H. Kurt

An active, deep marine strike‐slip basin along the North Anatolian fault in Turkey

The Tekirdag depression within the Marmara Sea in the Mediterranean region is an active, rhomb-shaped strike-slip basin along the North Anatolian fault with a basin floor at a water depth of −1150 m. New multichannel seismic reflection data and on-land geological studies indicate that the basin is forming along a releasing bend of the strike-slip fault and is filled with syntransform sediments of Pliocene-Quaternary age. The basin is bounded on one side by the North Anatolian fault and on the other side by a subparallel normal fault, which forms the steep submarine slope. In cross section the basin is strongly asymmetric with the thickness of the syntransform strata increasing from a few tens of meters on the submarine slope to over 2.5 km adjacent to the North Anatolian fault. Seismic sections also show that the slope-forming normal fault connects at depth to the North Anatolian fault, implying that the basin is completely detached from its substratum. The whole structure can be envisaged as a huge, rather flat, negative flower structure. The releasing bend of the North Anatolian fault, responsible for the formation of the basin, is flanked by a constraining bend. Along the constraining bend, the syntransform strata are being underthrust, implying a recent change in the direction of the regional displacement vector. This thrusting is responsible for the uplift of the submarine slope to a height of 924 m, possibly by a mechanism of elastic rebound. Regional geology suggests that most of the syntransform strata are lacustrine with only the topmost few hundred meters consisting of deep marine clays. The anomalous present depth of the Tekirdag depression is due to reduced Quaternary sedimentation coupled with high rates of displacement along the North Anatolian fault, which amounts to 20 mm/yr in the Marmara Sea region.

Investigation of the submarine active tectonism in the Gulf of Gökova, southwest Anatolia–southeast Aegean Sea, by multi-channel seismic reflection data

Submarine active tectonism in the Gulf of Gokova located at the southwest Anatolia–southeast Aegean Sea region was investigated by means of multi-channel seismic reflection data. The Gokova basin is filled by the latest Miocene–Pliocene–Quaternary sediments with maximum thickness of about 2.5 km. The Lycian Nappes, which predominantly cover extreme southwestern Anatolia, constitutes the basement rocks for the Gokova province. The gulf was mainly opened by a buried major listric normal fault, so-called Datca Fault, which has not been previously discussed in the literature. The north-dipping, mainly E–W-trending Datca Fault is located at the southern part of the gulf, whereas its associated antithetic faults are located at the north. The onset time of the opening of the gulf is possibly in the latest Miocene–Pliocene. In terms of local rather than regional effects, the activity of the Datca Fault has decelerated, possibly since the Pleistocene. The Datca Fault might have gained its curved fault plane as it evolved, beginning as planar and/or using antecedent planes of the Lycian Nappes in the area. As the extension progressed, i.e., as the hanging wall block slipped further north, gravity may have impeded rather than helped the faulting. On the other hand, continuing extension in the area may have initiated a second phase of faulting, i.e., WNW–ESE-oriented subgrabens in the gulf and major E–W normal faulting in the northeast margin. A bathymetric low in the mid-gulf area and a horst–graben system in the eastern part of the gulf are observable from the bathymetric data and are well correlated to the seismic data. Although the main orientation of the gulf is E–W, more recent WNW–ESE structures are remarkable in the mid-gulf and in the eastern part of the gulf. The latest WNW–ESE structures are also in agreement with the results of GPS and SLR studies as well as plate motion modelling by total moment tensor of earthquakes in the western Anatolia–Aegean Sea region, particularly in southwestern Anatolia. The amount of total N–S extension within the gulf is estimated as at least 5.5 km since the latest Miocene–Pliocene with overall constant extension rate of at least 1.1 mm/y where the estimated extension factor is about β=1.3.

Evidence for widespread creep on the flanks of the Sea of Marmara transform basin from marine geophysical data

“Wave” fi elds have long been recognized in marine sediments on the fl anks of basins and oceans in both tectonically active and inactive environments. The origin of “waves” (hereafter called undulations) is controversial; competing models ascribe them to depositional processes, gravity-driven downslope creep or collapse, and/or tectonic shortening. Here we analyze pervasive undulation fi elds identifi ed in swath bathymetry and new high-resolution multichannel seismic (MCS) refl ection data from the Sea of Marmara, Turkey. Although they exhibit some of the classical features of sediment waves, the following distinctive characteristics exclude a purely depositional origin: (1) parallelism between the crests of the undulations and bathymetric contours over a wide range of orientations, (2) steep fl anks of the undulations (up to ~40°), and (3) increases in undulations amplitude with depth. We argue that the undulations are folds formed by gravity-driven downslope creep that have been augmented by depositional processes. These creep folds develop over long time periods (≥0.5 m.y.) and stand in contrast to geologically instantaneous collapse. Stratigraphic growth on the upslope limbs indicates that deposition contributes to the formation and upslope migration of the folds. The temporal and spatial evolution of the creep folds is clearly related to rapid tilting in this tectonically active transform basin.

Exploring submarine earthquake geology in the Marmara Sea

The disastrous 1999 earthquakes in Turkey have spurred the international community to study the geometry and behavior of the North Anatolian Fault (NAF) beneath the Marmara Sea. While the area is considered mature for a large earthquake, the detailed fault geometry below the Marmara Sea is uncertain, and this prevents a realistic assessment of seismic hazards in the highly-populated region close to Istanbul. Two geological/geophysical surveys were recently conducted in the Marmara Sea: the first in November 2000 with the R/V Odin Finder, and the second in June 2001 with the R/V CNR-Urania. Both were sponsored and organized by the Institute of Marine Geology of the Italian National Research Council (CNR), in cooperation with the Turkish Council for Scientific and Technical Research (TUBITAK) and the Lamont-Doherty Earth Observatory of Columbia University Multi-beam bathymetry, multi-channel seismic reflection profiling, magnetometry high-resolution CHIRP sub-bottom profiling, and bottom imaging were carried out with a remotely operated vehicle (ROV). Over 60 gravity and piston cores were collected.

Petrophysical properties of three reservoir models by means of AVA data modeling

Summary Amplitude Versus Angle (AVA) modeling has been performed to find out effects of porosity and fluid type of three different reservoir rock types. We used two layer earth models with shale/limestone, shale/sandstone and shale/shaly sandstone horizontal interfaces. Our upper layer consists of nonpermeable shale unit and lower reservoir layer has permeable rocks with different percentage of porosities. Each interface is modeled with assumption of reservoir rock has porosities of 10%, 20% and 30%. Reservoir rocks are assumed with gas, oil and brine saturated. AVA gathers were created by using the matrix form of the Zoeppritz equations for these three trap models. Three dimensional graphs that show the reflection coefficients versus incident angle and porosity produced for better interpretations. Increasing porosity in all the models reduces the amplitudes from positive values to negative values. In every trap model increasing porosity ratios change the AVA curve significantly and thus the AVA class type can change especially in shale–sandstone interface model. Lithology and porosity are the main factors effecting on the angle dependent reflectivity for all the three models. Change in the rock fluid type is less effective for the AVA anomalies in these models.