LATE MIOCENE-PLIOCENE INCEPTION OF E-W EXTENSION IN TIBET AS EVIDENCED BY APATITE (U-TH)/HE DATA

Despite being the textbook example of a contractional orogenic zone, the salient feature observed within the Tibetan Plateau are N-S trending rift valleys. Since the recognition of these active N-S trending rift systems, numerous workers have investigated their development in an attempt to elucidate the uplift and elevation history of the Tibetan plateau. The age of initiation and spatial distribution of E-W extensional faulting, however, remains poorly understood. In order to understand the driving forces for the formation of the N-S trending rifts in Tibet and their tectonic implications for the Himalaya/Tibet orogenic system, it is of fundamental importance to constrain the timing and spatial distribution of rifting. Preliminary apatite (U-Th)/He data were collected from samples from several N-S trending rifts within the Lhasa terrane. (U-Th)/He data from a vertical transect collected from the western flank of the central Gulu rift (northern Yadong-Gulu rift) provide constraints on the onset of rifting. Apparent (U-Th)/He ages systematically decrease with decreasing elevation and exhibit a well-defined exhumed He partial retention zone (HePRZ). The inflection point at the base of the exhumed HePRZ indicates that extensional faulting and rift flank exhumation in the Gulu area started at ~7-5 Ma. Reproducible ages as young as 1.7 Ma from the structurally deepest samples also illustrate the substantial magnitude of Pliocene and younger exhumation and the continued rapid rift flank uplift. Samples analyzed from the southern Nyainqentanghla range (central Yadong-Gulu rift) range in age from ~10 Ma at higher elevations, to less than 0.5 Ma within the ductile mylonite zone at the base of the range, indicating substantial post-Miocene exhumation of the rift flank. A sample from the footwall of an E-dipping normal fault in the Daga Co rift, western Tibet, yielded a (U-Th)/He age of ~5 Ma, suggesting substantial Late-Miocene-Pliocene exhumation of the rift shoulder in response to E-W extension. Additional samples from the Tangra Yum Co and Pum-Qu-Xainza rift systems are currently being analyzed. These preliminary apatite (U-Th)/He data from several major rift systems within the Lhasa terrane indicate a late Miocene-Pliocene inception of E-W extension within the southern portion of the Tibetan plateau.