Gulf Of Saros Research Articles

Optically Stimulated Luminescence to Date Coastal Dunes and a Possible Tsunami Layer on the Kavak Delta (Saros Gulf, NW Turkey)

Optically stimulated luminescence (OSL) dating was used to determine the timing of initial dune formation and reconstruct the evolution of coastal dunes that developed on the Kavak Delta, Saros Gulf, Turkey. Along a 500-m-long representative transect, dune sands were extracted from foredune, semistable (grey) dune, stable (dark) dune and dune-swamp boundary defined by a scarp 50-75 cm high. The data obtained showed that dune drift initiated 670 years ago. A pumice layer 15-20-cm-thick interbedded with marine clay and sand showed an OSL age of 340 years coinciding with underlying dune sand. XRF analysis showed that pumices were of similar composition to those erupted by plinian activity of Thera (Santorini) in 1628 BC. On the basis of OSL ages, these deposits, which are widely distributed on the western Anatolian coasts of Turkey, might have presumably transported landward along tide channels on the delta during a tsunami event that occurred in 1672 near Bozcaada and Kos islands according to tsunami history of the Aegean Sea.

Water mass properties and chemical characteristics in the Saros Gulf, Northeast Aegean Sea (Eastern Mediterranean)

This work is a description of the water masses and chemical properties in the Northeast (NE) Aegean Sea, a region of recently renewed interest with respect to the hydrology of the Eastern Mediterranean Basin. Analysis of hydrological data from the study area reveals the presence of three water masses: The 20–40 m thick surface layer is occupied mainly by Black Sea Water, BSW, modified on its way through the Bosphorus, the Marmara Sea and the Dardanelles. Below the surface layer there is warm and highly saline water originating from the South Aegean and the Levantine basins, Levantine Intermediate Water (LIW), extending down to 350–400 m depth. Below this layer, the deeper than 400 m basins of the North Aegean contain locally formed, very dense water with different Potential temperature–Salinity characteristics at each sub-basin and called North Aegean Deep Water (NADW). Evidence of temperature-controlled dense water formation of the Gulf of Saros, in the NE Aegean was reported in January 2002. The nutrient concentrations varied between nitrate + nitrite = 0.19–0.37 µM, phosphate = 0.01–0.02 µM and silicate = 0.7–1.1 µM for the surface layer (BSW) during sampling periods. Dissolved nutrient concentrations in the NADW were: 1.7–3.7 µM for NO 3 + NO 2, 0.08–0.17 µM for PO 4 and 2.7–4.2 µM for silicate. During winter, relatively high nutrient levels are observed in the surface layers of the NE Aegean Sea, whilst phosphate and nitrates decrease in summer.

Faulting, mass-wasting and deposition in an active dextral shear zone, the Gulf of Saros and the NE Aegean Sea, NW Turkey

Structural, mass-wasting and sedimentation processes along an active dextral shear zone beneath the Gulf of Saros and the NE Aegean Sea were investigated on the basis of new high-resolution swath bathymetric data and multi-channel seismics. A long history of dextral shearing operating since the Pliocene culminated in the formation of a NE-SW-trending, ca. 800-m-deep basin (the so-called inner basin) in this region, which is bordered by a broad shelf along its northern and eastern sides and a narrow shelf at the southern side. The western extension of the North Anatolian Fault Zone (the Ganos Fault) cuts the eastern shelf along a narrow deformation zone, and ends sharply at the toe of the slope, where the strain is taken up by two NE-SW-oriented fault zones. These two fault zones cut the basin floor along its central axis and generate a new, Riedel-type pull-apart basin (the so-called inner depression). According to the bathymetric and seismic data, these basin boundary fault zones are very recent features. The northern boundary of the inner depression is a through-going fault comprising several NE-SW- and E-W-oriented, overlapping fault segments. The southern boundary fault zone, on the other hand, consists of spectacular en-echelon fault systems aligned in NE–SW and WNW–ESE directions. These en-echelon faults accommodate both dextral and vertical motions, thereby generating block rotations along their horizontal axis. As the basin margins retreat, the basin widens continuously by mass-wasting of the slopes of the inner basin. The mass-wasting, triggered by active tectonics, occurs by intense landsliding and channel erosion. The eroded material is transported into the deep basin, where it is deposited in a series of deep-sea fans and slumps. The high sedimentation rate is reflected in an over 1,500-m-thick basin fill which has accumulated in Pliocene–Quaternary times.

Toxic and trace element analysis of surface sediments from the Gulf of Saros by INAA and XRF methods

Some surface sediment samples, collected from 53 sampling sites of the Saros Gulf, were analyzed quantitatively by radioisotope energy dispersive X-ray fluorescence (EDXRF) and instrumental neutron activation analysis (INAA). Results indicated some correlations between Rb and Sr (r = 0.64), Fe2O3 and MnO (r = 0.59), Th and La (r = 0.71), Th and Ce (r = 0.64), Th and Sm (r = 0.60), concentration pairs. U and Th results are found to be compatible with those given in the literature for marine sediments.

A source study of the 6 July 2003 ( Mw 5.7) earthquake sequence in the Gulf of Saros (Northern Aegean Sea): Seismological evidence for the western continuation of the Ganos fault

The July 2003 sequence in the Gulf of Saros (Northeastern Aegean Sea) is investigated, in terms of accurate event locations and source properties of the largest events. The distribution of epicenters shows the activation of a 25-km long zone, which extends in depth between 9 and 20 km. The major slip patch of the 6 July 2003 M w 5.7 mainshock is confined in a small area (∼45 km 2), which coincides with the deeper (12–20 km) part of the activated zone. The epicenters of the sequence follow the northern margin of the Saros depression. This observation supports recent studies, according to which the continuation of the Ganos fault in the Gulf of Saros does not coincide with the fault along the northern coast of the Gelibolu peninsula, but it is located at the northern boundary of the Saros depression. This is further supported by the fact that the focal mechanisms of the mainshock and of the largest aftershocks of the 2003 sequence imply almost pure dextral strike-slip faulting, whereas the fault bounding the Gulf of Saros to the south appears as a normal fault on seismic sections. Thus, we infer that the principle deformation zone consists of a major strike-slip fault, which lies close to the northern margin of the Saros depression and this fault could be regarded as the continuation of the northern branch of the North Anatolian Fault into the Saros Gulf and North Aegean Trough as suggested by regional tectonic models. The northeastern extent of the 2003 sequence marks the western termination (at ∼26.3° E) of a long-term seismic quiescence observed in the period following the 1912 Ganos earthquake, which may be associated with the extend of the rupture of the particular earthquake.

Submarine fault scarps in the Sea of Marmara pull‐apart (North Anatolian Fault): Implications for seismic hazard in Istanbul

Earthquake scarps associated with recent historical events have been found on the floor of the Sea of Marmara, along the North Anatolian Fault (NAF). The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine‐scale morphology and geology of those scarps, their distribution, and geometry. The observations are consistent with the diversity of fault mechanisms and the fault segmentation within the north Marmara extensional step‐over, between the strike‐slip Ganos and Izmit faults. Smaller strike‐slip segments and pull‐apart basins alternate within the main step‐over, commonly combining strike‐slip and extension. Rapid sedimentation rates of 1–3 mm/yr appear to compete with normal faulting components of up to 6 mm/yr at the pull‐apart margins. In spite of the fast sedimentation rates the submarine scarps are preserved and accumulate relief. Sets of youthful earthquake scarps extend offshore from the Ganos and Izmit faults on land into the Sea of Marmara. Our observations suggest that they correspond to the submarine ruptures of the 1999 Izmit (Mw 7.4) and the 1912 Ganos (Ms 7.4) earthquakes. While the 1999 rupture ends at the immediate eastern entrance of the extensional Cinarcik Basin, the 1912 rupture appears to have crossed the Ganos restraining bend into the Sea of Marmara floor for 60 km with a right‐lateral slip of 5 m, ending in the Central Basin step‐over. From the Gulf of Saros to Marmara the total 1912 rupture length is probably about 140 km, not 50 km as previously thought. The direct observations of submarine scarps in Marmara are critical to defining barriers that have arrested past earthquakes as well as defining a possible segmentation of the contemporary state of loading. Incorporating the submarine scarp evidence modifies substantially our understanding of the current state of loading along the NAF next to Istanbul. Coulomb stress modeling shows a zone of maximum loading with at least 4–5 m of slip deficit encompassing the strike‐slip segment 70 km long between the Cinarcik and Central Basins. That segment alone would be capable of generating a large‐magnitude earthquake (Mw 7.2). Other segments in Marmara appear less loaded.

Tracing the geometry of an active fault using remote sensing and digital elevation model: Ganos segment, North Anatolian Fault zone, Turkey

The surface expression of the North Anatolian Fault between the Sea of Marmara and the Gulf of Saros, Turkey was determined using Landsat 5 Thematic Mapper (TM) and Système Probatoire de l'Observation de la Terre (SPOT) high resolution visible (HRV) Panchromatic images and a digital elevation model (DEM) based on 1 : 25 000 scale topographic maps. The remotely sensed data had been enhanced to reveal linear details. Between Gaziköy (Marmara Shore) and Saros two small releasing steps were identified, one to the east of Mürseli village and the other between Mürseli and Yayaköy villages. Hitherto, a single straight fault line had been drawn between Gaziköy and Saros on the geological map of this region. Between the Yeniköy and Sofular villages, the fault strike bends ∼7° further to the SE. The course of the fault between the Sea of Marmara and the Gulf of Saros is, therefore, more complicated than formerly believed and this is probably because the fault has utilized the pre-existing structures of the Neo-Tethyan subduction–accretion complex.

Extension of the North Anatolian Fault into the North Aegean Trough: Evidence for transtension, strain partitioning, and analogues for Sea of Marmara basin models

The North Aegean Trough is tectonically controlled by the projection of the North Anatolian Fault from western Turkey and the Sea of Marmara into the Aegean, where deformation is distributed in contrast to the simple strike‐slip fault model for central eastern Turkey. This region accommodates extensional and dextral displacement. New side‐scan sonar data coupled with new subbottom profiler and published seismic data reveal evidence for dextral displacement within the eastern trough and Gulf of Saros in the form of oblique en echelon and Riedel (R, R′, and P) fractures and displaced geomorphic features. Strain partitioning in the shallow crust allows the following two styles of faulting to coexist, supported by earthquake focal mechanisms: (1) transtensional basin‐bounding faults controlling the significant vertical topography of the basin and (2) dextral faulting within the central basin. These styles of faulting merge at greater depths into a single fault. A possible sinistral NW striking fault separates two subbasins and may indicate crustal block rotation with deformation focused in block corners. Such evidence and models from the north Aegean are similar to models of transtension and pull‐apart basins within the Sea of Marmara.

Kinematics and evolution of the northern branch of the North Anatolian Fault (Ganos Fault) between the Sea of Marmara and the Gulf of Saros

The WSW–ENE-trending Ganos Fault is a dextral strike-slip fault running parallel to and southwest of the West Marmara Trough and south of the Saros Trough. Dextral structures started evolving in the early Miocene, and at this time the Ganos fault system developed from a part of the Thrace–Eskişehir fault system. Beginning in the late Pliocene (∼3.5 Ma), the North Anatolian transform fault propagated into the Marmara region and captured the Ganos Fault. Subsequently, this fault has accommodated the westward movement of the Anatolian Block. Because of the curvature of the microplate boundary in this area, the Ganos fault system has tended to rotate counterclockwise. Farther west in the Gulf of Saros, the strike-slip motion was accommodated by a new fault on the northern margin of the gulf, rather than along the northern coast of the Gelibolu Peninsula as previously thought. This interpretation differs from previous assessments of the position of the northern strand of the North Anatolian fault (Marmara segment) in the Marmara Sea and in the Gulf of Saros. The role of the Ganos Fault proposed in this paper is considerably different from that proposed by earlier studies. While the revised orientation of the North Anatolian fault on land is about 7° different than specified by previous authors, at sea it is different by ∼32° counterclockwise and ∼23° clockwise in the West Marmara and Saros submarine depressions, respectively. The revised position of the Ganos Fault in the Marmara Sea, derived from shallow and conventional seismic reflection data, calls into question the validity of evolutionary models previously used in kinematic and stress-failure analyses. In particular, it is not possible to regard the Marmara Sea as a pull-apart basin and the Gulf of Saros as a transtensional half-graben. Furthermore, palinspastic maps taking into account the revised position of the Ganos Fault and GPS slip vectors support the idea that a dextral master fault is present to the north of the Saros Trough with a sinistral oblique fault dominated by normal offset (Gelibolu Fault) to its south. The Gelibolu Fault is reactivated in a limited region.

Distributions of heavy metals in the surface sediments of the Gulf of Saros, NE Aegean Sea

The Gulf of Saros in the northeastern Aegean Sea (Turkey) is a relatively unpolluted marine environment. There is almost no industry and only small settlements in the surrounding region. The main sources of fresh water and sediment to the Gulf are the Meriç River in the northwest and the Kavak Creek in the east. It has an asymmetric bathymetry with a 10-km-wide shelf to the north and up to 15-km-wide, 700-m-deep trough in the south. Water circulation in the Gulf is characterized by longshore currents and anticyclonic eddies. The Black Sea waters, leaving the Sea of Marmara through the Çanakkale Strait, are known to enter the southern Gulf during the summer. The surface sediments on the northern shelf consist mainly of sand, whereas those on the slope and the deep trough are mainly silt and clay. The ranges of Fe, Mn, Cu, Zn, Pb, Ni, and Hg contents of the surface sediments from the Gulf of Saros are 0.25–4.60%, 114–1740 ppm, 6–44 ppm, 23–154 ppm, 2–80 ppm, 14–145 ppm, and 10–130 ppb, respectively. These values are low compared to those from the other neighboring marine environments, indicating the pristine nature of the Gulf in terms of metal pollution. However, most metal and organic carbon (Corg) distributions in the Gulf sediments show the effect of anthropogenic and natural inputs to the northwestern and eastern shelf areas from the Meriç River and the Kavak Creek and to the deep trough from the Black Sea waters.

Paleoseismology of the Gazikoy-Saros segment of the North Anatolia fault, northwestern Turkey: Comparison of the historical and paleoseismic records, implications of regional seismic hazard, and models of earthquake recurrence

We excavated five trenches across the North Anatolia fault zone (NAFZ)along the Ganos fault (Gazikoy-Saros segment), which last produced surfacerupture in 1912, near Kavakkoy where the fault enters the Gulf of Saros. The trenches exposed faulted sediments in a flood-plain environment withabundant detrital charcoal and scattered land-snail shells. Twenty-tworadiocarbon dates place constraints on the ages of the exposed sediments,which range from less than a few hundred years to about 6000 years inage. In two closely spaced trenches, we identified five discrete earthquakeevent horizons in the upper 2.5 m of stratigraphy based on abruptupward termination of shear zones, folding, fissuring, and abruptstratigraphic thickening, four of which may corresponded to historicallyrecorded large regional earthquakes. The earliest of the identified eventsoccurs below an unconformity and dates to about 4 ka B.P. The morerecent four events all occurred within the past 1000–1200 years and maycorrespond to large earthquakes in A.D. 824, ca 1354, 1509, 1766 and1912 (Ambraseys and Finkel, 1987, 1991, 1995). In another trench,we identified at least two events that have occurred during the past 500years and probably correspond to the large events of 1766 and 1912. These observations support an average return period of about 250–300years for the Gazikoy-Saros segment of the NAFZ. They also suggest thatthis segment, which is bound both to the east and west by large releasingstepovers, behaves in a quasi-periodic fashion, at least for the past severalsurface ruptures.Most of the 23 mm/yr of dextral shear between Anatolia and Europeobserved by GPS occurs on the North Anatolian fault. We use18 mm/yr and the ∼ 250–300 year recurrence rate, as determined fromour trenching and the historical record, to suggest that each of theearthquakes observed in our trenches produced several meters of slip,consistent with their inferred sizes from the extent of historical damage. Considering that Istanbul has not suffered a large nearby event in theMarmara Sea since 1766, we suggest that about 4 m of strain hasaccumulated across faults in the Marmara during these past centuries. Thisis similar to the average slip in many of the large earthquakes on the NorthAnatolian fault this century. If released seismically, this could result in anearthquake in the M 7.2–M 7.6 range, similar to the August and November,1999 earthquakes east of the Marmara Sea.

North Anatolian Fault–Saros Gulf relationships and their relevance to hydrocarbon exploration, northern Aegean Sea, Turkey

The Saros Gulf is underlain by a graben formed since Oligocene period under the influence of movements of the Anatolian and Rhodope Massifs. The oil prospectivity of this area has been enhanced by the splays of the North Anatolian Fault (SNAF) which developed later between the Anatolian and Eurasian plates during the separation of the Arabian and African plates. The interpretation of well and seismic data reveals a SW–NE graben area in the Saros Gulf. Its long axis corresponds to the thick channel deposits of the Kirazli Formation, eroded and transported from the northern Korudag High. Subsidence studies show that this graben started to form during the Oligocene and was affected by the SNAF in Miocene time. Three sedimentary and tectonic phases can be observed in the Gulf: the first phase corresponds to the erosion of basement metamorphics and volcanics and the deposition of the reservoir and source rocks. The second phase is represented by the main subsidence phase, which caused the burial of the sediments in the basin. Due to the erosion of the Oligocene sediments, some abandoned channels have been formed at the northern and southern flanks of the graben area. The third phase was the establishment of the present form of the Saros Gulf under the influence of the SNAF and the divergence between the Anatolian and Rhodope Massifs. All these tectonic and sedimentary events resulted from the separation of the African and Arabian plates and collisions of the Arabian, Anatolian and Eurasian plates between Late Cretaceous (Maastrichtian) and Miocene times in Turkey. Studies of geothermal and heating gradients show that the deepest southern sector of the Saros Gulf produces gas below 3000 m, while the northern zone generates oil between 1500 and 2000 m. Evaluation of all the geological and geophysical data suggests some hydrocarbon exploration possibilities in the Saros Gulf.

Active submarine tectonism and formation of the Gulf of Saros, Northeast Aegean Sea, inferred from multi-channel seismic reflection data

Abstract Active submarine tectonism and the formation of the Gulf of Saros, northeast Aegean Sea, were investigated by means of multi-channel seismic reflection data. This area is a key in understanding the development of major structures resulting from the interacting regimes of the Western Anatolia–Aegean Sea and the North Anatolian Fault zone (NAF). The major feature forming the Gulf of Saros is the Ganos fault, which is one of the main segments of the northern strand of the NAF. In this study, a total of 159 km reflection seismic data were collected, processed and interpreted in seven lines in order to map active submarine faults which were used to propose a tectonic model for opening of the Gulf of Saros. Interpretation of the seismic sections and their correlation with the geological, morphological and earthquake data indicate that there are two main fault systems in the gulf: (i) strike-slip faults with normal components which bound the Saros trough in the north and south; and (ii) normal faults located within the trough. We propose that the Saros trough was first formed as a negative flower structure in response to the southward bending of the NAF zone possibly in Plio-Quaternary, then it evolved as a dilated negative flower structure as the Aegean extension intensified in late Pliocene–Quaternary.

Palaeogeographical evolution of the Thrace Neogene Basin and the Tethys–Paratethys relations at northwestern Turkey (Thrace)

The Thrace Neogene Basin situated in northwestern Turkey was initiated by strike-slip faulting that was active from the Early Miocene until the end of the Pliocene. During the Early Miocene, it began to form under the control of the Thrace–Eskisehir Fault Zone, initiated by continental collision in northwestern Anatolia (Late Oligocene–Early Miocene). During the late Early Miocene, the basin was a site of mainly fluviatile and limnic sedimentation to the west and marine sedimentation via the Paratethyan transgression in the north. With the onset of the Middle Miocene, the Thrace Block started to rotate in a counterclockwise sense and escaped westward with respect to the Strandja–Istanbul block owing to rejuvenation of fossil fault systems within it. In this period, warm marine conditions were also established around the Gulf of Saros through a Mediterranean originated transgression. During the Middle–Late Miocene fluviatile and limnic conditions were created over the western Thrace by the westerly propagation of the Thrace–Eskişehir Fault Zone. One of the principal results of the Early–Middle Miocene tectonics is the tilting of the Strandja–Istanbul Block to the south, severing the Tethys and the Paratethys. During the latest Miocene–Early Pliocene period, the Thrace–Eskisehir Fault Zone was deactivated because of the evolution of the North Anatolian Fault Zone to the south. The resurrected Ganos Fault Zone situated on the dissected fossil suture zone in the Sea of Marmara also joined the North Anatolian Fault Zone, uplifting the Gelibolu Peninsula and, thus, severing the connection that existed between the Sea of Marmara and the Paratethys. The Sea of Marmara eventually became an endemic basin by the activity of the North Anatolian Fault Zone.

Tectonic elements controlling the evolution of the Gulf of Saros (northeastern Aegean Sea, Turkey)

Tectonic elements controlling the evolution of the Gulf of Saros have been studied based upon the high-resolution shallow seismic data integrated with the geological field observations. Evolution of the Gulf of Saros started in the Middle to Late Miocene due to the NW–SE compression caused by the counterclockwise movement of the Thrace and Biga peninsulas along the Thrace Fault Zone. Hence, the North Anatolian Fault Zone is not an active structural element responsible for the starting of the evolution of the Gulf of Saros. The compression caused by the rotational movement was compensated by tectonic escape along the pre-existing Ganos Fault System. Two most significant controllers of this deformation are the sinistral Ganos Fault and the dextral northern Saros Fault Zone both extending along the Gulf of Saros. The most important evidences of this movement are the left- and right-oriented shear deformations, which are correlated with structural elements, observed on the land and on the high-resolution shallow seismic records at the sea. Another important line of evidence supporting the evolution of this deformation is that the transgression started in the early-Late Miocene and turned, as a result of regional uplift, into a regression on the Gelibolu Peninsula during the Turolian and in the north of the Saros Trough during the Early Pliocene. The deformation on the Gelibolu Peninsula continued effectively until the Pleistocene. Taking into account the fact that this deformation affected the Late Pleistocene units of the Marmara Formation, the graben formation of the Gulf of Saros is interpreted as a Recent event. However, at least a small amount of compression on the Gelibolu Peninsula is observed. It is also evident that compression ceased at the northern shelf area of the Gulf of Saros.

Geology of the Saros graben and its implications for the evolution of the North Anatolian fault in the Ganos–Saros region, northwestern Turkey

Palaeo- and neo-tectonic evolutions of the Gulf of Saros, northwestern Turkey, were investigated based on geological mapping, geomorphology, seismicity and GPS measurements. In this area three overlapping basins were differentiated: the Thrace, Enez and Saros basins. The Thrace basin opened during the middle Eocene on the continental Strandja Massif as a post-collisional, fault-controlled extensional basin after the closing of the Intra-Pontide Ocean to the south. This basin reached its greatest extent during the Late Eocene–Early Oligocene and then, turned into an intramontane terrestrial basin from the middle Miocene onwards. The Enez basin opened along the southern margin of the former Thrace basin as an E–W-trending half graben during the middle Miocene. The age and geometry of this basin corresponds to the extensional basins of the Aegean graben system to the south. Stratigraphy and structures related to the North Anatolian fault indicate that the fault started to be active since Pliocene and modified older structures. The fault zone evolved in two stages in and around the Gulf of Saros. During the initial stage the fault was trending as a single segment with a transpressional nature and without any stepping, and caused thrusting and folding subparallel to the main trace of the fault. Since the Late Pliocene the fault was left-stepping in the Saros area, giving rise to the Saros pull-apart basin. We modelled these two stages by using boundary element method, and found that there is a good correlation between the modelling results and geology and geomorphology.

Geological evolution of the Gulf of Saros, NE Aegean Sea

The Gulf of Saros is an Upper Miocene transtensional basin in NW Anatolia, formed by the interaction between the North Anatolian Fault and the N-S extensional tectonic regime of the Aegean. The present configuration of the basin evolved mainly during the Plio-Quaternary under the increased activity of the North Anatolian Fault. During the late Miocene-late Quaternary, no sedimentation took place on the shelves. After this long hiatus, an important change in tectonic style about 0.2 Ma BP allowed sedimentation to resume in the gulf.

GPS and geologic estimates of the tectonic activity in the Marmara Sea region, NW Anatolia

A dense network of 52 Global Positioning System (GPS) sites is used to determine the velocity field and strain rate pattern at the western end of the North Anatolian Fault Zone (NAFZ) in the Marmara Sea region, NW Anatolia. Based on four GPS campaigns carried out biannually between 1990 and 1996, the detailed kinematic field of crustal motion is calculated. The southernmost station shows an average rate of 22±3 mm/yr oriented westward relative to Istanbul (Eurasia). This value is indicative of the dextral strike‐slip motion of NW Anatolia relative to the Black Sea. The GPS velocity field reveals that the sites located on the Thrace‐Black Sea block show negligible motion relative to Istanbul. The GPS‐based deformation pattern of three sections crossing the westward prolongation of the NAFZ is compared with neotectonic data. Both the GPS and the neotectonic data give clear evidence that most of the deformation occurs along a relatively narrow E‐W oriented zone. This belt extends from the single fault trace of the NAFZ through the Gulf of Izmit, the Marmara Sea, the Şarköy region, and the Gulf of Saros into the North Aegean Trough (NAT). In addition, the cross sections show the increasing influence of the N‐S oriented extension in the Aegean Sea region. From the comparison of the directions and rates of shear and normal strain deduced from GPS data with geologic and seismic observations we conclude that the most active zone of dextral shear follows a line along Mudurnu Valley, the northern Marmara Sea and the Saros Trough where it joins the NAT.

The North Anatolian transform fault: its age, offset and tectonic significance

The North Anatolian transform fault is a morphologically distinct and seismically active strike-slip fault which extends for about 1200 km from Karliova to the Gulf of Saros along the Black Sea mountains of N. Anatolia. It takes up the relative motion between the Black Sea and the Anatolian plates, thereby connecting the E. Anatolian convergent zone with the Hellenic Trench through the complex plate boundary zone of the Aegean. For most of its length, the transform has a typical strike-slip fault zone morphology, characterized by narrow ‘rift zone,’ offset, captured and dammed streams, sag ponds and other deformed morphological features. The fault zone is a broad region of extensively crushed country rock cut by a number of parallel and/or anastomosing strike-slip faults. The transform has periods of seismic activity the last of which, from 1939 to the present, is characterized by frequent 6 ≤ M≤7earthquakes; these are separated by quiet periods of about 150 years. The crust along the fault zone is thinner than normal. The transform probably originated some time between the Burdigalian and the Pliocene and has an offset of about 85 km. Whether the offset of the fault changes systematically along its strike is not known. The North Anatolian transform fault seems to have originated as a consequence of the Arabia-Anatolia collision durmg the late (?middle) Miocene, when the Anatolian Plate originated and was wedged out into the oceanic tract of the E. Mediterranean from the converging jaws of Arabia and Eurasia to prevent excessive crustal thickening in E. Anatolia. The westerly motion of Anatolia with respect to Eurasia and Africa caused a great change in the tectonic evolution of the eastern Mediterranean, giving rise to the Aegean extensional regime and to internal deformation of Anatolia.