Upper Crustal Anisotropy in the Southeastern Termination of the Zagros Mountains and Qeshm Island

Summary We investigate upper crustal seismic anisotropy in the southeastern termination of the Zagros Mountains and Qeshm Island through shear wave splitting analysis on the aftershock records of 3 local earthquakes: the 2006 Tiab earthquake (Mw = 6) in the north of the Main Zagros Reverse Fault (MZRF), the 2006 Fin event (Mw = 5.9) in SE Zagros, and the 2005 Qeshm Island event (Mw = 5.8). The results show that north of the MZRF in the Faryab region, the local anisotropy as measured by fast axis orientations of the shear waves, is mainly controlled by crustal structures such as fault shear fabrics or plane fractures around a set of orthogonal active strike-slip faults of a dominantly NE-SW strike. Stress-induced anisotropy caused by fluid-filled microcracks aligned with local maximum horizontal compression seems to be of minor importance. In the vicinity of the MZRF and the Zendan-Minab-Palami Fault, another major fault of the region, local anisotropy is controlled by the structural fabric developed by those faults. Far from the faults, the fast orientations rotate to become aligned with the regional compressional stress. The MZRF is located on an old subduction suture, and it seems that the metamorphic or mylonite fabrics that prevail in this part have shaped the anisotropic structure in the upper crust. In the Simply-Folded Belt of the Zagros and in Qeshm Island, where the deformation of the upper crust is younger, local anisotropy is mostly stress-induced, and the role of fault structures, even though the region is affected by extensive and complex sets of active faulting, is relatively minor. The intensity of anisotropy, as witnessed by splitting delay times, decreases from the metamorphic belt north of the MZRF, to the deformation front in Qeshm Island. This observation is in agreement with the decrease in the accumulated deformation from north to south everywhere in the Zagros. Crustal anisotropy in the Zagros does not show large-scale uniform patterns, rather, it varies over relatively small distances. This is partly due to the fact that both stress-induced and structurally-controlled anisotropies are at work, and partly because of the short-distance variations of the modern stress field in the Zagros.