviewed the maps of the Marmara fault segments (e.g. [START_REF] Parke | Active faults in the Sea of Marmara, western Turkey, imaged by seismic reflection profiles[END_REF][START_REF] Parke | Interaction between faulting and sedimentation in the Sea of Marmara, western Turkey[END_REF]Le Pichon et al., 2003;[START_REF] Rangin | Strain localization in the Sea of Marmara: Propagation of the North Anatolian Fault in a now inactive pull-apart: Tectonics[END_REF], Grall et al., 2012, Sorlien et al., 2012) by using that database (Fig. 5A) consisting of 210 multichannel seismic reflexion profiles adding up to 6210 km profile length, so far unpublished except some segments of a number of the profiles that had been published by [START_REF] Ateş | Structural interpretation of the Marmara region, NW Turkey, from aeromagnetic, seismic and gravity data[END_REF] and high resolution bathymetry and chirp profiles reported in the literature to map all the faults that appear to cut the Miocene to Recent sedimentary rocks in the Sea of Marmara. We then distinguished those that cut only a portion of the sedimentary sequence and tentatively established a crude sequence of faulting events, according to the state of knowledge about the seismic stratigraphy of the Sea of Marmara (the crudeness being a function of the local paucity of direct litho-and biostratigraphic control). Finally, we have correlated the faults that cut the present-day sea-floor with the microearthquake epicentres between the years 2005 and 2011 plus the fault-plane solutions of all the earthquakes with magnitudes greater than 4 with a view to seeing which faults may have localised motion, and, where information is available, what kind, since 1960. In the sections that follow, we describe the faults illustrated in Fig. 5B and give examples of various faulting events established using seismic sequence stratigraphy.

Stratigraphic sequences that are related to 100 ka glacial/interglacial cyclicity over the last 450-540 ka are here depicted following the recent publications by Grall et al. (2012 and in press b) and [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF]. These authors show repetitions of onlap geometries (laterally correlated with condensed sections, Grall et al., in press b), and forset/toplap geometries [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF] and related them to variations of sedimentation rates during low-stand/high-stand transitions and low-stand deltas respectively. These authors use the following colour code associated to Marine Isotopic Stage (MIS) transitions: the Red MIS-5 (140 ka), the Blue MIS-7 (240 ka), the Yellow MIS-9 (340 ka), the Purple MIS-11(445 ka) and the Green MIS-13 (570 ka) transitions (Grall et al., in press b). We adopt this colour code, denoting by these colours the sequence on top of the sequence boundary. In this paper we designate our own figures with a capital F as Fig. and figures we cite from the liturature with a lower case f as fig.

THE EASTERN SECTOR Fig. 6 illustrates the faults we have mapped and Fig. 5A shows the locations of the multichannel seismic profiles that were used to identify them. Fig. 6 shows the eastern sector of our study area consisting essentially of the Çınarcık Basin (Le Pichon et al., 2001;[START_REF] Ateş | Structural interpretation of the Marmara region, NW Turkey, from aeromagnetic, seismic and gravity data[END_REF][START_REF] Carton | Études Tectoniques en Mediterranée Orientale par Analyse de Données de Sismique Réflexion: Mer de Marmara (Bassin de Çınarcık) et Marge du Liban: École Doctorale des Sciences de la Terre[END_REF][START_REF] Şengör | The North Anatolian Fault: A new look[END_REF][START_REF] Laigle | A first deep seismic survey of the Sea of Marmara: Deep basins and whole crust architecture and evolution[END_REF][START_REF] Uçarkuş | Active faulting and earthquake scarps along the North Anatolian fault in the Sea of Marmara[END_REF] and regions to its north [START_REF] Gökçeoğlu | Description of dynamics of the Tuzla landslide and its implications for further landslides in the northern slope and shelf of the Çınarcık basin (Marmara Sea, Turkey)[END_REF][START_REF] Özeren | Mathematical modeling of a potential tsunami associated with a late glacial submarine landslide in the Sea of Marmara[END_REF] for the neotectonics of the land area see Şengör, 2011) and south (including the İmralı Basin: [START_REF] Okay | Active faults and evolving strike-slip basins in the Marmara Sea, northwest Turkey: a multichannel seismic reflection study[END_REF][START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF]. Çınarcık Basin is a critical area because the exact manner of the westward prolongation of the faults from it to the Central High remains unclear.

In this sector we have chosen profiles 9 (Fig. 7) and 13 (Fig. 8) to illustrate the relative age sequence of the faults (for locations of the profiles, see Fig. 5A). Fig. 7 shows that along the profile 9, the main marginal fault cuts to the surface. The seismicity also shows it to be active localising almost exclusively right-lateral earthquakes (Figs. 6B and C;[START_REF] Örgülü | Regional moment tensor inversion for strong aftershocks of the August 17, 1999 İzmit earthquake (Mw=7.4)[END_REF][START_REF] Özalaybey | The 1999 İzmit earthquake sequence in Turkey: seismological and tectonic aspects[END_REF], 2003;Pınar et al., 2003). In Fig. 6, the focal mechanism solution 44 is not readily interpretable in terms of simple shear zone geometry, but it is still compatible with a tension gash orientation, albeit with a tiny sinistral component. We note that the term 'tension gash' is here used to denote a structure orientation as seen in micro-and meso-scales, where the structure was originally liquid-filled. The structures we describe are not necessarily 'tensional' but definitely 'extensional' with variable amounts of strike-slip associated with them (from 0% to 50% in some rare cases). As we have not read the seismograms ourselves we cannot further comment on that solution. If it is a 'tension gash' orientation normal fault only, then it is also compatible with pure strike-slip along the northern margin of the Çınarcık Basin.

The entire sedimentary fill seen in this basin is of Pleistocene age: In gravity core 27 (for location, see Fig. 7B), a radiocarbon age of 16.1 to 16.5 ky BP Cal. was obtained at a depth of 25 metres. In core 25, closer to the main bounding fault to the north, two ages of 14.3-16 and 15.0-16.5 ky BP Cal. were obtained from depths of 28 and 32 metres, respectively [START_REF] Beck | Late Quaternary co-seismic sedimentation in the Sea of Marmara's deep basins[END_REF]. They indicate a mean sedimentation rate of 1.5-2 mm/a during the Holocene within this basin. These ages make the entire sedimentary section as seen in Fig. 7 Pleistocene in age (Şengör et al., 2005) with the exception of the first 15 to at most 20 metres from the sea-floor, which represent Holocene sedimentation.

The fault to the south of the main fault in this profile (Fig. 7) cuts only up into a low amplitude layer at around 1.5 s twt, but does not penetrate farther up. On the evidence of the seismic profile that fault seems to have died within the Pleistocene (since 540-450 ka). Neither is that any evidence of its presence in the Chirp profile published by [START_REF] Zitter | Distribution, morphology and triggers of submarine mass wasting in the Sea of Marmara[END_REF]. However, there is seismicity correspondent to its location and available fault-plane solutions associated with that seismicity (solutions 33, 36, 39 in Fig. 6C) all indicate dextral strike-slip; the fault-plane solution 36 indicates a very slight extensional component either on a dextral Riedel shear (R) or on a sinistral anti-Riedel (R') shear, both entirely compatible with the main shear zone (the Y-shear direction) being a rightlateral one (see Fig. 9 for the terminology we use for various shear zone structures).

These earthquakes indicate a non-negligible strain being taken up on the fault that on the seismic profile 9 does not seem to reach the surface. The earthquakes' magnitudes are small enough and the data of the multichannel seismic profiles are coarse enough that the sea bottom's either not being breached or appearing not to have been breached is entirely understandable. That the fault does not show up even on the Marnaut 54 Chirp profile [START_REF] Zitter | Distribution, morphology and triggers of submarine mass wasting in the Sea of Marmara[END_REF] may suggest that in levels shallower than the green sequence the fault may branch out and even merge with the main boundary fault and reaches the surface via that route.

A south dipping fold and trust along the southern margin of the Çınarcık Basin separated two small basinal structures (Fig. 7 and Fig. 5B). Farther east there is an Ms 6.4 earthquake that gave a normal fault solution (solution labelled T in Fig. 7C: [START_REF] Taymaz | Active tectonics of the north and central Aegean Sea[END_REF] where shallow transtensive faults could be mapped on both bathymetry and chirp sub-bottom profiles, corroborating the view that at least some of the faults in this set are active (Fig. 5B). The microearthquake activity along these southern normal faults seems livelier than in the north since 2005, but the southern normal faults do not generate as large earthquakes as the northern main strike-slip strand, which is what is expected from their respective rates of motion (see Şengör et al., 2005).

Within the Çınarcık Basin, the northern master fault takes up by the largest portion of the movement which is nearly perfectly parallel with it as seen both from the earthquake fault plane solutions and from the GPS data (see Şengör et al., 2005 for a summary of the GPS vectors here) and there is no indication of major oblique-slip, contary to what is implied by Seeber et al. (2006), who over-estimated the oblique-slip component at the crustal level, as they did not take into account the compaction, which in a mature basin, could result in a seafloor subsidence 2 to 3 times larger than the subsidence of the basement. We have not taken into account in our study Hergert and Heidbach's (2010) modelling of the mechanical behaviour of the lithosphere in the Marmara Region which requires derivation of stresses. Such a derivation is hardly possible (except in a model), given the uncertainties concerning the properties of the real lithosphere and the stresses acting in it, but if one wished to invoke that model, the northern margin of the Çınarcık Basin would still require less than 4.5 mm/yr normal component of motion within its constraints, which we find reassuring.

Farther south, west of the Armutlu Peninsula, there is the southerly-to-southeasterlytilted İmralı Basin [START_REF] Okay | Active faults and evolving strike-slip basins in the Marmara Sea, northwest Turkey: a multichannel seismic reflection study[END_REF], also of Pleistocene age. Microearthquake activity (Fig. 6B andC) is much subdued here, only a few events falling directly on the profile. This shows that even the 'inactive' faults probably move, albeit rarely, during small magnitude earthquakes, but contribute essentially nothing to the further straining of the area. Along its easterly strike continuation, on the Samanlı Mountains (for their detailed geomorphology, see [START_REF] Bilgin | Samanlı Dağları-Coğrafî Etüd: İstanbul Üniversitesi Edebiyat Fakültesi yayını[END_REF], there is a livelier earthquake activity. The fault plane solutions available from there readily fall into two groups: a western group (fault plane solutions 16,45,47,49,51 in Fig. 6C) and an eastern group (solutions 53 and 54). It seems that the western group faults stretch the ground in a NNE-SSW direction and are related to those that created the İmralı Basin and the ones farther east cause extension in a NE-SW direction and are associated with the fault family that caused the Ms=6.4

Çınarcık earthquake farther north. Within the İmralı Basin, the main faults strike WNW-ESE and are related to the faults that cause the western group of earthquakes in the neighbouring Samanlı Mountains. But they are probably not mainly responsible for the origin of the İmralı Basin. Otherwise, the neighbouring Samanlı Mountains with similar and active faults would not have stood as high as they do now. Fig. 8A shows profile 13 which is farther west than the previous one (Fig. 7A) and Fig

8B our interpretation of it. The recognisable sequences indicate a much narrower basin

of a similar age to the one we saw farther east. Although this sub-basin is bounded by faults that cut to the sea-floor there is one between them that is much older, possibly pre-Pleistocene. The southerly-tilted basin here appears completely inactive, as none of the faults that appear related to its formation cut to the surface. The spread of microseismicity (Figs. 6B andC) corroborates this picture showing only a few scattered epicentres. [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF] have presented a meticulous and exhaustive analysis of the seismic stratigraphy of the Imrali Basin fill with a view to establishing a chronostratigraphy that they then used to argue that faulting continued at a constant rate since about half a million years ago. This is based on the date of what they call the red horizon, which has been correlated other most of the Sea of Marmara and has been attributed an age of 109 ka based on extrapolation from Marion-Dufresne cores [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF]Grall et al., in press b). Lower reflectors have been correlated to low stand deltas and age attributed with the assumption that the sediment input into the Imrali basin has been of the same order for each glacial cycle. However, this reasoning also led to the conclusion that the low-stand delta observed in stratigraphic succession do not systematically correspond to successive glacial cycles. In the absence of independent age constraints, ambiguities in the age attributions thus cannot be resolved. In detail, [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF] age model taken as face value does suggest some slowing down of the subsidence over the last 130 to 330 ka once compaction is taken into account. In both profiles it is difficult to ascertain whether the basin fill of the Çınarcık and the İmralı basins is conformable or unconformable with their stratigraphic basement. Because at least one fault is truncated by the bottom sequence (Fig. 8B) and some reflectors below the lowest sequence seem to have a geometry in places not exactly parallel with the basement layering, we assume an unconformity bounding the basin at the bottom, which is, however, not easy to defend conclusively on the basis of the available observations. Interestingly, once one goes southward over the Samanlı Mountains on the Armutlu Peninsula (SA in Fig. 5A), the microseismicity again becomes lively in the Gulf of Gemlik (GG in Fig. 5A). A major fault strand, which we here term the Gemlik-Bandırma Fault, goes from there to the Erdek tombolo (ET in Fig. 5) and then turns southwestward in the county Gönen into the Biga Peninsula (Fig. 5). Young scarps all along the fault suggest recent activity and at least two estimated M=7 earthquakes (128 and 543 CE) seem to have taken place on it during historical times (Fig. 4). [START_REF] Kuşçu | Recognition of active faults and stepover geometry in Femlik Bay, Sea of Marmara[END_REF] mapped a group of active faults in the Gulf of Gemlik and microearthquakes align along smaller faults on the isthmus separating Lake İznik and the Gulf of Gemlik, all betraying a lively activity although not accomplishing much strain (no earthquakes above magnitude 4), except obviously at intervals of a minimum or a millennium-anda-half. Although [START_REF] Barka | Strike-slip fault geometry in Turkey and its influence on earthquake activity[END_REF] called it the 'middle branch of the North Anatolian Fault', it is much less significant than the two major strands originally identified by [START_REF] Dewey | Aegean and surrounding regions: Complex multiplate and continuum tectonics in a convergent zone[END_REF] and Şengör (1979) and is not much different from the many deactivated faults populating the broad southern shelf of the Sea of Marmara (Fig. 5A). Therefore, we go along the recent trend in following [START_REF] Dewey | Aegean and surrounding regions: Complex multiplate and continuum tectonics in a convergent zone[END_REF] and Şengör (1979) and stick to two major strands, but we nevertheless point out that the Gemlik-Bandırma fault does maintain an activity, although much subdued.

THE CENTRAL SECTOR

This sector (Fig. 10), as herein used, corresponds mostly to what is called the Central High in the literature (it is the high region that completely surrounds the Kumburgaz Basin, KB; Le Pichon et al., 2001;[START_REF] Şengör | The North Anatolian Fault: A new look[END_REF] but that also includes the Kumburgaz Basin and almost the whole of the Central Basin (Le Pichon et al., 2001;[START_REF] Şengör | The North Anatolian Fault: A new look[END_REF]. The presence of the Kumburgaz Basin atop the Central High provides a sediment receptacle and thus a welcome means to date the faults cutting the Central High relative to one another (see especially Sorlien et al., 2012, fig. 6).

Profile 23 (Fig. 11A andB) is located in the eastern and central part of the basin and shows that both its bounding faults there cut to the surface (Fig. 11B). The basin also has other faults in it, but they seem to have died shortly after the basin initially formed. We also see that the basin was initially narrower. However, the faults that bound the basin on this profile are not parts of a single fault family: the northern fault is along the strike of the northern boundary fault of the Çınarcık Basin coming from the east, here forming a part of the main zone of displacement along the northern branch of the North Anatolian Fault (Şengör et a., 2005). The southern fault, however, has an WNW strike and seems to be a somewhat rotated, extensional Riedel shear so identified simply on account of its orientation. Immediately to its south there is another fault similar to it that also bounds the basin. Both of these faults cut to the surface and both are active as shown by the microseismicity they localise. To their east, the southern basin margin seems unfaulted as can be seen on fig. 6 by [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF]. [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF] carry what they call the Red-1 reflection into the Kumburgaz Basin where it is clearly cut twice, whereas in the İmralı Basin it is not, where it records the lower boundary of a non-faulted floor of a sedimentary infill. Therefore, although a throughgoing main strike-slip fault appears to have already formed in the Kumburgaz Basin, the auxiliary faults, such as the two Riedels just mentioned, appear not to have not terminated their activity. They probably do not localise major shocks (no earthquakes above magnitude 4) and therefore do not take up much displacement, but they nevertheless remain active and it is hard to tell up to what size earthquakes they might one day localise.

Profile 27 (Fig. 12A andB) shows a much wider basin, of almost bovine-head shape when considered together with the narrower fault trough underlying it, and numerous faults cut its most recent sequence. Although the major displacement is along the northern master fault bounding all the sequences and reaching the sea-floor, it seems that during the deposition of the youngest sequence, the basin had already become much wider. The numerous faults to the south, sub-parallel with the two previouslymentioned Riedel shears, are all either active or terminated their activity very recently during the deposition of the top sequence. Many of these faults seem to be short, therefore insignificant, in terms of the displacement they can accomplish, but they nevertheless are mostly active.

When we look farther south, an active fault that strikes about N45ºW, thus in an extensional orientation in the overall North Anatolian Shear Zone, is seen to be active with microearthquake epicentres on it. Yet it sits perfectly on strike with, and seems to be the northwesterly continuation of, a major inactive fault extending down into the Gulf of Gemlik.

Oblique (N120°-N140°) transtensional blind and deep-seated faults, identified in the western part of the Sea of Marmara, are part of a northeast dipping extensional fault system named the South Transtensional Zone (STZ) by [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian Fault[END_REF]. Many of these faults appear still active, although we do not know what portion of the total shear they accommodate. In particular, the western part of this fault system is suspected to play a rôle in the landslide activity in the Ganos area (in our western sector: [START_REF] Zitter | Distribution, morphology and triggers of submarine mass wasting in the Sea of Marmara[END_REF]. The long fault mentioned in the previous paragraph is the partly active eastward termination of this transtensional system providing a connection between the main northern branch of the North Anatolian Fault and the fault system to the south recently named the Southern Marmara Fault by Le Pichon et al. (in press). ~N100° striking, mainly wrench faults occur also in the eastern part of the Sea. These faults, which may be blind, are probably a part of the Central Fault System (CFS, [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian Fault[END_REF]. Other, shorter, faults close to and parallel with the inactive strand are, by contrast, still active.

A small family of short faults with the same orientation to the east of the meandering submarine canyon seen in Fig. 10C Curiously, the longer E-W orientated faults along Y-shear directions (cf. Fig. 9), some turning into Riedel and P-shear orientations at their ends, seem in some segments active and in others inactive.

Farther west, within the Central Basin, the microearthquake activity shows that the smaller spindle-shaped basin within the Central Basin is where most of the present activiy is located, but the northeastern border of the larger basin is also active (Figs. 10 B andC). The active, spindle-shaped basin within the larger Central Basin is in fact where the Holocene subsidence rate is the highest in the Sea of Marmara (reaching more than 7 mm/a, [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian Fault[END_REF]. This centralised subsidence is in stark contrast to the situation encountered in the Çınarcık and the Kumburgaz basins, where the main displacement zone is along the northern master faults of the basins.

So far, we had encountered no active faults in a shortening orientation. We encounter such a fault for the first time here, along the main displacement zone of the North Anatolian Fault in the Sea of Marmara just east of the Central Basin where the main strand is connected to the small spindle-shaped basinette in the middle of the Central Basin (Fig. 10B).

Within the southern Marmara Shelf, the available seismic profiles show that all the faults terminated their major movement at the end of the early Pliocene (3.5 Ma), but there is sparse, broadly spread microseismicity (Le Pichon et al., in press).

THE WESTERN SECTOR

The western sector (Fig. 13) houses what is termed the Western High and the Tekirdağ (ancient Rodosto) Basin. We selected two profiles to show here, one across the High (profile 70: Fig. 14A andB) and another, across the Basin (profile 78: fig. 16A andB).

Profile 70 (Fig. 14A) shows a much-faulted anticlinal ridge rising in the deepest part of a trough-shaped basin that is not seriously faulted at either end. Although what faults there are dip towards the basin, their vertical displacements are insignificant compared with the marginal faults of the Çınarcık, Kumburgaz and the Central basins. The basin in fact has the appearance of a sag in the midst of which the peculiar anticlinal ridge of the Western High rises. It seems that the only active faults are those that bound the Western High, the southern with a higher rate of vertical motion as judged from the thickness of the green sequence. It is that fault which coincides with the main displacement zone of the northern branch of the North Anatolian Fault here. Some of the faults that cut the high are active, others dead and yet both dead and active faults seem to belong to the same set of steep thrusts. This is another region where we see active faults in a thrust orientation. In fact, the entire geometry of the Western High, with its flanking basins, gives the impression of a shortening structure that originally consisted of a broad synclinal basin, in the middle of which an anticline later rose. This is also supported by the topography of the sediment/acoustic basement interface, which reveals a deep trough which extends from the Central Basin towards the east and to the Tekirdağ Basin towards the West, such as Bayraçkı et al. (in press) recently depicted. It seems that the throughgoing faulting chose the anticline to follow and in so doing sliced it with small and steep thrusts.

We have no data as to the nature of the sequences seen in the seismic profiles across the Western High making up its foundation. It is however very likely that they consist of the Ganos Flysch wedge that delimits the Thrace Basin to the south and the Mio-Pliocene gently-dipping sedimentary rocks covering the flysch wedge as one sees on the Gallipoli Peninsula, where even deeper portions of the accretionary wedge crop out (e.g., [START_REF] Şentürk | Blueschists discovered east of Saros Bay in Thrace[END_REF] report blueschists that crop out from under the flysch and the surrounding younger Cainozoic rocks)

The second profile, 78 (Fig. 15), goes across the Tekirdağ Basin. In principle it shows the same structures that one sees in the Western High, except the central anticline is here asymmetric and north-vergent, delimited against the northern, deeper half of the basin by a very steep, essentially vertical fault. [START_REF] Okay | An active, deep marine strike-slip basin along the North Anatolian Fault in Turkey[END_REF] interpreted it as a southvergent thrust, which is contrary to what is seen along profile 78 (Fig. 15A). The southern part of the anticline is not broken by faults (except two very shallow landslide faults) but has the form of a syncline. The sequences betray a faster subsidence in the basin centre than along its sides consistent with the syncline interpretation.

The steep fault is a continuation westwards of the main displacement zone. The smaller large fault to the north achieves not much of a vertical displacement and seems a part of a series of short, en-échelon faults that seem an abortive attempt to reach the northern end of the Ganos (MG in Fig. 13A) constraining bend thrust zone [START_REF] Dewey | Aegean and surrounding regions: Complex multiplate and continuum tectonics in a convergent zone[END_REF][START_REF] Şengör | The North Anatolian transform fault: its age, offset and tectonic significance[END_REF]; see Fig. 18 herein) by tearing a strike-slip fault from the northern basin north of the Western High across the northern part of the Tekirdağ Basin. That attempt seems to have been abandoned, however, while the youngest sequence was being deposited. There is essentially no microseismic activity along that trend of small faults.

Two other short trends of faults on the basin floor to the east of the middle part of the Ganos constraining bend suggest other attempts at reaching it farther north than its present southernmost end. Although both attempts seem to have failed, the latter group of faults are still active as indicated by a lively microseismic activity at their eastern ends, but this is probably because of the loading of the crust in which they reside by the Ganos Wedge (see Fig. 16 for the cross-sectional geometry of the Ganos overthrust zone along the restraining bend).

At the latitude of the Western segment, seismic activity on the South Marmara Shelf is considerably livelier than in the Central Segment. Here we have in fact four earthquakes with magnitudes larger than 6 (Fig. 17) on the Biga Peninsula, the western continuation of the southernmost part of the South Marmara Shelf. The microearthquake activity is also much higher than it is in the Central Sector.

In this sector, the Marmara Island (the ancient Proconnesos: the largest island in Fig. 17) is a divider: to the east nearly all major faults strike W to NW; to its west nearly all strike NE, indicating that the shear zone takes a fairly sharp bend to the southwest with accompanying appearance of major thrust earthquakes along the fault segments. The faults also acquire here a much greater continuity than they have in the east, as one would expect from thrust faults as opposed to normal faults which are numerous east of the Marmara Island. Interestingly, west of the Marmara Island all the major strands are active, presumably because here the 2.5 cm/a motion of the Marmara Block delimited by the northern and the southern strands of the North Anatolian Fault (Le Pichon et al. 2003) must be compensated in part by shortening and it is more economic to distribute the shortening and consequent crustal thickening on many faults.

However, here too the strike-slip faults display their characteristic complex strains by localising earthquakes of all three kinds of fault types (Fig. 17)!

DISCUSSION

Temporal evolution of the North Anatolian Fault in the Sea of Marmara

When we started this study, we were expecting, as expressed in Şengör et al. (2005), an orderly evolution of a shear belt in which an initially broad shear zone characterised by such 'secondary' or 'auxiliary structures' as Riedel (R) and Anti-Riedel (R') shears, X, and P shears and tension gashes (T) with shortening structures( S) gradually evolving and eventually abandoning the field of deformation to a narrow thorough-going fault consisting of segments of Y shears, much as in the experiments of [START_REF] Riedel | Zur Mechanik geologischer Brucherscheinungen[END_REF], [START_REF] Tchalenko | Similarities between shear zones of different magnitudes[END_REF], [START_REF] Wilcox | Basic wrench tectonics[END_REF], [START_REF] Bartlett | Experimental folding and faulting of rocks under confining pressure[END_REF], [START_REF] Naylor | Fault geometries in basementinduced wrench faulting under different initial stress states[END_REF] and [START_REF] Ahlgren | The nucleation and evolution of Riedel shear-zones as deformation bands in porous sandstone[END_REF] and as depicted in our Fig. 9. What we have found instead is a much more haphazard development. If one separates the fault families expected from the experiments into R, R', X, P and Y shears and T and S structures according to their orientations and, wherever available, from earthquake first-motion solutions and stratigraphic vertical separations and note their sequence of occurrence in the analogue experiments as R', R, P, X and, if the material being sheared is suitable to form them, first S and then T structures, Nature shows us that in the Sea of Marmara almost all of these structure types are active now. We have actually seen no X shears that can be confidently identified and some of the fault segments bending into a P-shear orientation are all now dead. What is even more peculiar is the observation that some faults may 2003) documented that the small strain was by no means negligible and is detectable by GPS observations. However, [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF] thesis that the faults have moved with a constant rate during the last half a million years, which implies in view of the ~ 55 km total offset for the entire Marmara area [START_REF] Akbayram | The cumulative offset of North Anatolian Fault in the Marmara region, northwest Turkey: EGU General Assembly[END_REF][START_REF] Şengör | The North Anatolian Fault: A new look[END_REF] that the Sea of Marmara has a total history of less than half a million years, which is certainly incorrect.

The difference between the experiments and Nature as it appears to us in the neotectonics of the Marmara region may be twofold: one is the time scale. What happens in nature in thousands to hundreds of thousands and even a couple of million years, happens in experiments in a matter of minutes or at most hours. We may be seeing a process of slow demise of the auxiliaries in Nature, which to us seem a time of activity in concert. In reality, the auxiliaries may be slowly passing away as the main displacement zone gathers the action onto itself. The fact that nearly all the main displacement today is concentrated on the almost completely throughgoing main displacement zone may be a result of its gradual climb to prominence, a climb that has not yet reached the summit.

The second difference pertains to the extreme heterogeneity of the natural material which the shear zone deforms as opposed to the nearly homogeneous materials used in the experiments. No experiment can hope to reproduce the multifarious natures of the materials making up the area in and around the Sea of Marmara. Entire domains consisting of very different types of materials dominate the Marmara area and the developing shear zone no doubt exploited every bimaterial surface to break through on all sorts of scales. That is mainly why we have been unable to use with confidence the kind of modelling represented by [START_REF] Hergert | Geomechanical model of the Marmara Sea region -II. 3-D contemporary background stress field[END_REF] paper, where rock properties in the sedimentary carapace are accepted to be distributed homogeneouslywhich is manifestly not the case, if for no other reason than the depth differences among basin basements, even within a single basin-and the great variations in the rock properties in the basement are not even considered. Şengör et al. (2005) pointed out that the North Anatolian Fault as a whole seems to have nucleated along the northern 'wall' of the Tethyside accretionary complexes against the more indurated basement of the Rhodope-Pontide Fragment (see Fig. 2), very much corroborating the theoretical model of Ben Zion and Andrews (1998) and Ben Zion (2001). There is little doubt that the sensitivity of bimaterial surfaces to preferentially produce rupture under stress is also present at much smaller scales between the tectonic units made up of different materials under and around the Sea of Marmara and between different rock packages within those tectonic units.

The origin of the Marmara Deeps

Both the Western and the Central Highs have east-northeasterly trends. Fig. 14A shows the Western High having an overall anticlinal structure albeit sliced by numerous faults. We had originally hypothesised that the anticline was a function of the shear evolution and asked ourselves that if we reverse the shear along the North Anatolian Shear Zone, what orientation the high would acquire. Our expectation was that it would rotate into a more northeasterly direction and we expected a similar result from the Central High. Fig. 18A shows the present day fault geometry within the Sea of Marmara and the outlines of the two highs in blue. Fig. 18B shows the situation if a homogeneous strain is imposed on the system to reverse a 55 km offset. Because the overall ofset along the North Anatolian Shear Zone is 75 to 80 km along its entire length (Şengör et al., 2005), we subtracted the offset of some 26 km seen along the Pamukova strand leading to the main southern strand (Şengör et al., 2005) from that to obtain the 55 km left-lateral offset on the entire Sea of Marmara fault family to reverse the present right lateral shear for the purposes of Fig. 18. This seems consistent with the 52 km offset reported by [START_REF] Akbayram | The cumulative offset of North Anatolian Fault in the Marmara region, northwest Turkey: EGU General Assembly[END_REF] from the northern strand east of the Sea of Marmara. The timing of the offset reported by [START_REF] Akbayram | The cumulative offset of North Anatolian Fault in the Marmara region, northwest Turkey: EGU General Assembly[END_REF] is problematic, however, because some of it could have formed just after the medial Eocene collision along the Intra-Pontide suture in a completely different system of strike-slip faults [START_REF] Akbayram | The evolution of the Intra -Pontide suture: implications of the discovery of late Cretaceous -early Tertiary melanges[END_REF]. It is clear, however, that at least the Marmara Fault family north of the South Marmara Shelf takes up a displacement that farther east is concentrated on the northern strand of the North Anatolian Fault and that offset is an absolute maximum of 57 km. Fig. 18C shows what is seen when we superimpose Figs.

18A and 18B keeping the northern boundary fixed. The highs do not rotate as we expected, but acquire a lumpier appearance! In fact, as a result, they present an optical illusion as if they rotated into a more E-W orientation when the displacement is reversed. This to us suggests that the highs, or their much more subdued forerunners, were probably there before the shear began and were similar to gentle axial culminations along outer arc highs in present-day forearcs, in this case possibly either accentuated or even initially caused by the collision. When the shear began they were shortened into S structures and became elongated in an ENE orientation. The Western High was probably only a gentle 'suggestion' with a broad basin, a simple 'undulation,' of the basin floor, which later became accentuated as the shear evolved. The fig. 6 in Sorlien et al. (2012) very nicely shows the anticlinal structure of the Central High and the smaller wavelength folds right across it involving even the youngest sediments, although there is a clear decrease in folding amplitude up section (the reverse of what one sees in the so-called 'generative folds'; our sorts of folds may thus be called 'degenerative') indicating a waning of the shortening commensurate with the concentration of the shear along the northern branch of the North Anatolian Fault.

The Çınarcık Basin is clearly not a part of this system as indeed it does not sit in front of the Thracian fore-arc basin, but in front of the more solid and homogeneous İstanbul Zone (Şengör 2011, fig. 1) and in a totally different orientation, viz WNW. We think that the interpretation of it by [START_REF] Uçarkuş | Active faulting and earthquake scarps along the North Anatolian fault in the Sea of Marmara[END_REF] as a fault wedge basin sensu [START_REF] Crowell | Origin of late Cenozoic basins in southern California[END_REF] is so far the best explanation for the origin and the present structure of the basin, so we do not further elaborate on it here, except to say that it is a completely different sort of basin from the other, more westerly, basins of the Sea of Marmara. (with the exception of the present microseismic activiy), but that more activity is seen in the northern auxiliary structures, on both seismic and stratigraphic [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF] evidence, may be an indication of the fact that the displacement is gradually gathering itself to the northern branch alone.

The offset along the northern branch of the North Anatolian Fault

CONCLUSIONS

The present day fault families younger than about the medial Miocene in the Sea of Marmara are all parts of the North Anatolian Shear Zone and they all seem to have shared among themselves a total offset of some 55 km along this zone since the later medial Miocene. Contrary to expectation from analogue claycake and sandbox experiments, a regular development from distributed structures to a single, throughgoing strand cannot now be established, but this may be more a function of the vastly different time-scales between the experiments and Nature herself than a fundamental difference in the evolutionary sequence. That the Southern Shelf fault family is now dead and that no major earthquake seems to be nucleated on the auxiliaries (except in the southern part of the Çınarcık Basin) may indicate a gradual localisation of the offset along the single strand of the Northern Brach of the North Anatolian Fault.

This study gives support to the older ideas that intracontinental transform faults begin their lives as broad shear zones and become gradually converted into single-(or at most a few-) strand structures. What such a broad shear zone at depth looks like may be displayed by the 15-km wide mylonite zone seen along the giant Irtysh shear zone (displacement ≈ 2000 km) in Central Asia [START_REF] Şengör | Palaeotectonics of Asia: Fragments of a synthesis: in Tectonic Evolution of Asia[END_REF]. The numerous active fault zones we see at the surface no doubt broaden with depth (earthquake hypocentres are hardly deeper than some 10 km under the Sea of Marmara; below that the deformation is most likely ductile) and given their density at the surface, they most likely coalesce into a very broad ductile shear zone. Le Pichon et al.'s (2003) study has corroborated the presence of such a general, distributed deformation south of the Northern Branch of the North Anatolian Fault. Fig. 19 shows a situation where the faults depicted on Fig. 18A have been widened to shear zones of some 4 km width, the presumed with of the San Andreas Fault below 10 km (e.g., [START_REF] Mcbride | Reanalysis of the cocorp deep seismic reflection profile across the San Andreas fault, Parkfield, California[END_REF]. In these regions of amphibolite grade metamorphism what a shear zone looks like is shown in the inset from the Diancan Shang late Cainozoic metamorphic shear zone Ailao-Shan along the Red River strikeslip fault in Southeast Asia (copied from [START_REF] Cao | Exhumation of the Diancang Shan metamorphic complex along the Ailao Shan-Red River belt, southwestern Yunnan, China: Evidence from 40 Ar/ 39 Ar thermochronology[END_REF]. We have flipped Cao et al.'s figure to make the shear right-lateral and the similarity of the hand sample to the map pattern displayed by the presumed depth-map of the Sea of Marmara fault family is staggering: it is one of the best examples of Pumpelly's rule that small structures mimic big structures and a further demonstration of [START_REF] Tchalenko | Similarities between shear zones of different magnitudes[END_REF] point that shear structures are independent of scale. . We thus think that at a depth below 10 km, the Sea of Marmara basement consists of sheared rocks of the kind seen below the Ailoa Shan strike-slip fault, which, by an interesting coincidence, is also nucleated on a suture zone bearing steep serpentinite screens [START_REF] Şengör | Palaeotectonics of Asia: Fragments of a synthesis: in Tectonic Evolution of Asia[END_REF]Burchfiel and Chen, 2012, chapret 15, especially fig. 15-1). In deeply-eroded Precambrian terrains, the presence of such broad, vertical, anastomozing ductile shear zones are most likely expressions of large strike-slip faults at the surface, many of which were probably transform faults. As transform faults are dependable indicators of relative plate motion directions, this is one way to get a handle on such old plate motion directions. (Wilson, 1965, fig. 3). For 'arc' read 'trench' to m), which is consistent the mean sedimentation rates in this area (Seeber et al., 2006); as well as the sequences jump correlated by Sorlien et al., (2012, fig. 8). C. Same as Fig. 8B, but with no vertical exaggeration to show the 'real' geological structure. We have done this only for this profile to give an idea about the difference between the vertically exaggerted and non-exaggerated sections for those not used to looking at seismic profiles. horizon (also drawn on the fig. 4 in [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF]. We assigned the same sequence boundary as [START_REF] Sorlien | Uniform basin growth over the last 500ka, North Anatolian Fault[END_REF] in this area, and we note that onlap geometry on the sequences boundaries, are also observed in this basin. However, this is only an instantaneous picture, because as the shear zone evolves, all structures will rotate and strains will increase as we now see in the Sea of Marmara case. Key to lettering: F is fold axial trace, P is P-shear, R is a Riedel shear, R' is an anti-Riedel shear, T is a tension gash (in meso and macro examples, depending on the complexity of the stystem, the stress along the 'tension gash orientation' line may not necessarily be purely tensional, but there must be a major component of extension), X is an X shear. φ is the angle of internal friction of the material being sheared. Our interpretation of the geology along line 78. Note the fault that bounds the anticline to the north. It now seems vertical, but the internal structure of the anticline implies that it might have originated as a very-steeply south-dipping thrust fault, contrary to the interpretation by [START_REF] Okay | An active, deep marine strike-slip basin along the North Anatolian Fault in Turkey[END_REF]. 1.5 s of coherent sedimentary section is imaged across the Tekirdağ Basin, on which onlap sequences (indicated by black arrows) occur periodically, as in the Central Basin [START_REF] Grall | Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian Fault[END_REF], and in the Çınarcık (Fig. 8A). The first Red onlap surface (dated at ~130 ka) is at around 300 ms twt (roughly 230 m) below the seafloor within the depocentre. This age estimation is thus consistent with the mean sedimentation rates (1.5-2mm/a) derived from the last marine/lacustrine transition (Mercier de Lepinay, pers. comm). The transparent lens in green (430-540 ka interval) in the basin as well as in the bounded anticline, looks very similar to the one encountered on the Western High and elsewhere in the Sea of Marmara (Grall et al., in press b). The Red and the Blue horizons have been correlated from the basin to the southern anticline across the faults. The Red Horizon is tentatively proposed on the base of sediments affected by gravitay-gliding, as it has been shown that main slides occur preferentially during glacial/interglacial transitions (Grall et al., in press a).

FIGURE CAPTIONS

Nonetheless, we cannot exclude that the prominent reflector on the base of this sliding mass, which apparently functions as a décollement could have no stratigraphic meaning.

Besides, it is also probably interesting to note that across this basin as across other deep basins, no mass transports deposits have yet been recognized on top of the violet horizon, despite the fact that they have been recognized in the ponded basins of the Western High (Grall et al., in press b), as well as on the margins of the basin within which the Western High seems to rise (Fig. 13B). F I G U R E S

  are all active and clearly localise microearthquakes. By contrast, some abutting against the bounding faults of the spindle-shaped basin within the Central Basin appear dead, perhaps because they have been recently decapitated by the main northern branch of the North Anatolian Fault. All of the faults belonging to Grall et al.'s Central Fault System belong to the tension-gash-orientation (refer to Fig. 9 to see what that orientation means) normal faults related to the North Anatolian Shear Zone.

  have both dead and active segments along them (within the time frame of a part of the Pleistocene, mostly even late Pleistocene-Holocene)! An extreme example has been recently documented by LePichon et al. (in press), who showed that what they called a South Marmara Fault under the late Pliocene beds in the South Marmara Shelf appears entirely inactive there at least for the last 2.5 or 2.6 Ma, yet both at its eastern and western ends it is today active![START_REF] Kuşçu | Recognition of active faults and stepover geometry in Femlik Bay, Sea of Marmara[END_REF] showed that the faults in the Gulf of Gemlik are active and the prolongation of the South Marmara Fault into the Aegean in the west just localised a Mw 6.2 dextral slip earthquake on 8 th January 2013(Le Pichon et al., in press)! This suggests that some dead fault strands may be connected to active ones by as yet unrecognised intermediate segments.An extremely interesting observation has been reported recently by[START_REF] Shillington | Evidence for widespread creep on the flanks of the Sea of Marmara transform basin from marine geophysical data[END_REF]: they showed the presence of folds forming by downslope creep on steep (3º to 10º) slopes in and around the main trough of the Sea of Marmara. Interestingly, with the exception of the slope north of the Armutlu Peninsula and to the south of the Central Basin, all of the slopes on which such creep folds have formed and in places continue to form are on the slopes of structures that are in a shortening orientation and have been so since the shear began. The Çınarcık exception is easy to explain in terms of the active normal faulting going on at the foot of the Armutlu Peninsula. The slope south of the Central Basin may have been accentuated by the faults that there exist along the South Marmara Shelf (see LePichon et al., in press), but the data there are not adequate to address this question.[START_REF] Shillington | Evidence for widespread creep on the flanks of the Sea of Marmara transform basin from marine geophysical data[END_REF] work is a very nice possible example of structures forming concurrently with growing folds on a large scale, perhaps not dissimilar to the situation as documented by the progressive growth in time (late Cretaceous to Eocene) of the Hathira (Makhtesh-Hagadol or Kurnub) anticline in southern Israel[START_REF] Eyal | The Syrian Arc Fold System: age and rate of folding[END_REF].Although most of the elements of the shear zone in the Sea of Marmara are active now, the strain they accomplish is extremely variable. It seems that almost all the motion of the Eurasia/Marmara Block (the area delimited by the northern and the southern strands of the North Anatolian Fault: Le Pichon et al., 2003) is taken up by the Y-shears making up what Le Pichon et al. (2001) and Şengör et al. (2005) termed the main displacement zone forming, from east to west, the main northern bounding faults of the Çınarcık and the Kumburgaz basins, the fault cutting through the Central Basin and bounding the small spindle-shaped basin in the middle, and the fault delimiting the Western High and the Tekirdağ Basin to their south. All other active faults seem to cut to the sea-floor and nucleate microseismic shocks with few exceptions, such as the normal faults south of the Çınarcık Basin that nucleated an Ms=6.4 earthquake and possibly another one in 1894 that greatly damaged İstanbul. They contribute very little to the overall strain, yet Le Pichon et al. (

Fig. 18 C

 18 Fig. 18 C further shows that the statement by Le Pichon et al. (2001) that the offset along

Fig. 1 .

 1 Fig. 1. Wilson's illustration of the kinds of transform faults(Wilson, 1965, fig. 3). For

  Fig. 2. The North Anatolian Fault, forming in reality a family of faults that constitute the

Fig. 3 .

 3 Fig. 3. The North Anatolian Shear Zone (NASZ: delimited by discontinuous lines) and

Fig. 4 .Fig. 6 .

 46 Fig. 4. Historical earthquake epicentres in and around the Sea of Marmara compiled by

Fig. 8A .

 8A Fig. 8A. Line 13 showing another cross section farther west than line 9 across the

Fig. 9 .

 9 Fig. 9. Development of a shear zone (from Şengör, in press). Brown areas are structures

Fig. 10 .

 10 Fig. 10. Maps of the central sector of our study area: A. Location map of the profiles

Fig. 12A .

 12A Fig. 12A. Line 27 showing another cross section farther west than line 23 across the

Fig. 14A .

 14A Fig. 14A. Line 70 showing a cross section across the Western High showing its faulted

Fig. 15A .

 15A Fig. 15A. Line 78 showing another cross section farther west than line 70 across the

Fig. 16 .

 16 Fig. 16. Profile 84 across the Mt. Ganos shelf and slope showing the underthrusting of

Fig. 17 .

 17 Fig. 17. The locations, magnitudes and the first-motion solutions of the major shocks

Fig. 18A .

 18A Fig. 18A. A simplified map of the faults reported in this paper. The two light blue

Fig. 19 .

 19 Fig. 19. The presumed geometry of the fault network depicted in Fig. 18A at a depth of

ACKNOWLEDGEMENTS

This paper was written upon the invitation of Professor Ali Polat to whom we are grateful for the honour of being included in a volume in memory of Tuzo Wilson.

Professor Namık Çağatay was a great source of knowledge and critique in all matters stratigraphic. MARSITE funding made substantial parts of this study possible. All marine work in the Sea of Marmara since 2000 forming the basis of this study was undertaken with the active and cheerful support of the Turkish Navy. Detailed and constructive reviews by Professors Kevin Burke and Mark Zoback substantially improved the original presentation.