Transpressional deformation in the lithospheric mantle beneath the North Anatolian Fault Zone

Abstract

We studied mantle xenoliths sampled from two late Miocene volcanic centers located immediately north of the North Anatolian Fault Zone (NAFZ) in western Turkey. The spinel lherzolite and harzburgite xenoliths equilibrated at temperatures ranging from 757 to 1019°C, estimated from two-pyroxene geothermometry. Pressures estimated from the Cr-in-clinopyroxene geobarometer and pseudosection modelling range from 1.0 to 2.1 GPa, which correspond to depths of 38–75 km. We used X-ray computed tomography to quantify the three-dimensional shape preferred orientation of spinel grains. Spinel displays dominantly oblate fabric ellipsoids, consistent with flattening strain. Olivine crystallographic preferred orientation (CPO), determined by electron backscatter diffraction, exhibits chiefly axial-[010] and A-type patterns. The axial-[010] CPO pattern is consistent with the spinel fabric that shows flattening strain, and is primarily developed in xenoliths from the volcanic center nearer to the NAFZ. Obliquity of olivine CPO to the spinel lineation decreases with proximity to the NAFZ, suggesting an increase in shear strain. Our results favor an interpretation of transpressional deformation in the upper mantle below the NAFZ. We attribute this pattern to a restraining bend in the mantle section of the NAFZ.