Deep crustal structure of northwestern part of Turkey

Abstract

The deep crustal structure of the Polatli-Nallihan-Karasu (NW Turkey) area was investigated by Magnetotelluric and Transient Electromagnetic measurements. All measurements were acquired as part of a national project called Crustal Structures of Turkey undertaken by the General Directorate of Mineral Research and Exploration of Turkey (MTA). Magnetotelluric data were collected from 44 stations located along a 250 km northwest to southeast profile crossing in succession the Istanbul Zone, Intra-Pontid Suture, Sakarya Continent, Izmir-Ankara Suture, Tavsanli Zone (the Tauride-Anatolide Belt), Intra-Pontid Suture and Sakarya Continent. After decompositional analysis and strike direction estimation, static shift is corrected by using transient electromagnetic data from MT apparent resistivity data. The processed MT data were inverted to derive a two-dimensional geoelectrical model. The estimated model is interpreted with the help of gravity data observed along the MT lines and using earthquake epicenter which occurred around MT measurement lines. Four layers with distinctive resistivity values dominate the geoelectrical section. The uppermost layer, having resistivity values of 5-35 ohm m can be interpreted as the sedimentary fill. This layer reaches down to about 10 km depth in Haymana Basin. The second layer has resistivity values higher than 110 ohm m and reaches 27 km depth. The first and second layers can be inferred as constituting the upper crust in the general model of the Earth's lithosphere. The third layer underlying the upper crust has resistivity values of 10-30 ohm m corresponding to the lower crust. The fourth layer having high resistivity values (>225 ohm m) can be considered as the upper mantle. The depth of the boundary separating the crust from the upper mantle (Moho) varies between 20 and 40 km. High conductive zones between the Istanbul Zone and Sakarya Continent, Sakarya Continent and Tavsanli Zone coincide with high Bouguer gravity anomaly values. It can be inferred from these properties that these zones correspond to the Intra-Pontide Suture and the Izmir-Ankara Suture. Noticeable differences in tectonic characteristics are observed between the southern and northern part of Sakarya Continent in the geoelectrical section. The northern part of the continent is separated by high dip slip faults while the faults in the southern part do not affect the high resistive zone in the deeper part of the section. These faults controlled the development of the Haymana Basin but they do not alter the high resistive zone to form separate blocks. The majority of these faults, which are clearly seen in the geoelectrical section are not evident in the sedimentary fill. These buried faults have not been studied in detail prior to the present study. (C) 2010 Elsevier B.V. All rights reserved.