TECTONIC EVOLUTION OF THE CONTINENTAL SE ASIA: FRAGMENTS OF A SYNTHESIS

Since Argand, numerous analogue and numerical experiments have been performed to characterize the deformation of the Asian continental lithosphere, by assuming a continuity of the style of deformation. However, the great amount of observational evidence that has been accumulated in the past three decades, indicate a more heterogeneous style of deformation, which also changed through time, due to a complex interaction of several driving forces, including crustal thickening, temporal changes in lithospheric rheologies, lateral extrusion, and rotation which may have effected different parts of the lithosphere at different times. Early Tertiary shortening in east-central Tibet, along with early Tertiary contraction in central Tibet along the Bangong suture zone and in the Himalayan fold-thrust belt, probably accommodated several hundred kilometers of India-Asia convergence during the early phase of the collision, while the region between the Ailao Shan and Gaoligong Shan shear zones, extruded to the SE, possibly due to the limited compressibility of Asia. As the indentation continued, and structures became deflected, two important changes occurred: (1) deformation spread north of Tibet and south into the Himalayas, and (2) extrusion phase ended by ~17 Ma as a result of the bending of the structures which bound the extruding fragment, and the change of boundary conditions in eastern Asia with the collisions of the Australian and Philippines Sea plates. Convergence continued to be accommodated by thrusting along the periphery of the Tibetan Plateau from ca. 20 to 10 Ma, while the region between the Indoburman Ranges and the North and South China blocks as a broad right-lateral transfer zone. Since about 10 Ma, however, the surface deformation on the plateau has strongly decoupled from motion of the lower crust, where its northern section is dominated by north- to northeast-directed shortening, and its southern section, bounded by the Xianshuihe fault zone to the north and east, undergoes clockwise rotation. Identifying and determining the relative importance and temporal variability of the structures in the Tibetan Plateau as well as SE Asia, and mechanisms responsible for accommodating convergence during the Indo-Asian collision are fundamental to understanding the geologic evolution of Tibet and the Himalaya.