Seismic Anisotropy and Its Geodynamic Implications in Iran, the Easternmost Part of the Tethyan Belt

Abstract

In this study, we use the results of seismic anisotropy as inferred from shear wave splitting analyses of SKS phases to propose a geodynamical model of the Arabia-Eurasia collision zone. A detailed analysis of the 202 non-null splitting and 196 null splitting measurements obtained from a dense temporary network are utilized to investigate the possibility of lateral and vertical variations in the anisotropic parameters and the hypothesis of a dipping anisotropic layer. A 2D geodynamical model of the western part of the collision zone is constructed. The preferred 2D model suggests that the belt-parallel orientation of fast axes in the western Zagros originates from a lithospheric transpressional deformation. The plate motion-parallel pattern in central Iran and western Alborz coincides with the decrease in the lithospheric thickness. Thus, we believe this trend has its origin in the asthenosphere. A combination of the keel effect of the thickened Zagros lithosphere, the asthenospheric edge-driven convection flow and the lithospheric deformation in the shear zones can cause the NW-SE oriented splitting pattern reported in some parts of central Iran. The asthenospheric flow beneath the thinner lithosphere to the south of the Bitlis suture in northern Iraq is likely the causative mechanism for our observed plate motion-parallel splittings there. The variation of the convergence obliquity along the Alborz and Zagros inferred from analysis of geodetic data implies that a change in the pattern of lithospheric deformation and the consequent anisotropy is expected.