Postseismic Deformation of the 2008 Wenchuan Earthquake Illuminates Lithospheric Rheological Structure and Dynamics of Eastern Tibet

Abstract

AbstractTectonic research of the Tibetan Plateau has long focused on its deformation style and mechanisms. The 2008 Mw7.9 Wenchuan earthquake ruptured the Longmen Shan fault located at the eastern rim of the plateau and excited a viscoelastic response of the lithosphere. We infer a three‐dimensional (3D) rheological structure of eastern Tibet from modeling nine years of postseismic displacements observed by GPS. Our solution provides tight constraints on the lower‐crustal and upper‐mantle steady‐state viscosities of the Songpan‐Ganzi Terrane as (5.0 ± 0.7) × 1018 and (1.3 ± 0.3) × 1019 Pa s, respectively, consistent with a “jelly sandwich” model of Tibet, but not with some crustal channel flow models featuring much lower viscosities. The inferred lower‐crustal and upper‐mantle transient viscosities are (5.0 ± 1.3) × 1017 and (5.0 ± 1.5) × 1018 Pa s, respectively, suggesting nonlinear deformation mechanisms. The adjacent West Qinling and Sichuan blocks feature an order‐of‐magnitude higher rheological strength, which is consistent with the changes in the crustal material properties and interseismic deformation style across the East Kunlun‐Tazang and Longmen Shan faults. Our results enable us to propose a conceptual 3D tectonic deformation model, in which the eastward extrusion of Tibet is absorbed in the Songpan‐Ganzi crust mainly by E‐W shortening and N‐S extension, accommodated through faulting of conjugate strike‐slip faults in the upper crust and distributed shear in the lower crust.