Crustal structure beneath the eastern margin of the Tibetan Plateau and its tectonic implications

Two crustal cross sections through the eastern margin of the Tibetan Plateau are jointly determined from deep seismic sounding. The E - W trending line AA' passes through the western Sichuan plateau (including the Songpan- Garze terrane and the Longmenshan fault belt) and ends in the Sichuan basin (a part of the Yangtze craton). Line BB' has a trend of NNE and crosses the Songpan- Garze terrane. Two- dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data. Our seismic velocity models indicate that the western Sichuan plateau and the Sichuan basin have crustal thicknesses of 62 and 43 km, average crustal P wave velocities of 6.27 and 6.45 km/s and lower crustal (V-p > 6.5 km/s) thicknesses of 27 and 15 km, respectively. Density models constructed from the seismic velocity models are consistent with observed Bouguer gravity anomalies. We infer that collision between the Tibetan Plateau and the Yangtze craton has caused thickening of the lower crust and uplift of the western Sichuan plateau. We detect a low- velocity layer in the upper crust of the western Sichuan plateau but observe no equivalence in the Sichuan basin; west dipping thrusts may detach into this low- velocity layer. The seismic phase PmP in the western Sichuan plateau has low amplitude, suggesting high attenuation in the lower crust (Q(p) of 100 - 300). We suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western Sichuan plateau.