The project INDEPTH (International Deep Profiling of Tibet and the Himalaya), phase IV, seismology group collected an abundance of active- and passive-source seismic data in north Tibet from 2007-2009, and I will present results from those investigations and implications for crustal structure and tectonics. Specifically, we derived seismic structure and P-wave velocities from the 270-km long, roughly north-south, active-source, wide-angle reflection and refraction profile crossing the Songpan-Ganzi terrane, Kunlun Mountains, and Qaidam Basin using first arrivals tomography and ray tracing. Then we calculated P-to-S receiver functions using linear and regional arrays of passive seismic stations in the same areas to provide independent constraints on crustal thickness, lithospheric structure, and Vp/Vs ratios.
The wide-angle reflection and refraction velocity model shows a crustal thickness change from 70 km beneath the Kunlun Mountains to 50 km beneath central Qaidam. Crustal P-wave velocities in the thickened Songpan-Ganzi terrane and Kunlun Mountains exhibit lower velocity crust also characteristic of southern Tibet, whereas crustal velocities in the central Qaidam Basin resemble average continental crust. In contrast to previous work, we relocate the 20-km crustal thickness change to ~40 km north of the Kunlun Mountains topographic front, in a region of overlapping bright Moho reflectors at ~70 km and ~50 km. P-receiver functions calculated for passive seismic stations coincident with the active-source profile show a very similar Moho structure. P-receiver functions calculated for stations scattered along-strike from 91.5° to 98°E near the Kunlun-Qaidam boundary show that the width of the overlapping region ranges from ~10-40 km at different locations along the boundary, and near Golmud the deeper reflector may dip northwards. The crustal thickness change appears unrelated to the strike-slip North Kunlun Fault. At depths shallower than 100 km, we also see no evidence of southward subduction of Eurasian lithosphere. Crustal velocities and impedance contrasts suggest that instead weak Tibetan lower crust is injected northward beneath stronger Qaidam crust.
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