Knowledge of the structure of continental strike-slip faults within the lithosphere is essential to understand where the deformation occurs and how strain localizes with depth. With the aim to improve the constraints on the lower crust and upper mantle structure of a major continental strike-slip fault, we present a high-resolution teleseismic tomography of the North Anatolian Fault Zone (NAFZ) in Turkey. Our results highlight the presence of a relatively high velocity body between the two branches of the fault and significant along-strike variations in the NAFZ velocity structure over distances of ~20 km. We interpret these findings as evidence of laterally variable strain focussing caused by preexisting heterogeneity. Low velocities observed in the crust and upper mantle beneath the NAFZ support the presence of a narrow shear zone widening in the upper mantle, where we constrain its width to be ~50 km.Major funding was provided by the UK Natural Environment Research Council (NERC) under grant NE/I028017/1. Equipment was provided and supported by the NERC Geophysical Equipment Facility (SEIS-UK). This project is also supported by Bogaziçi University Scientific ˘ Research Projects (BAP) under grant 6922 and Turkish State Planning Organization (DPT) under the TAM project, number 2007K120610
