Structure of the crust beneath the southeastern Tibetan Plateau from teleseismic receiver functions

Abstract

Southeastern Tibet marks the site of presumed clockwise rotation of the crust due to the India–Eurasian collision and abutment against the stable Sichuan basin and South China block. Knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region. Here, we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method. We find that the crustal thickness decreases gradually from the Tibetan Plateau proper to the Sichuan basin and Yangtze platform but that significant (intra-)crustal heterogeneity exists on shorter lateral scales (<1000 km). Most receiver functions reveal a time shift of ∼0.2 s in the direct P arrival and negative phases between 0 and 5 s after the first arrival. Inversion of the receiver functions yields S-velocity profiles marked by near-surface and intra-crustal low-velocity zones (IC-LVZs). The shallow low-velocity zones are consistent with the wide distribution of thick surface sedimentary layers. The IC-LVZ varies laterally in depth and strength; it becomes thinner toward the east and southeast and is absent in the Sichuan basin and the southern part of the Yangtze platform. Results from slant-stacking analysis show a concomitant decrease in crust thickness from ∼60 km in the Songpan-Ganze fold system to ∼46 km in the Sichuan basin and ∼40 km in the Yangtze platform. High Poisson's ratios (>0.30) are detected beneath the southeastern margin of Tibet but in the Sichuan basin and southeastern Yangtze platform the values are close to the global average. Combined with high regional heat flow and independent evidence for mid-crustal layers of high (electric) conductivity, the large intra-crustal S-wave velocity reduction (12–19%) and the intermediate-to-high average crustal Poisson's ratios are consistent with partial melt in the crust beneath parts of southeastern Tibet. These results could be used in support of deformation models involving intra-crustal flow, with the caveat that significant lateral variation in location and strength of this flow may occur.