THE LATE CENOZOIC TECTONIC EVOLUTION OF THE GURLA MANDHATA DETACHMENT SYSTEM, SOUTHWEST TIBET

Active deformation in the southern Tibetan Plateau is dominantly characterized by east-west extension along approximately north-striking normal faults. Timing constraints for these structures are key to understanding the tectonic history of this region. In southwest Tibet, Gurla Mandhata is thought to be a metamorphic core complex bounded on its west side by a moderately west-dipping normal (detachment) fault juxtaposing mylonitic gneisses against Quaternary alluvium. The Gurla Mandhata detachment (GMD) system traverses the Indus-Yarlung suture zone, which is a structural boundary in between the Tibetan Plateau and the High Himalaya. The Gurla Mandhata detachment system is located at the southeast terminus of the right-lateral Karakoram fault, exhuming lesser Himalayan affinity rocks. Currently, a debate exists with regard to the kinematic linkage between the GMD system and the Karakoram fault. Two end member models have been proposed for this system: (1) the Karakoram fault system transfers slip eastward into and along the Indus-Yarlung suture zone, and (2) the GMD system is an extensional step-over of the Karakoram fault system, redirecting slip southeastward into the High Himalaya. We evaluate these two end member possibilities by comparing the late Miocene fault slip rate history of the GMD system to that of the Karakoram fault. To address this question (U-Th)/He thermochronometric data from several east-west footwall transects are used to constrain the late Miocene thermal history and extension rate. Preliminary zircon (U-Th)/He thermochronometric data from two 40 km long horizontal transects across the GMD footwall yield cooling ages ranging from 3.7 +/- 0.3 Ma to 8.6 +/- 0.7 Ma. Our preliminary results suggest that the Late Miocene extension rate for the GMD system is relatively high exceeding 10 mm/yr. This extension rate matches slip rate estimates obtained for the Karakoram fault at similar times scales (e.g. Lacassin et al., 2004; Valli et al., 2007) supporting the second proposed model. However, additional analyses are required to rigorously examine the possibility for along-strike variations in extension rates and to better constrain a complete thermal history.