Asymmetric slip partitioning in the Sea of Marmara pull‐apart: a clue to propagation processes of the North Anatolian Fault?

Between 1939 and 1999 the North Anatolian fault (NAF) experienced a westward progression of eight large earthquakes over 800 km of its morphological trace. The 2000-km-long North Anatolian transform fault has also grown by westward propagation through continental lithosphere over a much longer timescale (∼10 Myr). The Sea of Marmara is a large pull-apart that appears to have been a geometrical/mechanical obstacle encountered by the NAF during its propagation. The present paper focuses on new high-resolution data on the submarine fault system that forms a smaller pull-apart beneath the Northern Sea of Marmara, between two well-known strike-slip faults on land (Izmit and Ganos faults). The outstandingly clear submarine morphology reveals a segmented fault system including pull-apart features at a range of scales, which indicate a dominant transtensional tectonic regime. There is no evidence for a single, continuous, purely strike-slip fault. This result is critical to understanding of the seismic behaviour of this region of the NAF, close to Istanbul. Additionally, morphological and geological evidence is found for a stable kinematics consistent both with the long-term displacement field determined for the past 5 Myr and with present-day Anatolia/Eurasia motion determined with GPS. However, within the Sea of Marmara region the fault kinematics involves asymmetric slip partitioning that appears to have extended throughout the evolution of the pull-apart. The loading associated with the westward propagation process of the NAF may have provided a favourable initial geometry for such a slip separation.     

Structural interpretation of the Marmara region, NW Turkey, from aeromagnetic, seismic and gravity data

The seismically active Marmara region, located in NW Turkey, lies on the westward end of the North Anatolian Fault (NAF). The NAF is well defined on land. Previous investigations of its extension in the Marmara Sea include marine bathymetry, seismological activity and seismic profiles. In this study, faults and their configurations identified inland are extended into the Marmara Sea by means of aeromagnetic anomalies, as well as seismic and gravity profiles. The deep structure was resolved by constructing a map of the Tertiary bottom. Shallow Curie isotherm was determined by spectral analysis, indicating a thinner crust in the northern Marmara depression area with respect to the continental crust. A combination of the geophysical data allows us to propose the existence of subsidence and isostatic equilibrium in the northern Marmara Sea. A less-active zone identified in the central high zone dividing the Marmara Sea into two parts may also be deduced from the seismic data. This structural arrangement may play a key role in earthquakes that will affect the surrounding regions.     

Stress modeling to determine the through-going active fault geometry of the Western North Anatolian Fault,Turkey

The North Anatolian Fault (NAF) is a 1200 km long dextral strike-slip fault which is part of an east-west trending dextral shear zone (NAF system) between the Anatolian and Eurasian plates. The North Anatolian shear zone widens to the west, complicating potential earthquake rupture paths and highlighting the importance of understanding the geometry of active fault systems. In the central portion of the NAF system, just west of the town of Bolu, the NAF bifurcates into the northern and southern strands, which converge, then diverge to border the Marmara Sea. At their convergence east of the Marmara Sea, these two faults are linked through the Mudurnu Valley. The westward continuation of these two fault traces is marked by further complexities in potential active fault geometry, particularly in the Marmara Sea for the northern strand, and towards the Biga Peninsula for the southern strand. Potential active fault geometries for both strands of the NAF are evaluated by comparing stress models of various fault geometries in these regions to a record of focal mechanisms and inferred paleostress from a lineament analysis. For the Marmara region, the best-fit active fault geometry consists of the northern and southern bounding faults of the Marmara basin, as the model representing this geometry better replicated primary stress orientations seen in focal mechanism data and stress field interpretations. In the Biga Peninsula region, the active geometry of the southern strand has the southern fault merging with the northern fault through a linking fault in a narrow topographic valley. This geometry was selected over the other two as it best replicated the maximum horizontal stresses determined from focal mechanism data and a lineament analysis.     

Neotectonics of Turkey – a synthesis

Turkey forms one of the most actively deforming regions in the world and has a long history of devastating earthquakes. The better understanding of its neotectonic features and active tectonics would provide insight, not only for the country but also for the entire Eastern Mediterranean region. Active tectonics of Turkey is the manifestation of collisional intracontinental convergence- and tectonic escape-related deformation since the Early Pliocene (∼5 Ma). Three major structures govern the neotectonics of Turkey; they are dextral North Anatolian Fault Zone (NAFZ), sinistral East Anatolian Fault Zone (EAFZ) and the Aegean–Cyprean Arc. Also, sinistral Dead Sea Fault Zone has an important role. The Anatolian wedge between the NAFZ and EAFZ moves westward away from the eastern Anatolia, the collision zone between the Arabian and the Eurasian plates. Ongoing deformation along, and mutual interaction among them has resulted in four distinct neotectonic provinces, namely the East Anatolian contractional, the North Anatolian, the Central Anatolian ‘Ova’ and the West Anatolian extensional provinces. Each province is characterized by its unique structural elements, and forms an excellent laboratory to study active strike-slip, normal and reverse faulting and the associated basin formation.     

Neotectonics of Turkey – a synthesis

Abstract

Turkey forms one of the most actively deforming regions in the world and has a long history of devastating earthquakes. The belter understanding of its neotectonic features and active tectonics would provide insight, not only for the country but also for the entire Eastern Mediterranean region. Active tectonics of Turkey is the manifestation of collisional intracontinental convergence- and tectonic escape-related deformation since the Early Pliocene (~5 Ma). Three major structures govern the neotectonics of Turkey; they are dextral North Anatolian Fault Zone (NAFZ), sinistral East Anatolian Fault Zone (EAFZ) and the Aegean–Cyprean Arc. Also, sinistral Dead Sea Fault Zone has an important role. The Anatolian wedge between the NAFZ and EAFZ moves westward away from the eastern Anatolia, the collision zone between the Arabian and the Eurasian plates. Ongoing deformation along, and mutual interaction among them has resulted in four distinct neotectonic provinces, namely the East Anatolian contractional, the North Anatolian, the Central Anatolian ‘Ova’ and the West Anatolian extensional provinces. Each province is characterized by its unique structural elements, and forms an excellent laboratory to study active strike-slip, normal and reverse faulting and the associated basin formation. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Structural relationship between the Sea of Marmara Basin and the North Anatolian Fault Zone

The North Anatolian Fault (NAF) zone is 1500 km long, extending almost up to the Greek mainland in the west. It is a seismically active right-lateral strike-slip fault that accommodates the relative motion between the Turkish block and Black Sea plate. The Sea of Marmara lies along the western part of the NAF and shows evidence of subsidence. In this area pure strike-slip motion of the fault zone changes into extensional strike-slip movement that is responsible for the creation of the Sea of Marmara and the North Aegean basins. The northern half of the Sea of Marmara is interpreted as a large pull-apart basin. This basin is subdivided into three smaller basins separated by strike-slip fault segments of uplifted blocks NE-SW. Basinal areas are covered by horizontally layered sedimentary sequences. Uplifted blocks have undergone compressional stress. All the blocks are subsiding and are undergoing vertical motions and rotations relative to one another. The uplifted blocks exhibit positive Bouguer gravity anomalies. According to gravity interpretation, there is relative crustal thinning under the Sea of Marmara. The northern side of the Sea of Marmara is marked by a distinctive deep-rooted magnetic anomaly, which is dissected and shifted southward by strike-slip faulting. The southern shelf areas of the Sea of Marmara are dominated by short-wavelength magnetic anomalies of shallow origin.     

A study of microearthquake seismicity and focal mechanisms within the Sea of Marmara (NW Turkey) using ocean bottom seismometers (OBSs)

We have carried out seismological observations within the Sea of Marmara (NW Turkey) in order to investigate the seismicity induced after Gölcük–İzmit (Kocaeli) earthquake (M_w 7.4) of August 17, 1999, using ocean bottom seismometers (OBSs). High-resolution hypocenters and focal mechanisms of microearthquakes have been investigated during this Marmara Sea OBS project involving deployment of 10 OBSs within the Çınarcık (eastern Marmara Sea) and Central-Tekirdağ (western Marmara Sea) basins during April–July 2000. Little was known about microearthquake activity and their source mechanisms in the Marmara Sea. We have detected numerous microearthquakes within the main basins of the Sea of Marmara along the imaged strands of the North Anatolian Fault (NAF). We obtained more than 350 well-constrained hypocenters and nine composite focal mechanisms during 70 days of observation. Microseismicity mainly occurred along the Main Marmara Fault (MMF) in the Marmara Sea. There are a few events along the Southern Shelf. Seismic activity along the Main Marmara Fault is quite high, and focal depth distribution was shallower than 20 km along the western part of this fault, and shallower than 15 km along its eastern part. From high-resolution relative relocation studies of some of the microearthquake clusters, we suggest that the western Main Marmara Fault is subvertical and the eastern Main Marmara Fault dips to south at 45°. Composite focal mechanisms show a strike-slip regime on the western Main Marmara Fault and complex faulting (strike-slip and normal faulting) on the eastern Main Marmara Fault.     

Late Cenozoic tectonics and stratigraphy of northwestern Anatolia: the effects of the North Anatolian Fault to the region

In northwest Anatolia, there is a mosaic of different morpho-tectonic fragments within the western part of the right-lateral strike-slip North Anatolian Fault (NAF) Zone. These were developed from compressional and extensional tectonic regimes during the paleo- and neo-tectonic periods of Turkish orogenic history. A NE-SW-trending left-lateral strike-slip fault system (Adapazari-Karasu Fault) extends through the northern part of the Sakarya River Valley and began to develop within a N–S compressional tectonic regime which involved all of northern Anatolia during Middle Eocene to early Middle Miocene times. Since the end of Middle Miocene times, this fault system forms a border between a compressional tectonic regime in the eastern area eastwards from the northern part of the Sakarya River Valley, and an extensional tectonic regime in the Marmara region to the west. The extension caused the development of basins and ridges, and the incursions of the Mediterranean Sea into the site of the future Sea of Marmara since Late Miocene times. Following the initiation in late Middle Miocene times and the eastward propagation of extension along the western part of the NAF, a block (North Anatolian Block) began to form in the northern Anatolia region since the end of Pliocene times. The Adapazari-Karasu Fault constitutes the western boundary of this block which is bounded by the NAF in the south, the Northeast Anatolian Fault in the east, and the South Black Sea Thrust Fault in the north. The northeastward movement of the North Anatolian Block caused the formation of a marine connection between the Black Sea and the Aegean/Mediterranean Sea during the Pleistocene.     

The termination of the North Anatolian Fault System (NAFS) in Eastern Turkey

ABSTRACT

The present-day tectonic framework of Turkey comprises mainly two strike-slip fault systems, namely dextral North Anatolian and sinistral East Anatolian faults. They are considered as the main cause of deformation patterns in Anatolia. These two mega shear systems meet at Kargapazar? village of Karl?ova county. The area to the east of the junction has a transpressional tectonic regime between the Eurasian and Arabian plates and is characterized, based on field observation, by a network of faults defining a typical horsetail splay structure. The horsetail splay is interpreted as marking the termination of the North Anatolian Fault System (NAFS), which continues eastward into the Varto Fault Zone (VFZ) and then dies out. The present study reveals that the VFZ is made up of two main parts, namely the principal displacement zone (PDZ) and the transpressional splay zone (TPSZ), both characterized by the right-lateral strike-slip with reverse motion. However, the area to the east of Varto is characterized dominantly by reverse-thrust faults and E–W-trending faults as shown by focal mechanism solutions. The generation of the VFZ as a transpressional termination to the NAFS can be related directly to the block movements of the Eurasian, Anatolian, and Arabian plates.     

Counterclockwise rotation of the Anatolian block

Seismological and other data from the boundaries of Anatolia are used to discuss the motion and plate tectonics of this block. Models which include a rotation of Anatolia around a single pole are shown to be inconsistent with much of this data. In particular a westward motion of Anatolia, which is the most commonly used model, should be reconsidered. Models which use a single pole of rotation fail because they assume the North Anatolian Fault to be an ideal transform fault which describes all of the motion between Anatolia and the Black Sea. In effect, internal deformation in the form of extension in western Anatolia and for the most part strike-slip faulting in eastern Anatolia, are important and account for some of the relative motion. Thus, a more appropriate model for this region is one which not only stresses the dominance of the North Anatolian Fault, but also recognises the importance of internal deformation and the limitation of classic plate tectonic models which this deformation implies. The preferred model for the motion of Anatolia is a nonuniform, tight, counterclockwise rotation approximated by the shape of the North Anatolian Fault as well as by the faults which splay from it to the south. Such a model is consistent with data from the boundaries of Anatolia including the source mechanism of earthquakes and depth of Benioff zone along the southern boundary of the block. Counterclockwise rotation of Anatolia is the natural consequence of the collision of Arabia with eastern Anatolia, coupled with subduction in the eastern Mediterranean.     

Middle Pleistocene bivalves of the İznik lake basin (Eastern Marmara, NW Turkey) and a new paleobiogeographical approach

?znik Lake is a tectonically originated basin mainly controlled by the E–W trending middle strand of the North Anatolian Fault (NAF) system. Pleistocene sediments occurring in front of the faults are well exposed both in the northern and in the southern shorelines of the basin. In this study, two endemic brackish water bivalve species, Didacna subpyramidata Pravoslavkev 1939 and Didacna nov. sp. were found in the oldest terrace of the northern Pleistocene sequence. Having characterized morphology, these species serve as stratigraphic indicators in the regional Pleistocene stratigraphy of the Ponto-Caspian region, and thus are well correlated to the assemblages of the early Khazarian subhorizon (Middle Pleistocene). Hence, these data demonstrate that the early Khazarian brackish water sea covered the study area. Additionally, a model for the formation of the basin is proposed: the ?znik lake basin was a gulf of the former Marmara Sea in the early Khazarian, connecting the Marmara to the Black Sea and the Caspian Sea. The subsequent regional prograding uplifts, main dextral strike-slip fault and many normal faults of the NAF Zone cut off the marine connections to the basin, leading to its present location and topographic level.     

The Bekten Fault: the palaeoseismic behaviour and kinematic characteristics of an intervening segment of the North Anatolian Fault Zone,Southern Marmara Region,Turkey

The Bekten Fault is 20-km long N55°E trending and oblique-slip fault in the dextral strike-slip fault zone. The fault is extending sub-parallel between Yenice-Gönen and Sar?köy faults, which forms the southern branch of North Anatolian Fault Zone in Southern Marmara Region. Tectonomorphological structures indicative of the recent fault displacements such as elongated ridges and offset creeks observed along the fault. In this study, we investigated palaeoseismic activities of the Bekten Fault by trenching surveys, which were carried out over a topographic saddle. The trench exposed the fault and the trench stratigraphy revealed repeated earthquake surface rupture events which resulted in displacements of late Pleistocene and Holocene deposits. According to radiocarbon ages obtained from samples taken from the event horizons in the stratigraphy, it was determined that at least three earthquakes resulting in surface rupture generated from the Bekten Fault within last ~1300 years. Based on the palaeoseismological data, the Bekten Fault displays non-characteristic earthquake behaviour and has not produced any earthquake associated with surface rupture for about the last 400 years. Additionally, the data will provide information for the role of small fault segments play except for the major structures in strike-slip fault systems.     

Strong ground motion estimation in the Sea of Marmara region (Turkey) based on a scenario earthquake

We perform a broadband frequency bedrock strong ground motion simulation in the Marmara Sea region (Turkey), based on several fault rupture scenarios and a source asperity model. The technique combines a deterministic simulation of seismic wave propagation at low frequencies with a semi-stochastic procedure for the high frequencies. To model the high frequencies, we applied a frequency-dependent radiation pattern model, which efficiently removes the effective dependence of the pattern coefficient on the azimuth and take-off angle as the frequency increases. The earthquake scenarios considered consist of the rupture of the closest segments of the North Anatolian Fault System to the city of Istanbul. Our scenario earthquakes involve the rupture of the entire North Anatolian Fault beneath the Sea of Marmara, namely the combined rupture of the Central Marmara Fault and North Boundary Fault segments. We defined three fault rupture scenarios based on the location of the hypocenter, selecting a preferred hypocentral location near a fault bend for each case. We analysed the effect of location of the asperity, within the Central Marmara Fault, on the subsequent ground motion, as well as the influence of anelasticity on the high-frequency attenuation characteristics. The fault and asperity parameters for each scenario were determined from empirical scalings and from results of kinematic and dynamic models of fault rupture. We calculated the resulting time series and spectra for ground motion at Istanbul and evaluated the sensitivity of the predictions to choice of model parameters. The location of the hypocenter is thus shown to be a critical parameter for determining the worst scenario earthquake at Istanbul. We also found that anelasticity has a significant effect on the regional attenuation of peak ground accelerations. Our simulated ground motions result in large values of acceleration response spectra at long periods, which could be critical for building damage at Istanbul during an actual earthquake.     

Uplift and subsidence from oblique slip: the Ganos–Marmara bend of the North Anatolian Transform, western Turkey

Bends that locally violate plate-motion-parallel geometry are common structural elements of continental transform faults. We relate the vertical component of crustal motion in the western Marmara Sea region to the NNW-pointing 18° bend on the northern branch of the North Anatolian Fault (NAF-N) between the Ganos segment, which ruptured in 1912, and the central Marmara segment, a seismic gap. Crustal shortening and uplift on the transpressive west side of the bend results in the Ganos Mountain; crustal extension and subsidence on the transtensional east side produce the Tekirdağ Basin. We propose that this vertical component of deformation is controlled by oblique slip on the non-vertical north-dipping Ganos and Tekirdağ segments of the North Anatolian Fault. We compare Holocene with Quaternary structure across the bend using new and recently published data and conclude the following. First, bend-related vertical motion is occurring primarily north of the NAF-N. This suggests that this bend is fixed to the Anatolian side of the fault. Second, current deformation is consistent with an antisymmetric pattern centered at the bend, up on the west and down on the east. Accumulated deformation is shifted to the east along the right-lateral NAF-N, however, leading to locally opposite vertical components of long- and short-term motion. Uplift has started as far west as the landward extension of the Saros trough. Current subsidence is most intense close to the bend and to the Ganos Mountain, while the basin deepens gradually from the bend eastward for 28 km along the fault. The pattern of deformation is time-transgressive if referenced to the material, but is stable if referenced to the bend. The lag between motion and structure implies a 1.1–1.4 Ma age for the basin at current dextral slip rate (2.0–2.5 cm/year). Third, the Tekirdağ is an asymmetric basin progressively tilted down toward the NAF-N, which serves as the border fault. Progressive tilt suggests that the steep northward dip of the fault decreases with depth in a listric geometry at the scale of the upper crust and is consistent with reactivation of Paleogene suture-related thrust faults. Fourth, similar thrust-fault geometry west of the bend can account for the Ganos Mountain anticline/monocline as hanging-wall-block folding and back tilting. Oblique slip on a non-vertical master fault may accommodate transtension and transpression associated with other bends along the NAF and other continental transforms.     

Closing-up structures, alternatives to pull-apart basins; the effect of bends in the North Anatolian Fault, Turkey

Fault blocks passing bends or stepovers in a fault zone must adapt their margins to the uneven fault trace. Two cases of adaption are distinguished for extensional bends or stepovers (transtension): (1) The fault margins close up behind a single bend ('knickpoint') of a strike-slip fault and a 'closing-up structure' (new term) arises or (2) fault-block margins are extended behind a releasing bend (double bend) or stepover parallel to the displacement and a pull-apart basin originates. The dosing up described here is accomplished by acute-angled synthetic strike-slip faults that dissect the straight fault in front of a knickpoint to form a zig-zag block boundary behind it. Crustal extension is also involved in the closing-up structure, but in a different way from typical pull-apart basins.
The closing-up structure illustrated was developed behind an extensional knickpoint in the North Anatolian Fault west of Lake Abant, NW Turkey, where the process of closing up continues to this day. The kinematic model of this closing-up structure is supported by displacements and ruptures observed during the 1967 Mudurnu valley earthquake and the 1957 Abant earthquake.     

The South Marmara Fault

We use about 800 km of multichannel exploration seismic reflection profiles of the seventies as well as the results of three drill holes that penetrated the sedimentary cover down to the Upper Cretaceous basement to describe a continuous gently curvilinear, south-concave zone of deformation about 10 km wide that extended over the whole southern shelf of the Sea of Marmara from the Gulf of Gemlik to the Dardanelles Straits in Lower Pliocene time, about 4 Ma. We call this zone of deformation the South Marmara Fault (SMF) system and propose that the SMF was then a branch of the dextral North Anatolian Fault. This branch passed to the north of the Marmara Island Eocene block and thus had a south-facing concavity. This curvature resulted in a significant component of shortening in the western part of the fault. The SMF was deactivated at the end of Lower Pliocene, about 3.5 Ma, except for its easternmost branch between the Gulf of Gemlik and ?mral? Island where about 5 mm/year of dextral motion is still occurring today.     

Seismic observations at an overstep of the western North Anatolian Fault (Abant–Sapanca region, Turkey)

The course of the active North Anatolian Fault system from Lake Abant to Lake Sapanca was traced by its high micro-earthquake activity. If approaching from the east this section includes a broad south to north overstep (fault offset) of the main fault. Local seismicity has been recorded in this area by a semi-permanent network of 8 stations since 1985 within the frame of the Turkish–German Joint Project for Earthquake Research. The effect of the overstep and its complex fracture kinematics are reflected by the seismicity distribution, the variations of composite fault-plane solutions, and by the spatial coda-Q distribution. Areas of different stress orientation can be distinguished and assigned to different groups of faults. The stresses and the tectonic pattern only in part correspond to a simple model of an extensional overstep and its correlative pull-apart basin. Other types of deformation involved are characterized by normal faulting on faults parallel to the general course of the main strike-slip fault and by synthetic strike-slip faults oriented similar to Riedel shears. Shear deformation by this fault group widely distributed in an area north and east of the main fault line may play an important role in the evolution of the overstep. The development of a pull-apart basin is inhibited along the eastern half of the overstep and compatibility of both strands of the main fault (Bolu–Lake Abant and Lake Sapanca– Izmit–Marmara Sea) seems to be achieved with the aid of the fault systems mentioned. The extension of the missing part of the pull-apart basin seems to be displaced to positions remote from the Lake Abant–Lake Sapanca main fault line, i.e. to the Akyaz?–Düzce basin tract. Highest Q-values (lowest attenuation of seismic waves) were found in the zone of highest seismicity north and west of the overstep which is the zone of strongest horizontal tension. If high coda-Q is an indicator for strong scattering of seismic waves it might be related to extensional opening of fractures.     

The geology and morphology of the Antakya Graben between the Amik Triple Junction and the Cyprus Arc

In southeastern Turkey, the NE-trending Antakya Graben forms an asymmetric depression filled by Pliocene marine siliciclastic sediment, Pleistocene to Recent fluvial terrace sediment, and alluvium. Along the Mediterranean coast of the graben, marine terrace deposits sit at different elevations ranging from 2 to 180 m above present sea level, with ages ranging from MIS 2 to 11. A multisegmented, dominantly sinistral fault lying along the graben may connect the Cyprus Arc in the west to the Amik Triple Junction on the Dead Sea Fault (DSF) in the east. Normal faults, which are younger than the sinistral ones, bound the graben’s southeastern margin. The westward escape of the continental ?skenderun Block, delimited by sinistral fault segments belonging to the DSF in the east and the Eastern Anatolian Fault in the north caused the development of a sinistral transtensional tectonic regime, which has opened the Antakya Graben since the Pliocene. In the later stages of this opening, normal faults developed along the southeastern margin that caused the graben to tilt to the southwest, leading to differential uplift of Mediterranean coastal terraces. Most of these normal faults remain active. In addition to these tectonic movements, Pleistocene sea level changes in the Mediterranean affected the geomorphological evolution of the area.     

Fault kinematic and Plio-Quaternary paleostress evolution of the Bakırçay Basin,Western Turkey

ABSTRACT

At the end of the Cenozoic, western Turkey was fragmented by intense intra-continental tectonic deformation resulting in the formation of two extensional areas: a transtensional pull-apart basin systems in the northwest, and graben systems in the central and southwest areas. The question of the connection of this Late Cenozoic extensional tectonics to plate kinematics has long been an issue of discussion. This study presents the results of the fault slip data collected in Bak?rçay Basin in the west of Turkey and addresses changes in the direction of extensional stresses over the Plio-Quaternary. Field observations and quantitative analysis show that Bak?rçay Basin is not a simple graben basin that has evolved during a single phase. It started as a graben basin with extensional regime in the Pliocene and was transformed into a pull-apart basin under the influence of transtensional forces during the Quaternary. A chronology of two successive extensional episodes has been established and provides reasoning to constrain the timing and location of subduction-related back-arc tectonics along the Aegean region and collision-related extrusion tectonics in Turkey. The first NW–SE trending extension occurred during the Pliocene extensional phase, characterized by slab rollback and progressive steepening of the northward subduction of the African plate under the Anatolian Plate. Western Turkey has been affected, during the Middle Quaternary, by regional subsidence, and the direction of extension changed to N–S, probably in relation with the propagation of the North Anatolian Fault System. Since the Late Quaternary, NE–SW extension dominates northwest Turkey and results in the formation and development of elongated transtensional basin systems. Counterclockwise rotation of Anatolian block which is bounded to the north by the right-lateral strike-slip North Anatolian Fault System, accompanies to this extensional phase.     

Kinematic analysis and palaeoseismology of the Edremit Fault Zone: evidence for past earthquakes in the southern branch of the North Anatolian Fault Zone,Biga Peninsula,NW Turkey

The Edremit Fault Zone (EFZ) forms one of the southern segments of the North Anatolian Fault Zone (NAFZ) at the northern margin of the Edremit Gulf (Biga Peninsula, South Marmara Region, Turkey). Stratigraphic, structural and kinematic results indicate that basinward younging of the fault zone, in terms of a rolling-hinge mechanism, has resulted in at least three discrete Miocene to Holocene deformational phases: the oldest one (Phase 1) directly related to the inactive Kazda? Detachment Fault, which was formed under N–S trending pure extension; Phase 2 is characterised by a strike-slip stress condition, probably related to the progression of the NAFZ towards the Edremit area in the Plio–Quaternary; and Phase 3 is represented by the high-angle normal faulting, which is directly interrelated with the last movement of the EFZ. Our palaeoseismic studies on the EFZ revealed the occurrence of three past surface rupture events; the first one occurred before 13178 BC, a penultimate event that may correspond to either the 160 AD or 253 AD historical earthquakes, and the youngest one can be associated with the 6 October 1944 earthquake (M_w = 6.8). These palaeoseismic data indicate that there is no systematic earthquake recurrence period on the EFZ.