Electromagnetic imaging of the Thrace Basin and intra-pontide subduction zone, northwestern Turkey

Magnetotelluric data were acquired at nine stations along an similar to80 km profile crossing the Thrace Basin and the Istranca Massif. These data were modeled using two-dimensional inverse techniques. The main findings are: (1) In the northernmost part of the profile, a wide (similar to21 kin) zone of the Istranca Massif with very high resistivity (>2000 ohm m) was imaged at a depth of 2.5 km, extending to similar to35 km depth. (2) Sedimentary rocks of the similar to63 km wide Thrace Basin (<75 ohm m) extend to a maximum depth of about 8 kin along the profile. (3) Below the Thrace Basin, an undulating similar to10 km thick zone of conductive lower crust with low resistivity (<75 ohm in) is present. (4) The depth to the lithospheric upper mantle (similar to250 ohm m) is about 45 kin beneath the Istranca Massif in the northernmost part of the profile, and rises to a lowest value of 17 km toward the southeast part of the, possibly indicating mantle uplift. (5) The location and existence of the Intra-Pontide subduction zone in the Thrace Basin is the subject of debate. We imaged for the first time a slab subducting to the north beneath the Istranca massif in the Thrace Basin. (6) Our georesistivity model also indicates that, in contrast to western Turkey, all electrically conductive asthenosphere is not present in northwestern Turkey

Magnetotelluric images of the crust and mantle in the southwestem Taurides, Turkey

Inversion of the magnetotelluric data across the southwestern Taurides reveals two subzones of crust with varying thicknesses: conductive lower crust (350 Omega m) upper crust, with four resistive cores (>2000 Omega m) separated by three relatively conductive vertical zones. The first and second vertical zones coincide with surface faults interpreted in Anatolia, such as Fethiye Burdur Fault Zone. The third one is the most conductive and lies in continuity with the Strabo Fault Zone in the Mediterranean Sea. A hypocentral cross section of earthquakes along the profile shows more dense seismic activity in the second resistive core where the conductive crust is not present beneath it. The depth of the crust/upper mantle boundary varies between 30 and 50 km and has an undulating character. The resistivity of the upper mantle reaches 500-1000 Omega m. (C) 2004 Elsevier B.V. All rights reserved

An approach for heat flow determination in the absence of geothermal gradient measurements: west Anatolia example

In western and southwestern Anatolia, the geothermal gradient cannot be accurately measured in many wells due to the complex horizontal water cycle. The aim of this study is to calculate the geothermal gradients by using a new approach for such wells. In this case, the first step is to calculate the geothermal gradients in order to estimate the heat flow values around the wells. Naturally, a direct heat flow estimate cannot be made for wells without a geothermal gradient. However, the heat flow values can be indirectly obtained by different methods. Curie point depth, silica geothermometer, and magnetotelluric conductive layer depth are some of these indirect methods. In this study, we used the heat flow data based on the Curie depth. In this approach, Curie temperature gradients are calculated using the Curie heat flow values and thermal conductivities for problematic wells. An empirical relationship between the Curie temperature gradient and the correctly measured temperature gradients in the region was established. With this relationship, it is possible to make an approach from the current Curie temperature gradient to the normal temperature gradient. In this way, heat flow can also be calculated for areas where no geothermal gradient can be obtained, and heat flow distribution over the whole area can be given. This proposed approximation, to obtain thermal gradient and heat flow, can also be extended to any other region

The deep resistivity structure of southwestern Turkey: Tectonic implications

Magnetotelluric data along two profiles, across the Lycian nappes and the Beydag autochthon of the southwestern Taurides in the eastern Mediterranean region images shallow and deep crustal structures. Inversion of the magnetotelluric data from profiles K and H reveals two subzones of the crust of varying thickness; the first is the conductive and viscoelastic lower crust (< 75 ohm in), whereas the second is the resistive (500-7500 ohm in) and brittle upper crust. The thickness of the uppermost conductive units, the Lycian nappes, is found to be 3.5-4 kin in the northwest and 0.5 kin in the southeast. The total thickness of the autochthon that forms the resistive upper crust varies from 7-16 kin beneath the Lycian nappes to 11-20 kin beneath the Korkuteli region along profile K. The depth to the upper/lower crust boundary varies from 10 to 30 km in the region. The resistive upper crust is interrupted by more conductive vertical zones. One of these zones along profile K coincides with the Fethiye Burdur fault zone (FBFZ), one of the most prominent geological structures in the region. The FBFZ lies in continuity with the Pliny and the Strabo subduction zones in the Mediterranean Sea. Projections of all the vertical resistivity discontinuities in the NIT images onto the gravity map with the main surface faults show an alignment parallel to the FBFZ, and the Pliny and the Strabo trenches. The Bouguer gravity map shows a low gravity zone between the towns of Fethiye and Burdur. This gravity low also coincides with the alignment of projections of vertical high conductivity zones onto the map. Almost no earthquakes occur in the conductive lower crust, whereas significant earthquakes occur in the resistive upper crust, and the resistive lithospheric upper mantle (similar to250 ohm in) in the southeastern part of the region along the profile K. In contrast, in the northwestern part of the profile, the upper mantle is conductive (80 ohm in), indicating a viscoelastic character

On the Salt Water Intrusion into the Durusu Lake, Istanbul: A Joint Central Loop TEM And Multi-Electrode ERT Field Survey

Durusu Lake is the biggest and most important freshwater source supplying drinking water to the European side of Istanbul. In this study, electrical resistivity tomography (ERT) and transient electromagnetic (TEM) measurements were applied to detect a possible salt water intrusion into the lake and to delineate the subsurface structure in the north of Durusu Lake. The ERT and TEM measurements were carried out along six parallel profiles extending from the sea coast to the lake shore on the dune barrier. TEM data were interpreted using different 1-D inversion methods such as Occam, Marquardt, and laterally constrained inversion (LCI). ERT data were interpreted using 2-D inversion techniques. The inversion results of ERT and TEM data were shown as resistivity depth sections including topography. The sand layer spreading over the basin has a resistivity of 150-400 Omega m with a thickness of 5-10 m. The sandy layer with clay, silt, and gravel has a resistivity of 15-100 Omega m and a thickness of 10-40 m followed by a clay layer of a resistivity below 10 Omega m. When the inversion of these data is interpreted along with the hydrogeology of the area, it is concluded that the salt water intrusion along the dune barrier is not common and occurs at a particular area where the distance between lake and sea is very close. Using information from boreholes around the lake, it was verified that the common conductive region at depths of 30 m or more consists of clay layers and clay lenses

A VLF survey using current gathering phenomena for tracing buried faults of Fethiye-Burdur Fault Zone, Turkey

Fethiye-Burdur Fault Zone (FBFZ) produces moderate to large earthquakes which can severely affect the surrounding cities spreading towards the basins of the region. In this study, with the aim of feature city planning, very low frequency electromagnetic (VLF) survey was carried out across (FBFZ) to locate subsurface faults covered by basin fills. VLF parameters such as the apparent resistivity, phase (VLF-R response) real and imaginary parts of tipper (VLF-EM response) were obtained using GBZ, a well known radio station in England at Oxford, which give a strong signal in our survey area with a suitable frequency (19.6 kHz) for our purposes. VLF method is called as VLF-EM, when it is only relying on the measurement of horizontal and vertical magnetic fields. However, when it includes measurement of horizontal electric field, providing resistivity values, it is referred as VLF-R method. In this study, the direction of horizontal electric field is perpendicular to fault strike describing the H-polarization mode. Although this mode produces strong VLF-R (resistivity) response, we also obtained quite clear and characteristic VLF-EM anomalies along some of the survey profiles. Both VLF-R and VLF-EM responses with the contribution of the vortex and gathered (direct) currents, in E and H-polarization modes, are reviewed and VLF anomalies observed over FBFZ are discussed. In FBFZ, current gathering anomalies become stronger in the presence of high conductivity contrast and in the presence of parallel fault branches separating the medium into resistors in series. In these conditions, changing electrical charge at the contacts of faults cause successive variations in E-field. As a result VLF-R response is strengthened and also secondary magnetic field which causes VLF-EM anomalies may be enhanced, in H-polarization mode. Interpretation of resistivity, phase and tipper data with current density pseudosections, over our survey lines, showed the location of buried faults of FBFZ beneath the sedimentary covers of the basins. (C) 2009 Elsevier B.V. All rights reserved

Plio-Quaternary kinematic development and paleostress pattern of the Edremit Basin, western Turkey

WOS: 000378177700015The Edremit Basin and Kazdag High are the most prominent morphological features of the Biga Peninsula in northwest Anatolia. There is still no consensus on the formation of Edremit Basin and debates are on whether the basin evolved through a normal, a right-lateral or a left-lateral strike-slip faulting. In this study, the geometric, structural and kinematic characteristics of the Edremit Basin are investigated to make an analytical approach to this problem. The structural and kinematic features of the faults in the region are described according to field observations. These fault-slip data derived from the fault planes were analyzed to determine the paleostress pattern of faulting in the region. According to the performed analysis, the southern end of the Biga Peninsula is under the influence of the ENE-WSW-trending faults of the region, such as the Yenice-Gonen, the Edremit, the Pazarkoy and the Havran-Balikesir Fault Zones. The right step-over geometry and related extension caused to the development of the Edremit Basin as a transtensional pull-apart basin between the Havran-Balikesir Fault Zone and the Edremit Fault Zone. Field observations showed that the Plio-Quaternary faults at the Edremit Gulf and adjacent areas are prominently right-lateral strike-slip faults. Our paleostress analyses suggest a dominant NE-SW extension in the study area, as well as NW-SE direction. This pattern indicates the major effects of the North Anatolian Fault System and the component of Aegean Extensional System in the region. However, our kinematic analysis represents the dominant signature of the North Anatolian Fault System in basin bounding faults. The field observations and kinematic findings of this study are also consistent with the regional GPS, paleomagnetic and seismological data. This study concludes that the North Anatolian Fault System is the prominent structure in the current morphotectonic framework of the Edremit Gulf and adjacent areas. (C) 2016 Elsevier B.V. All rights reserved.Scientific and Technical Research Council of Turkey (TUBITAK) [CAYDAG-111Y258]The study is supported by The Scientific and Technical Research Council of Turkey (TUBITAK) with grant no: CAYDAG-111Y258. The authors are grateful to Bahadir Aktug (AU) for interpretations on the GPS derived data of the region. Constructive comments made by Uwe Ring and two anonymous reviewers greatly improved the manuscript

Mohr-circle-based rotational invariants of a magnetotelluric data set from the Thrace region of Turkey: Geological implications

In the summer of 1995, magnetotelluric data were acquired at forty sites along a southwest-northeast profile crossing northwestern Turkey in continental Europe. The traverse, 204 km in length, crossed the boundary between a metamorphic mass called the Istranca Massif and a large Tertiary sedimentary basin called the Thrace Basin. The pseudosections of Mohr-circle-based rotational invariant characteristics of the magnetotelluric data such as central impedance (d(3)), anisotropy angle (lambda) and phase of central impedance (phi(d3)) were prepared with classical magnetotelluric parameters (apparent resistivity and impedance phase), and detailed Mohr-circle displays were shown for selected stations as a function of the Bostick depth. The pseudosections and the Mohr-circle displays suggest that the northeast Thrace region is geologically more complex than the southwest region, where the Istranca Massif bounds the Thrace Basin by a steep NW-SE-oriented fault, possibly with a considerable component of normal slip. In the southwest Thrace region, NE-SW-oriented faults with a component of strike slip occur. forming a broad structural uplift to the north of Saroz Bay. We also find that the most evident changes in geoelectrical strike directions are related to changes in thickness of the upper crust and electromagnetic dimensionality effects, knowledge of which is extremely important in modelling and inversion of data, are predominant on the magnetotelluric data set

Late Cenozoic shift from extension to strike-slip stress regime in the west of the Biga Peninsula, NW Turkey

To the west of the Biga Peninsula, there are Paleozoic-Mesozoic metamorphic massifs and young fault systems that developed in different directions and periods. We investigated the types, formation times, and kinematics of these faults. Our field observations and kinematic studies show that there were three stress phases in the region between the Neogene and Quaternary. According to the analysis, in the first phase during the Miocene-Pliocene, the region was under NW-SE extension which affected western Anatolia. Right-lateral faults with left step-overs developed with the entrance of the North Anatolian Fault System into the Marmara region in the Middle-Late Pleistocene. This system started the second stage in the Late Pleistocene, and a short-lived NNE-SSW compression regime prevailed in the region. As a result, reverse faults and thrusts developed. In the third phase, During the Late Pleistocene-Holocene, the direction of extension turned to the NE-SW as the North Anatolian Fault System affected the region. The data we obtained from the region show that the West Anatolian Extensional System was effective during the Miocene-Pliocene. Then, particularly within the Late Pleistocene-Holocene period, the North Anatolian Fault System played the main role in the morphotectonic development of the Biga Peninsula and its surroundings

Conductivity structure along the Gediz graben, west Anatolia, Turkey: Tectonic implications

Groups of grabens in west Anatolia have contrasting E-W and NE-SW orientations and are the subject of debate as to their relative ages and relationships. We investigated the E-W-trending Gediz graben and its neighboring NE-SW-trending Gordes, Demirci, and Selendi grabens, which form an important graben system representative of the region. We studied gravity data from one profile and magnetotelluric (MT) data from two profiles, 73 km and 93 km long. The data supports the hypothesis that the Gediz graben was superimposed onto the (older) NE-SW grabens. 2D gravity and MT modelling revealed an undulating graben floor, varying in depth between 500 and 30004000 in (gravity-MT); within the graben two apparent basins 3-4 and 1.5-2.5 kin deep (gravity-MT) are separated by a subsurface horst. The residual gravity map appears to indicate the continuation of NE-SW grabens from north of Gediz graben to beyond its southern border