Young, active conjugate strike-slip deformation in West Sichuan: Evidence for the stress-strain pattern of the southeastern Tibetan Plateau

The active kinematics of the eastern Tibetan Plateau are characterized by the southeastward movement of a major tectonic unit, the Chuan-Dian crustal fragment, bounded by the left-lateral Xianshuihe-Xiaojiang fault in the northeast and the right-lateral Red River-Ailao Shan shear zone in the southwest. Our field structural and geomorphic observations define two sets of young, active strike-slip faults within the northern part of the fragment that lie within the SE Tibetan Plateau. One set trends NE-SW with right-lateral displacement and includes the Jiulong, Batang, and Derong faults. The second set trends NW-SE with left-lateral displacement and includes the Xianshuihe, Litang, Xiangcheng, Zhongdian, and Xuebo faults. Strike-slip displacements along these faults were established by the deflection and offset of streams and various lithologic units; these offsets yield an average magnitude of right- and left-lateral displacements of 15-35 km. Using 5.7-3.5 Ma as the time of onset of the late-stage evolution of the Xianshuihe fault and the regional stream incision within this part of the plateau as a proxy for the initiation age of conjugate strike-slip faulting, we have determined an average slip rate of 2.6-9.4 mm/year. These two sets of strike-slip faults intersect at an obtuse angle that ranges from 100 to 140 facing east and west; the fault sets define a conjugate strike-slip pattern that expresses internal E-W shortening in the northern part of the Chuan-Dian crustal fragment. These conjugate faults are interpreted to have experienced clockwise and counterclockwise rotations of up to 20. The presence of this conjugate fault system demonstrates that this part of the Tibetan Plateau is undergoing not only southward movement, but also E-W shortening and N-S lengthening due to convergence between the Sichuan Basin and the eastern Himalayan syntaxis.