MIOCENE EXHUMATION OF MID-CRUSTAL ROCKS ALONG THE XIANSHUIHE FAULT, NEAR

New field observations along the active, sinistral Xianshuihe fault, near the Gonga Shan region of eastern Tibet, reveal the exhumation of mid-crustal rocks by extensional processes. This region records a spatial association between high topography, intense erosion, rapid exhumation and extensional faulting. The massif is a region of anomalously high topography, rising 1-3 km above the surrounding Tibetan plateau surface.  The northern margins of the massif is bounded by a north-northwest dipping low-angle extensional detachment. A moderately to steeply dipping fault contact on the eastern border of the massif separates high-grade rocks in the massif from low-grade rocks bordering the massif to the east. This eastern contact has been modified by brittle structures related to the Xianshuihe fault. Exhumed rocks observed within the massif include sillmanite-grade metasediments and meta-basalts. PTt estimates indicate that these rocks were exhumed from pressures of 3-9 kbar and temperatures of >700°C, and achieved peak metamorphic conditions at ~170 Ma. Additional thermochronologic constraints (Roger et al., 1995) suggest that these rocks then cooled slowly (2°C/My) until middle Miocene time. Onset of rapid exhumation of the Gonga Shan massif is estimated to have occurred between 10 and 15 Ma, on the basis of low temperature thermochronometry (e.g. Xu and Kamp, 2000), and the timing of emplacement of syn- and post-exhumational granites (Roger et al., 1995). Rates of exhumation are estimated to be on the order of 25°C/My, an order of magnitude greater than pre-Miocene rates. In contrast, Jurassic plutons exposed on the Tibetan plateau surface to west of Gonga Shan indicate of continued slow (2°C/My) cooling from Jurassic to present time. These observations together suggest that the Gonga Shan massif has been rapidly exhumed from mid-crustal depths since mid-Miocene time, in contrast to surrounding terrain. The Xianshuihe fault that bounds the eastern margin of the exhumed terrain has clearly played a role in the late-stage brittle exhumation of the massif; however, the role of strike-slip faulting in the earlier, ductile stage of exhumation remains to be resolved.