Present-day tectonic regime and stress patterns from the formal inversion of focal mechanism data, in the North of Central–East Iran Blocks

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The kinematic models and the associated orientation of compressional stress in northeastern Iran have long been the subject of debate. Previously proposed models have relied on the interpretation of overall fault geometry and geological fault slip data. These models generally suffer from low resolution of the temporal and spatial changes in the stress field and geodynamic regime. In the recent years there has been a significant increase in the number of focal mechanisms available for the study area, and it is now possible to estimate the present-day stress field in relative detail based on the formal stress inversion method. We compiled 59 earthquake focal mechanisms from the Global/Harvard CMT catalogue and various other sources. These we grouped into 7 separate regions (boxes). For each region reduced stress tensors have been obtained by formal stress inversion using the Win-Tensor program (Delvaux and Sperner, 2003). The present-day stress state obtained from earthquake focal mechanisms shows that the North of Central–East Iran Blocks (NCEIB) is presently subjected to a transpressional tectonic regime (R′ = 1.78 ± 0.26), with an N37 ± 7° E direction of horizontal principal compression. This stress state is consistent with the direction of convergence between the Arabian and Eurasian plates. In this part of northeastern Iran this convergence is accommodated by clockwise rotations. The Doruneh fault system (DFS) curves ESE–WSW, perpendicular to relative motion. This orientation results in oblique compression with a significant along-belt component of left-lateral shear in the western and central segments and belt-perpendicular compression in the eastern segment.