Lithospheric deformation and seismotectonics in the southeastern Tibetan Plateau: A holistic perspective from joint inversion of multi-geophysical data

Abstract

The southeastern Tibetan Plateau (SETP) plays a pivotal role in accommodating the crustal deformation between the complex Tibet Plateau and the South China Block during the Cenozoic associated with the India-Asia Convergence. In this study, we establish a high-resolution 3-D lithospheric structure model of the SETP through joint inversion of observable multi-geophysical data, to understand its Cenozoic progressive deformation processes, deep seismotectonic environment, as well as regional geodynamic mechanism. We identify two low-density zones within the mid-lower crust that are separated by a pronounced high-density body beneath the inner zone of the Emeishan Large Igneous Province (ELIP). We also image an interconnected channel flow in the lower crust beneath the SETP. To further confirm the relationship between Cenozoic deformation propagation and deep lithospheric architecture, we adopt a holistic perspective from joint inversion of observable multi-geophysical data, coupled with integrated analysis on geometric-kinematic characteristics of major strike-slip fault zones and regional tectonics in the SETP. The results show that the mechanically weak mid-lower crust of the SETP is characterized by low effective elastic thickness (Te), high heat flow, low-density/velocity and low-viscosity, which might accommodate the ductile flow and provide an important channel for the lateral extrusion of crustal materials from the Tibetan Plateau, and ultimately contribute to the episodic lithospheric deformation of the SETP. We trace three main phases of deformation within the SETP during the Cenozoic: the Eocene-early Oligocene latitudinal crustal shortening and thickening, the late Oligocene-early Miocene clockwise rotation and lateral extrusion along major strike-slip faults, and the mid-late Miocene lower crustal flow accompanied with regional kinematic reversal. The lithospheric deformation, the invasion of fluids and the upwelling of deep molten materials are conducive to strain accumulation, which might also explain the occurrence of large earthquakes. Geodynamically, we consider that both the spatio-temporal variance of convergent rates, subduction angle, and processes of the India-Asia Convergence may be associated with episodic crustal deformation and intense seismicity in the SETP. The aqueous fluids in the weak mid-lower crust may have propagated outward due to the long-term gravitational driving forces and contributed to the lithospheric deformation and seismicity of the SETP. Furthermore, the retreat of the subducted Indian slab as well as the rollback and back-arc spreading of the western Pacific Plate also provided favorable conditions for the eastward extrusion of the Tibetan Plateau.