Radially anisotropic lithospheric shear wave velocity structure of the Pamir-Hindu Kush region and its implications for continental subduction and intra-plate intermediate-depth seismicity

Since the collision of the Indian and Eurasian plates, the Pamir-Hindu Kush region has experienced dynamic processes of terrane accretion, crustal thickening and surface uplift similar to the Tibetan plateau. During this period, subduction processes have accommodated a large amount of crustal shortenings and induced most intense intra-plate intermediate-depth seismicity worldwide. However, it is a matter of debate over the subduction patterns and the morphology of the subduction plates under the Pamir-Hindu Kush region as well as the causes of the intra-plate intermediate-depth seismicity. A detailed image of lithospheric structure could offer new insights on above-mentioned debates.

In this study, we constructed a lithospheric scale three-dimensional (3-D) radially anisotropic shear wave velocity model beneath the Pamir-Hindu Kush region based on 2-D Rayleigh wave and Love wave phase velocity maps at periods of 20-167 s. Our results image a low velocity anomaly in the mid-lower crust from 15 km to the Moho beneath the northwest of the Pamir plateau, probably indicating the existence of widespread crustal partial melting in this region. Another low-velocity anomaly is observed in the uppermost mantle (at depths from the Moho to 140 km) of the northern and central Pamir. We interpret this anomalous low-velocity as the lower crustal material subducting into the mantle with the Asian lithospheric mantle. Additionally, the lower crust of the southeastern Pamir and lithospheric mantle of the south Pamir are characterized with high shear wave velocities. It likely implies that the Indian lithospheric mantle is subducting to the north of the south Pamir, while its lower crust is underthrusting beneath the base of the lower crust of the south Pamir. The subduction of the Indian lithosphere thickens the crust of Pamir which may induce eclogitization in the lower crust of the Indian plate. The eclogitized lower crust and the Indian lithospheric mantle under the Pamir may contribute to this fast velocity anomaly in the lower crust and the upper mantle of the south Pamir. We will further derive the radial anisotropy from inverted Vsh and Vsv structure. Both of shear wave velocity and radial anisotropy models will shed new lights on the continental subduction and intra-plate intermediate-depth seismicity.