Block rotation in western Turkey: 1. Observational evidence

Abstract

Western Turkey is a region of rapid continental extension, where extension rate increasing westward causes counterclockwise vertical vorticity and hence counterclockwise rotation around vertical axes on a regional scale. Field observations and seismicity of the past century are used to deduce positions and slip senses of major active oblique normal faults in this region, which take up extension in the upper crustal brittle layer and allow fault‐bounded blocks to rotate around vertical axes. The principal active normal faults in western Turkey strike roughly east‐west near the Aegean coast, turning gradually toward westnorthwest‐eastsoutheast strike across ∼200 km distance inland to the east. Slip vector azimuths indicate typical extension direction on these faults is ∼S18°±8W. They truncate a second set of faults with smaller displacement and roughly orthogonal strike and slip vector azimuth. As well as subdividing the brittle layer into blocks that show little internal deformation, some major oblique normal faults subdivide it into domains that show different Neogene rotation around vertical axes. Some domains contain sets of major active oblique normal faults that bound blocks that are elongated, angular, and shaped like tilted dominoes. These faults take up extension, and can also take up rotation around vertical axes provided they and the blocks between them rotate around vertical axes at the same rate. Where adequate paleomagnetic observations are available, they indicate these domains have rotated counterclockwise, as expected, by up to ∼40°: roughly equal to the change in strike of the principal set of active faults between the Aegean coast and the eastern edge of the actively extending zone. However, evidence is scarce for the right‐lateral strike slip expected in some kinematic models on a set of oblique normal faults that takes up extension and counterclockwise rotation; instead, each fault appears to have the component of strike slip necessary to enable it, given its local strike, to slip in Ue direction in which the region is extending. Two domains that have rotated clockwise are approximately circular, in plan view with diameter ∼90 km, and are bounded by faults with variable orientation; each comprises a single block with no internal major active faults.