Spatial variation in seismic anisotropy beneath the eastern and northeastern Iranian plateau and its geodynamic implications

The Iranian plateau is a natural laboratory for studying the early stage of continental collision and plateau development. The collisional front and northern plateau are the major areas accommodating the Arabia-Eurasia convergence. GPS observations suggest that the blocks of central Iran with minor shortening may be relatively rigid. However, recent seismic imaging results suggest that the lithosphere in this region might not be rigid for it is thin and not seismically fast. Widespread mantle-derived magmatism since Middle Miocene also lends support to a relatively hot and weak lithosphere. It may raise a question of why these blocks could behave rigidly when transmitting stresses to the north.

Deformation patterns of the lithosphere and asthenosphere in the northeastern and eastern Iranian plateau, which can be constrained by seismic anisotropy, may help to understand the nature of the lithosphere within the continental interior and its responses to the Arabia-Eurasia collision. We studied the seismic anisotropy of the region via teleseismic shear-wave splitting analysis on dense array data and compared the new results with multidisciplinary observations, particularly the surface strain rates and the structure of the lithosphere-asthenosphere system. In northeastern Iran around the Paleo-Tehtys suture, the dominant fast polarization direction (FPD) is NW-SE, subparallel to the strikes of thrust faults and orogenic belts. This combined with the relatively higher strain rates and thicker crust and lithosphere suggests that northeastern Iran with pre-existing weakness may have experienced considerable lithospheric shortening. The Lut block, which is a major block of eastern Iran bounded by large-scale strike-slip faults and previously assumed rigid, shows a complex anisotropic structure. In its northern part where the strain rates are low, the average NE-SW FPD has no obvious link to active faults but is roughly parallel to the collision-induced asthenospheric flow. The area to the south around the Dasht-e-Bayaz fault shows high strain rates and a complex structure of Moho. The generally NW-SE FPDs are subparallel to the direction of the surface right-lateral shear, possibly reflecting a fault-controlled lithospheric deformation pattern. Further south is the central Lut area with moderate strain rates. It is characterized by a two-layer structure of anisotropy, with the FPDs in the upper and lower layers being similar to those of the area around the Dasht-e-Bayaz fault and the northern Lut block, respectively. This feature indicates that the anisotropy and deformation of the central Lut area could be affected by both large-scale strike-slip faults and collision-induced mantle flow.

Collectively, our observations suggest that both the collisional processes at the plate boundary and the nature and structural heterogeneities of the continental lithosphere may control the intracontinental deformation of the Iranian plateau. The observed minor deformation of the Lut block and also other blocks within this young plateau does not necessarily mean that these blocks are rigid, but is probably because of significant deformation preferentially taking place at not only the collision front but also mechanically weak zones in the hinterland, which may have accommodated most of the Arabia-Eurasia convergence.