Probing the Lithospheric Rheology Across the Eastern Margin of the Tibetan Plateau Based on Postseismic Deformation

Tuesday, 16 December 2014: 11:50 AM
Mong-Han Huang, University of California Berkeley, Berkeley, CA, United States, Roland Burgmann, Univ California Berkeley, Seismological Laboratory, Berkeley, CA, United States and Andrew Mark Freed, Purdue Univ, West Lafayette, IN, United States
The fundamental geological structure, geodynamics, and rheology of the Tibetan Plateau have been debated for decades. Two end-member models have been proposed: (1) the deformation of Tibet is broadly distributed and associated with ductile flow in the mantle and middle or lower crust, (2) the Tibetan Plateau formed during interactions between rigid lithospheric blocks with localization of deformation along major faults. The nature and distribution of continental deformation are governed by the varying rheology of rocks and faults in the lithosphere. Insights into lithospheric rheology can be gained from observations of postseismic deformation, which represents the response of the Earth’s interior to coseismic stress changes. Here we use up to 2 years of InSAR and GPS measurements to investigate postseismic displacements following the 2008 Mw 7.9 Wenchuan earthquake in eastern Tibet and probe the differences in rheological properties across the edge of the Plateau. We find that near-field displacements can be explained by shallow afterslip on the Beichuan Fault, which is anti-correlated with the coseismic slip distribution. Far-field displacements cannot be explained by a homogeneous rheology, but instead require a viscoelastic lower crust (from 45 to 60 km depth) beneath Tibet with an initial effective viscosity of 4.4×1017 Pas and a long-term viscosity of 1018 Pas, whereas the Sichuan Basin block has a high-viscosity upper mantle (>1020 Pa s) underlying an elastic 35-km-thick crust. The inferred strong contrast in lithospheric rheologies between the Tibetan Plateau and the Sichuan Basin is consistent with models of ductile lower crustal flow that predict maximum topographic gradients across the Plateau margins where viscosity differences are greatest.