Outward growth of Tibetan Plateau: Insights from joint inversion of lithosphere structure

Abstract:

Models for the growth of the Tibetan plateau have been a subject of great debate for decades. Here we add to the debate with new insights from an advanced imaging of the lithosphere structure of the western China. To resolve the ambiguity of model parameters and to improve resolution in seismic imaging, a combination of different data sets that have sensitivities to different parameters is required. We’ve recently developed joint-inversion methods involving Pn travel times, receiver functions, and surface wave dispersion measurements and applied them systematically to seismic stations in western China to obtain improved 3D P and S lithosphere models. Our models show significantly higher Vp/Vs ratios in northern Himalaya Block and southern Qiangtang Block than in the Lhasa Block. Mid-crustal low velocity zone (LVZ) is observed under much of the Tibetan Plateau. However, it is much more pronounced under the outer regions of the plateau (the Himalaya and Qiangtang Blocks) than under its interior (the Lhasa Block). The location of pronounced mid-crustal LVZ correlates (anti-correlates) with the distribution of seismicity in the plateau; the low seismicity areas have more pronounced LVZs and greater Vp/Vs values and the high seismic areas have less pronounced LVZs and smaller Vp/Vs values. The observations support the existence of a proto-Tibetan Plateau core and the outward growth of the margins at a later stage after the India-Eurasian collision (Wang C.S. et al., PNAS, 2008). The proto-plateau is more rigid and can sustain stress to cause brittle failure of earthquakes while the materials at the margins are weaker and can be subject to plastic deformation with much less seismicity. In this model, the proto-plateau core acts as an efficient medium for the stress transfer from the collision front to the margins for the outward growth of the plateau. The crust and mantle lithosphere act as a coherent unit as indicated by the consistent pattern of seismic anomalies with depth. We speculate that such a stress-transfer mechanism from strong lithosphere blocks (e.g., major basins surrounding the plateau such as Tarim, Qaidam, and Sichuan) may have played a strong role in generating the deformation and seismicity in Asia.