Crustal composition model across the Bangong–Nujiang suture belt derived from INDEPTH III velocity data

Abstract

The Bangong–Nujiang suture (BNS) belt separates the Lhasa and Qiangtang plates and, as indicated from previous teleseismic tomography, receiver functions, and active source seismic experiments, record the convergence and collision between the Indian and Eurasian plates. In this study, we construct a crustal composition model along the 400 km long NNW–SSE-oriented INDEPTH III profile by comparing the P-wave velocity (VP ), S-wave velocity (VS), VP /VS ratio, density (ρ) and Lam´ e impedances (ρλ, ρμ) to those determined by laboratory measurements of different crustal rock types. Corrections for pressure and temperature are performed in order to directly compare the physical properties determined from the INDEPTH III data with these laboratory measurements made at room temperature and variable pressures. Inversion of the INDEPTH III data set to rock compositions indicates that there are a number of differences in the crustal composition of the Lhasa and Qiangtang plates. For example, while the average composition of the upper crust in both plates is mainly characterized by granite–granodiorite and granite gneiss, these lithologies are much thicker beneath the Lhasa plate. Metagreywacke provides a best fit lithology for the upper crust beneath the BNS. The middle crust in both plates is somewhat similar, with diabase being the best fit for the Lhasa plate and basalt for the Qiangtang plate. The lower crust is quite distinctive for the two plates, with mafic garnet granulite, gabbronorite or troctolite providing a best fit for the Lhasa plate, whereas mafic garnet granulite, marble and anorthositic granulite being the best fit for the Qiangtang plate. While the difference in composition between the two plates may be affected by the different thermal regimes determined for them, it nevertheless appears that the BNS clearly forms a boundary between the physical properties and therefore the composition of the Lhasa and Qiangtang plates.