STRUCTURAL AND GEOCHRONOLOGICAL EVIDENCES FOR MULTIPLE EPISODES OF TERTIARY DEFORMATION ALONG THE AILAO SHAN-RED RIVER SHEAR ZONE, SOUTHEASTERN ASIA, SINCE EARLY PALEOCENE TIME

Structural and Geochronological analyses show that, contrary to published reports, the Ailao Shan-Red River Shear Zone (ASRRSZ) of Southeastern Asia experienced three episodes of Tertiary left-lateral slip. The first episode of such deformation occurred throughout the eastern high-grade belt of the ASRRSZ under a transtensional environment and produced L tectonites of amphibolite grade. The second episode of left-lateral slip overprinted parts of the high-grade belt, but was concentrated along its tow margins. Its deformational mechanism was near simple shear and the rocks that were deformed are L-S mylonites of greenschist grade. The third episode of left-lateral slip took place chiefly in a western lower-grade belt of the ASRRSZ. This deformation occurred in a transpressional environment and produced phyllonites of lower-greenschist grade and sinistral oblique thrusts. Geochronological data document ages for the three episodes of left-lateral slip as ~58-56Ma, 24-22Ma and 13-12Ma respectively. Thus, the ASRRSZ experienced a multiple deformational history from early Paleocene to late Miocene time. The first episode of slip in the ASRRSZ appears to correspond with the initial collision of India and Asia at ~60Ma. The second episode took place at almost the same time as the most intensive compression and uplift in Tibet. The latest event may represent a accelerated west to east material flow in Tibet after ~16-13Ma.