Block rotation: Tectonic response of the Sichuan basin to the southeastward growth of the Tibetan Plateau along the Xianshuihe‐Xiaojiang fault

Abstract

AbstractGPS 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 Ms 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).