Interpretation of Cenozoic Tectonics in the Right-Lateral Accommodation Zone Between the Ailao Shan Shear Zone and the Eastern Himalayan Syntaxis

Abstract

The region between India and South China has evolved tectonically as an accommodation zone during postcollisional intracontinental deformation between the Indian and Eurasian plates. During postcollisional convergence, and as India rotated in a counterclockwise direction, the eastern Himalayan syntaxis migrated northward, and crustal material between India and South China was subject to increasing amounts of shortening and right-lateral shear. Crustal fragments were extruded to the southeast from north of the syntaxis, bounded on the west by the right-lateral Gaoligong fault and on the east by the left-lateral Ailao Shan shear zone. Between these two shear zones, crustal material was deformed by shortening, strike-slip faulting, and clockwise rotation. Clockwise rotation caused bending and superposed shortening of early structures, leading to the development of NE-trending left-lateral shear zones and E-W extension. Extrusion of crustal material was accompanied by significant internal deformation of crustal fragments. How much of the extrusion was absorbed in intracrustal deformation and how much occurred by movement on strike-slip faults remains unknown at present. Pre-Cenozoic crustal anisotropies played an important role in the formation of crustal fragments, inasmuch as many of the large zones of strike-slip displacement and shortening are localized along older suture boundaries. Most of the major shear zones and faults within the accommodation zone are transfer structures in which strike-slip displacement was transferred into shortening structures, and both the transfer and shortening structures were subject to clockwise rotation. Development of the Ailao Shan shear zone was at least partially coeval with the adjacent Simao fold-and-thrust belt; oblique shear was partitioned into strike-slip along the shear zone and thin-skinned shortening in the fold-and-thrust belt. Relations between the two styles of deformation suggest that the Ailao Shan shear zone may lie above a subhorizontal zone of intracrustal detachment. There is no simple and direct relation among intracontinental convergence in the India/Eurasia collision zone, the development of the Ailao Shan shear zone, and the formation of the South China Sea. Extrusion of crustal material to the southeast, north of the eastern Himalayan syntaxis, was absorbed by internal deformation in terranes southwest of the Ailao Shan shear zone, and an unknown but limited amount of left-lateral shear extends into the South China Sea. The courses of the major rivers that flow south from Tibet were subject to deformation within this accommodation zone. Their close spacing in western Yunnan and right-stepping bends in southern Yunnan are caused by (1) shortening and horizontal shear and (2) clockwise rotation, respectively. The southerly flow indicates that a topographic gradient from the Tibetan Plateau may be at least as old as the early Miocene. If the rivers are that old, major southeastward extrusion has not occurred across them since at least early Miocene time.