Seismic Anisotropy and Its Geodynamic Implications in Iran

Abstract

We used the results of seismic anisotropy as inferred from shear wave splitting analyses of the core-refracted phases and combined them with the quasi-Love wave observations and geodetic measurements to propose a geodynamical model of the Arabia-Eurasia collision zone. A detailed analysis of the non-null splitting and null splitting measurements obtained from a dense temporary network is utilized to investigate the possibility of lateral and vertical variations in the anisotropic parameters and the hypothesis of a dipping anisotropic layer (Sadeghi-Bagherabadi, 2018a; 2018b). The preferred 2-D geodynamical model of the western part of the collision zone 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. The quasi-Love wave observations also indicate that the fast-axis orientations are highly affected by the lateral variations of the LAB depth in the region.