Deep burial of Asian continental crust beneath the Pamir imaged with local earthquake tomography

摘要

An inclined zone of intermediate-depth seismicity beneath the Pamir orogen in Central Asia has been interpreted as southward subduction of a slab of Asian lithosphere. However, it is not known whether Asian lithosphere subducts intact or only partially. We used arrival times of shallow and intermediate-depth earthquakes, recorded with a temporary (2008-2010) seismic network in this region, to invert for 3D models of seismic velocities in an attempt to answer this question. With local seismicity reaching depths of up to 240 km, the deep structure of the Pamir could be illuminated with high resolution.The resulting velocity models show a north-south contrast in crustal seismic velocities in the Pamir, with very low P velocities (5.7-5.9 km/s at 15-30 km depth), coupled with relatively low vp/vs (<1.70), at mid-crustal levels in the southern part of the orogen. At sub-Moho depths, we image an arcuate high-velocity (8.2-8.6 km/s) slab dipping south in the eastern Pamir and east in the Pamir's southwest, underlying the intermediate-depth earthquakes. On top of the high-velocity slab and just above the onset of deep seismicity, between a depth of 60 to 100 km, very low compressional wavespeeds (around 7.1 km/s) and high vp/vs ratios (≥1.80) attest to subducted crustal rocks. Additionally, we carried out 2D numerical thermomechanical modeling of the continental collision in the Pamir, focusing on the fate of the crust and mantle lithosphere of the Asian and Indian plates. Seismic velocities were computed from the modeling results, and the resulting images were compared with the velocity distributions obtained from seismic traveltimes.Combining tomography and modeling results, we infer that a substantial amount of crustal material is pulled down beneath the Pamir by cold mantle lithosphere to depths of at least 80-100 km. From there on, only lower crust and mantle lithosphere continue their subduction, and earthquakes occur inside the lower crustal layer probably due to metamorphic reactions.