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

Abstract

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.