Nature and structural heterogeneities of the lithosphere control the continental deformation in the northeastern and eastern Iranian plateau as revealed by shear-wave splitting observations

Abstract

To investigate the intracontinental deformation during the early stage of Arabia-Eurasia collision, we studied the seismic anisotropy of the northeastern and eastern Iranian plateau via teleseismic shear-wave splitting analysis using a dense array of 65 stations. While the average delay times of ∼0.75 s for most stations suggest a moderate magnitude of anisotropy, small-scale variations in fast polarization directions (FPDs) indicate complex anisotropic features that may be related to the nature and structural heterogeneities of the lithosphere. The variations in FPDs are correlated with tectonic boundaries, including the northeastern plateau around the Paleo-Tethys suture and large-scale strike-slip faults bounding the Lut block in the eastern plateau. This suggests that the weak boundary zones may have significantly impacted the lithospheric deformation and anisotropy in response to the ongoing continental collision. Within the Lut block, the insignificant surface deformation suggests that the upper crust might not be noticeably affected by collision, and the fault-induced deformation mainly occurs at depth. The regional northeastward asthenospheric flow driven by the lithospheric thickness contrast across the collisional front could contribute but to different extents among areas. The asthenosphere-induced anisotropy has a magnitude (delay time) comparable to that from lithospheric deformation in central Lut, consistent with the two-layer model observed there. It dominates the seismic anisotropy in northern Lut where the thin lithosphere may contribute less but probably facilitates melt concentration on the top of the asthenosphere and thus enhances anisotropy through aligned melt-filled pores. The complex deformation pattern of the study region suggests that the intracontinental deformation may be controlled by not only the collisional processes at the plate boundary but also the nature and structural heterogeneities of the continental lithosphere itself, and the Arabia-Eurasia convergence may be more preferentially accommodated along tectonic boundaries and at depth.