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37 Introduction e fundamental geological structure, geodynamics, and rheology of the Tibetan Plateau have been debated for de- cades� 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 rig- id lithospheric blocks with localized deformation along major faults� e nature and distribution of continental deformation is 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 represent the response of the Earth’s interior to coseismic stress changes� Here we use up to 2 years of interferometric synthetic aperture radar (InSAR) and GPS measurements to investigate postseis- mic displacements following the 2008 M w 7�9 Wenchuan earth- quake 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 aſterslip on the Be- ichuan Fault (BCF), which is anti-correlated with the coseismic slip distribution� Far-field displacements cannot be explained with a homogeneous rheology, but instead require a visco- elastic lower crust (from 45–60 km depth) beneath Tibet and a relatively strong Sichuan block� e inferred strong contrast in lithospheric rheologies between the Tibetan Plateau and the Sichuan Basin is consistent with models of ductile lower crust- al flow that predict maximum topographic gradients across the Plateau margins where viscosity differences are greatest� Postseismic Deformation A number of processes contribute to postseismic deforma- tion� Aſterslip is the continuous slip of the fault aſter the main- shock and is oſten found to occur downdip of the fault rupture zone� We use a dislocation model in a layered Earth structure to investigate the aſterslip distribution by inverting the geodet- ic data� We modify the fault geometry proposed by Shen et al. (2009) and extend the fault depth to 65 km depth for aſterslip at the downdip extension (Figure 2�13�1a)� e aſterslip occurs on both shallow and deep parts of the BCF that represent the fit to the near- and far-field displacements� We use a 3D finite element model (Huang et al., 2014) to construct a regional rheologic model composed of an elastic Tibet upper crust and Sichuan crust, a viscoelastic Tibet lower crust, and a viscoelastic upper mantle� We use the bi-viscous Burger’s rheology to represent the transient and steady state pe- riods of the postseismic deformation� e Burger’s rheology is composed of a Maxwell fluid connected in series with a Kelvin solid to represent the steady state and transient viscosities� e best-fitting model involves a low-viscosity lower crust in Tibet between 45 and 60 km in depth� Constraining Tibet’s Lithospheric Rheology A Maxwell fluid with a constant viscosity fails to explain the postseismic displacement rate changes, and shows the need for a model in which the effective viscosity increases with time� e change of effective viscosity implies either transient rheology or stress-dependent power-law rheology or both� In this study, we try to distinguish the main mechanism that contributes to the postseismic displacements and the contrasting rheology be- tween Tibet and Sichuan, and thus adopt a simple bi-viscous Burgers rheology� As the viscoelastic relaxation model can explain most of the early postseismic transients in the middle field, we can rule out aſterslip as being the major cause of the initial rapid displacements� e best-fitting model predicts a transient viscosity (ηK) of 1017�9 Pa s and a steady-state vis- cosity (ηM) of 1018 Pa s, whereas the Sichuan Basin block has a high-viscosity upper mantle (> 1020 Pa s) underlying an elastic 35 km-thick crust Models of Tibetan lower crustal channel flow predict that the Plateau margins are steepest where the viscosity of the sur- rounding blocks are highest, and thus impede and divert the flow (Clark et al., 2005)� ese models predict the strongest viscosity contrasts with the Sichuan and Tarim Basin blocks (η = 1016–18 Pa s in a 15–20 km thick lower crustal layer versus ~1020–21 Pa s in adjacent crust), where topographic gradients are greatest� Our preferred viscosity structure deduced from the postseismic deformation transients across the Longmen Shan is consistent with such contrasting lithospheric rheology and de- formation between eastern Tibet and the Sichuan Basin� Acknowledgements We thank D� Dreger, Z� Shen, I� Ryder, and F� Pollitz for dis- cussions and constructive suggestions� is work is support by the National Science Foundation grant (EAR-1014880)� References Clark, K� M�, Bush, J� W� M�, and Royden, L� H� Dynamic topog- raphy produced by lower crustal flow against rheological strength heterogeneities bordering the Tibetan Plateau� Geophys. J. Int., 162, 575-590, 2005� Huang, M�-H, Bürgmann, R�, and Freed, A� M� Probing the litho- spheric rheology across the eastern margin of the Tibetan Plateau� Earth Planet Sci. Lett., 396, 88-96, 2014� Shen, Z�K�, J� Sun, P� Zhang, Y� Wan, M� Wang, R� Bürgmann, Y� Zeng, W� Gan, H� Hiao, and Q� Wang, Slip maxima at fault junctions and rupturing of bariers during the 2008 Wenchuan earthquake, Nat. Geosci., 2, 718-724, 2009� 13 Probing the Lithospheric Rheology Across the Eastern Margin of the Tibetan Plateau Mong-Han Huang, Roland Bürgmann, Andrew M. Freed (Dept. of Earth, Atmospheric, and Planetary Sciences, Purdue University, IN)
