Eastward expansion of the Tibetan Plateau by crustal flow and strain partitioning across faults

摘要

The lateral expansion of the southeastern Tibetan Plateau causes devastating earthquakes, but is poorly understood. In particular, the links between regional variations in surface motion~(1–3) and the deeper structure of the plateau are unclear. The plateau may deform either by movement of rigid crustal blocks along large strike-slip faults~(4,5), by continuous deformation~(6,7), or by the eastward flow of a channel of viscous crustal rocks~(8,9). However, the importance of crustal channel flow was questioned in thewake of the 2008 Wenchuan earthquake~(10–12). Controversies about the style of deformation have persisted, in part because geophysical probes have insufficient resolution to link structures in the deep crust to the observed surface deformation. Here we use seismic data recorded with an array of about 300 seismographs in western Sichuan, China, to image the structure of the eastern Tibetan Plateau with unprecedented clarity. We identify zones of weak rocks in the deep crust that thicken eastwards towards the Yangtze Craton, which we interpret as crustal flow channels. We also identify stark contrasts in the structure and rheology of the crust across large faults. Combined with geodetic data, the inferred crustal heterogeneity indicates that plateau expansion is accommodated by a combination of local crustal flow and strain partitioning across deep faults. We conclude that rigid block motion and crustal flow are therefore not irreconcilable modes of crustal deformation.