GPS field and seismic data show that the southeastern margin of the Tibetan Plateau is tectonically and seismically active. This activity is due to the southeastward extrusion of the Chuandian fragment, a large crustal block rotating clockwise around the northeastern syntaxis of the Himalayas. The eastern boundary fault of this fragment is defined by the left-lateral Xianshuihe-Xiaojiang fault, which abruptly truncates the Sichuan basin of the Yangtze block. Our paper presents evidence indicating that the Sichuan basin experienced right-lateral shear along its margin, including the Longmen Shan fault belt, as shown by the presence of a large number of interference deformation features, including S-shaped and Z-shaped folds and faults, aligned in an en echelon pattern. This study hypothesizes that the Sichuan basin experienced counterclockwise rotation, dragged by the left-lateral movement along the Xianshuihe fault, and it is this rotation that was the underlying cause of the 12 May 2008 Wenchuan M s 7.9 earthquake. During the rotation, the Sichuan basin decoupled along a subhorizontal decollement fault zone that developed along Triassic gypsum-and coal-bearing rocks, at a mean depth of~5000 m, below which the Paleozoic rocks experienced much more intense deformation than the overlying Mesozoic rocks, suggesting that the lower part of the basin experienced a larger-scale rotation relative to the uppermost part of the basin. Based on thermal data from the western margin of the Sichuan basin and from along the Xianshuihe fault, the counterclockwise bending/rotation of the Sichuan basin initiated in late Cenozoic time (~13 Ma).

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Editor: A. YinKeywords: crustal strength Tibetan Plateau lake shoreline effective elastic thickness viscosity Controversial end member models for the growth and evolution of the Tibetan Plateau demand quantitative constraints of the lithospheric rheology. Direct determinations of bulk crustal rheology, however, remain relatively sparse. Here we use the flexural rebound of lacustrine shorelines developed during the Lingtong highstand around Siling Co, in central Tibet, to place bounds on the effective elastic thickness (T e ) and viscosity of Tibetan crust. Shoreline features associated with the Lingtong highstand complex ∼60 m above present lake level are deflected from horizontal by 2-4 m over wavelengths of ∼200 km. Optically stimulated luminescence dating of aggradational shoreline deposits indicates that these lake levels were reached at 6-4 ka. Assuming that surface loads were entirely supported by an elastic layer overlying an inviscid fluid, the range and spatial distribution of variations in shoreline elevation are consistent with deflections predicted by a uniform elastic plate with thickness, T e of 20-30 km. If viscoelastic relaxation in response to lake withdrawal is complete, our data suggest an average viscosity ≤10 19 Pa s. These results imply that the apparent viscosity of the lower crust inferred over millennial timescales is comparable with that estimated from post-seismic relaxation over decadal timescale.Published by Elsevier B.V.