As the western end point of continental collision between the Indian and Eurasian plates, Pamir is an ideal place to research uplifting mechanisms in the Tibetan plateau. In this study, 141 644 Pn arrivals were used to obtain seismic wave velocities and anisotropy in the uppermost mantle beneath Pamir and its adjacent regions by performing tomographic inversion of Pn travel times. The data were selected from multiple databases, including ISC/EHB, the Annual Bulletin of Chinese Earthquakes, and regional bulletins of Xinjiang. The tomography results reveal significant features with high resolution and correlate well with geological structures. The main results are as follows： （1） The Pn wave velocities are particularly high in the old stable blocks such as Tarim basin, Indian plate and Tajik basin, while the low Pn velocities always lie in tectonically active regions like the western Tibetan plateau, Pamir, Tianshan and Hindu Kush. （2） Strong Pn anisotropy is found beneath the Indian-Eurasian collision zone; its direction is parallel to the collision are and nearly perpendicular to both the direction of maximum compression stress and relative crustal movement. The result is probably caused by the pure shear deformation in the uppermost mantle of the collision zone. （3） A geodynamic continent-continent collision model is proposed to show anisotropy and collision mechanisms between the Indian plate and the Tarim and Tajik basins.
This study investigated the crustal attenuation structures of Sg and Lg waves of the northeastern Tibetan Plateau.We collected ML amplitude data recorded at 168 permanent stations between 1985 and 2016 and 11 temporary broadband stations between 2014 and 2016.Detailed Q0 variation maps of Sg and Lg waves were obtained by applying ML amplitude tomography.The average Q0 values of the Sg and Lg wave were 440 and 220,respectively.Relatively high attenuation anomalies of both waves appeared in the central and eastern regions of the Bayan Har Block and the east edge of the Qiangtang Block,which may be related to partial melting,high geotemperature,and strong tectonic processes.High attenuation anomalies were also found in the Qilian Orogenic Belt and Hetao Graben,which may be related to their active tectonic behavior and densely distributed faults.The relatively low attenuation anomalies of both waves were revealed in the Alax and Ordos blocks,Qaidam,Tarim,Qinghai Lake,and Gonghe basins,which can be explained by the tectonically stable properties and ancient composition of geological elements.These results indicate that the path between the highly attenuated lower crust of the Bayan Har Block and the Qilian Orogenic Belt is obstructed by three adjacent low attenuated areas(i.e.,the Qilian,Qinghai Lake,and Gonghe basins);thus,it appears unlikely that a crustal flow channel from the interior of the Tibetan Plateau to the Qilian Orogenic Belt will form.
