YOUNG AND ACTIVE TECTONISM IN THE EASTERN TIBETAN PLATEAU AND EASTERN CHINA: CONSTRAINTS AND INTERPRETATIONS

The Cenozoic tectonics of Southeast Asia/Tibetan plateau is a natural laboratory for study of processes of post-collisional, intracontinental deformation and landscape evolution. The rapidity of tectonic activity in the region facilitates the study of tectonic processes, but its scale, difficult access, structural complexity, and only 25 years of modern study make syntheses preliminary. Only a few data are reasonably well established: 1. India collided with Eurasia along a north-dipping subduction zone, but the time of collision remains controversial, (@ 50 Ma=post-collisional convergence at east and west syntaxes is 3000±250 and 2500±200 km, @ 70 Ma=5400±300 and 4500±200 km ), 2. Post-collisional deformation created a ~ 5 km high plateau of low relief, and the Tien Shan farther north, 3. The N and S margins of the plateau are well defined by steep topographic gradients, but the eastern (and western) margins are very irregular, and in eastern China two N-S trending topographic gradients step elevations down into the flanking marginal seas. Since any hypothesis for the evolution of the SE Asian area must accommodate present day morphology, lithospheric structure, and present-day kinematics, one research approach examines the young (from ~Late Miocene time) and active tectonics of the region coupling geological, geophysical, neotectonic, geodetic, and landscape analysis to develop kinematic patterns and dynamic interpretations. Analysis of latest Cenozoic-Recent data permits comparison over time scales that differ by several orders of magnitude, and has the best chance to establish contemporaneous events. Although data have added new constraints for the youngest SE Asian tectonic events, they have not discriminated clearly among differing kinematic and dynamic hypotheses. Combined elements from different hypotheses appear to explain surface and shallow crustal data, but deeper lithospheric processes remain controversial. Geodetic velocities and neotectonic deformation suggests inhomogeneous upper crustal deformation at different time and spatial scales, and the resultant landscape evolution is likewise inhomogeneous. Early Cenozoic tectonic evolution appears similarly inhomogeneous.