Does middle-lower crustal flow exist in the eastern Tibetan Plateau? Insights from finite-element modeling and geodetic observations

Abstract

It remains unknown whether middle-lower crustal flow exists in the eastern Tibetan Plateau. More and more geodetic data have been acquired during the past several tens of years, especially after the 2008 Mw 7.9 Wenchuan earthquake. Some important questions are proposed: can we use the geodetic data to determine if middle-lower crustal flow exists in the eastern Tibetan Plateau, and can we estimate the patterns and magnitudes of the crustal flow in this region if it exists? To investigate these questions, we construct a three-dimensional viscoelastoplastic finite-element model to simulate regional crustal deformation during the seismic cycle of 2008 Wenchuan earthquake in cases with and without middle-lower crustal flow, and analyze and compare these model results with geodetic observations. From modeling the co- and post-seismic deformation, we have suggested a steady-state (Maxwell) viscosity of 1.1 × 1019 Pa s within a 40-km-thick middle-lower crust beneath the eastern Tibetan Plateau. Through inter-seismic deformation, we find that the eastern Tibetan Plateau, in crustal shortening model, has undergone pure shear deformation, leading to an even-distributed surface uplift rate of 0.5 mm/yr. It is significantly lower than the average uplift rate of 2.2 mm/yr from the observed leveling data. On the other hand, the hybrid crustal flow model can predict a faster surface uplift rate with the fastest uplift rate localized at the flow front, which is more consistent with the observed leveling data. Based on the model results and the leveling data, we infer that middle-lower flow may exist within a wide area beneath the eastern Tibetan Plateau.