Comprehensive crustal structure and seismological evidence for lower crustal flow in the southeastern margin of Tibet revealed by receiver functions

Abstract The lower crustal flow model has become an interesting proposal to describe the growth and expansion of eastern Tibet. To date, however, the geophysical results are not definitive due to the non-uniqueness of the solutions found through data inversion and the resolution limitations. In this study the polarity of P-to-S converted phases at intra-crustal interfaces are analyzed in order to reveal evidence for low seismic velocity and lower-crustal flow in southeastern Tibet. We are interested in a joint study that is both a mapping of the crustal structure (Moho topography and Poisson's ratio) and an attempt to constrain an ICLVZ. We have determined the crustal thickness and Poisson's ratio from P (PRFs) and S (SRFs) receiver functions acquired in 108 permanent seismic stations deployed in the Sichuan and Yunnan areas. Also, we have obtained 98 measurements of Pms-phase splitting parameters at the array stations. The crustal thickness provided by PRFs is consistent with that given by SRFs. It varies from ~ 60 km in the Songpan-Ganzi (SG) fold system and the northern part of the Sichuan-Yunnan diamond-shaped block (SYDSB), near the Eastern Himalayan Syntaxis, to ~ 33 km in southern Yunnan. The Poisson's ratio varies mostly from 0.24 to 0.32; the highest values 0.28–0.32 are found along the axis formed by the Longmenshan fault, the Lijiang-Jinhe fault and in the Tengchong volcano area. The value of ~ 0.28 observed in SYDSB and the SG fold system is attributed to the presence of a low-velocity zone in the lower crust, and partly to the great sediment thickness in Sichuan Basin. The crust is clearly anisotropic in the southeastern margin of Tibet: the fast-wave polarization direction exhibits a NW-SE regional bias, while the time delay varies between 0.07 s and 1.27 s, being the average 0.54 s ± 0.12 s, implying the existence a strong shear zone in the lower crust. Based on the negative polarity observed after the direct P-wave in teleseismic receiver functions, we have detected an intra-crustal low-velocity zone (ICLVZ) in a wide region including east and southeast of Tibet and southern Yunnan, so both the shear zone and ICLVZ suggest the existence of lower crustal flow. The reverberation PpPs and PsPs + PpSs phases at the Moho are clearly weaker in SYDSB and the SG fold system than in the Indochina Block and eastern Yunnan, implying that SYDSB has a structure and tectonic history different from Indochina on its west side and eastern Yunnan on its east side. The above results provide seismological evidence for a lower crustal flow spreading from the eastern Tibet and that in its advance southward has invaded SYDSB widely, crossed the Lijiang-Jinhe fault and reached southern Yunnan.