Aysan Gürer

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 Gördes, 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 3000-4000 m (gravity-MT); within the graben two apparent basins 3–4 and 1.5-2.5 km 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. The MT model revealed three main zones of varying thickness within the crust. The britde upper crust comprises two zones: sedimentary fill (apparent resistivity 15-50 ohm.m) and Menderes massif basement (200 ohm.m). The third zone is highly conductive lower crust (10 ohm.m), identified by our MT modeling at an average depth of 10 km. This conductive layer was considered in conjunction with two other regional features, high heat flow values and shallow earthquake focal depths. A heat flow map shows a very high average value of 108 mWm−2 for west Anatolia and 120-300 mWm−2 for the Gediz graben area specifically, compared with the world average of 80 mWm−2. Seismological records showing shallow earthquake focal depths together with the high conductivity zone were taken to indicate a partially melted, viscoelastic lower crust.

Deep Conductivity Structure of the North Anatolian Fault Zone and the Istanbul and Sakarya Zones Along the Gölpazari-Akcaova Profile, Northwest Anatolia

Magnetotelluric (MT) measurements at six locations along a 90-km profile across the area between Golpazari and Akcaova have been modeled in two dimensions to increase understanding of the deep conductivity structure of the western part of the North Anatolian fault zone (NAFZ) and the Istanbul and Sakarya zones. It is well known from surface geology that the branch of the NAFZ that passes through the Pamukova Valley with an E-W strike separates the region into two sub-areas, containing contrasting sets of geological features. These two areas also exhibit significant differences in terms of their deep conductivity structure. Electrical resistivity is quite low (10 ohmm) south of the fault at an approximate depth of 26 km, compared to the area north of the fault zone. This low-resistivity zone may provide an indication of partial melting at this depth. In the northern part of the profile beneath Ucgaziler (DUC) and Akcaova (DAK), a five-layered conductivity sequence obtained by magnetotelluric modeling and t...

Geoelectromagnetic and geothermic investigations in the Ihlara Valley geothermal field

Abstract The Ihlara Valley is situated within a volcanic arc that is formed by the collision of the eastern Mediterranean plate system with the Anatolian plate. In this study we will present data from a reservoir monitoring project over the Ihlara-Ziga geothermal field, located 22 km east of Aksaray, in central Anatolia. Although identified geothermal resources in the Ihlara Valley are modest, substantial undiscovered fields have been inferred primarily from the volcanic and tectonic setting but also from the high regional heat flow (150–200 mWm−2) on the Kirsehir Massif. In 1988 and 1990, geoelectromagnetic surveys were undertaken by MTA-Ankara to confirm the presence of a relatively shallow (≈ 0.5–1 km), hydrothermally caused conductive layer or zone. CSAMT and Schlumberger resistivity data show good correspondence with each other, and 2-D geoelectric models are also in harmony with geologic data and gravity anomalies. The depth of the resistive basement, which is interpreted as Paleozoic limestone, is 200–250 m in the western part and increases eastward (≈ 600–750 m). This may imply N-S-oriented normal faulting within the survey area. The parameters of the top layer are a resistivity of 25 to 95 ohm m and a thickness of between 100 and 250 m. The thickness of the conductive tuffs between the top layer and the basement, whose resistivity is about 4–5 o hmm, also increases eastward (from 100 to 450 m). The apparent resistivity maps for the frequencies between 32 and 2 Hz reveal a localized low resistivity anomaly to the east of Belisirma.

Electromagnetic Imaging of the Thrace Basin and Intra-Pontide Subduction Zone, Northwestern Turkey

Magnetotelluric data were acquired at nine stations along an ˜80 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 (˜21 km) zone of the Istranca Massif with very high resistivity (>2000 ohm m) was imaged at a depth of 2.5 km, extending to ˜35 km depth. (2) Sedimentary rocks of the ˜63 km wide Thrace Basin (<75 ohm m) extend to a maximum depth of about 8 km along the profile. (3) Below the Thrace Basin, an undulating ˜10 km thick zone of conductive lower crust with low resistivity (<75 ohm m) is present. (4) The depth to the lithospheric upper mantle (˜250 ohm m) is about 45 km 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 profile, 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, an electrically conductive asthenosphere is not present in northwestern Turkey.

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 m), whereas the second is the resistive (500-7500 ohm m) and brittle upper crust. The thickness of the uppermost conductive units, the Lycian nappes, is found to be 3.5-4 km in the northwest and 0.5 km in the southeast. The total thickness of the autochthon that forms the resistive upper crust varies from 7-16 km beneath the Lycian nappes to 11-20 km 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 MT 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 (~250 ohm m) 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 m), indicating a viscoelastic character.

Development of Evaporites and the Counterclockwise Rotation of Anatolia, Turkey

In western and central Anatolia, numerous basins developed during Tertiary time. The major basins generally are not connected to one another, and cover large areas containing substantial sedimentary deposits. These basins include the following: Thrace, Gediz, Buyuk Menderes, Beypazari, Tuz Golu, Ulukisla, Sivas, Erzincan, Mus, and Adana, as well as others. Two significant differences can clearly be seen in the basin sequences: (1) the age of evaporites—in central Anatolia, evaporites developed during the Oligocene, whereas in western Anatolia they formed during the late Miocene; and (2) the character of the deposition—in central Anatolia, Oligocene redbed molasse deposits are abundant, whereas in western Anatolia they developed in the late Miocene and the beds are thin and not very abundant. Climate is one of the governing factors of deposition, controlled by elevation and latitude. Therefore, the difference in sequences between central and western Anatolia depends on these two parameters. In this study, ...

Upper crustal electrical resistivity structures in the vicinity of the Çatalca Fault, Istanbul, Turkey by magnetotelluric data

A magnetotelluric survey was performed at the Çatalca Region, west of Istanbul, Turkey with the aim of investigating geoelectrical properties of the upper crust near the Çatalca Fault and its vicinity. Broadband magnetotelluric data were collected at nine sites along a single southwest-northeast profile to image the electrical resistivity structure from surface to the 5 km depth. The dimensionality of the data was examined through tensor decompositions and highly two-dimensional behavior of the data is shown. Following the tensor decompositions, two-dimensional inversions were carried out where E-polarization, B-polarization and tipper data were utilized to construct electrical resistivity models. The results of the inversions suggest: a) the Çatalca Fault extends from surface to 5 km depth as a conductive zone dipping to southwest; b) the thickness of the sedimentary cover is increasing from SW to NE to 700 m with low resistivity values between 1–100 Ωm; c) the crystalline basement below the sedimentary unit is very resistive and varies between 2000–100000 Ωm; d) a SW-dipping resistivity boundary in the northeastern part of our profile may represent the West Black Sea Fault.