SPATIAL AND TEMPORAL VARIABILITY IN DEFORMATION, METAMORPHISM, AND DISPLACEMENT ALONG THE SOUTH TIBETAN DETACHMENT SYSTEM; INSIGHTS INTO THE EXHUMATION OF THE GREATER HIMALAYAN SLAB, TIBET

The South Tibet detachment system (STDS) is proposed to have accommodated extrusion and exhumation along the upper margin of the Greater Himalayan Slab (GHS) during the Miocene. Many models for the STDS suggest that exhumation of the GHS was relatively uniform along orogenic strike. New macro-and micro-scale structural analysis combined with thermobarometry from three transects across a 70 km-long section of the detachment provide insights into the spatial and temporal evolution of this orogen-scale feature. Exposed ~ 40 km north-northwest of the summit of Cho Oyo is a newly documented section of the STDS we term the Gondasampa transect. Here, unlike other sections of the STDS, garnet bearing schist and gneiss are present within a 150 m-thick mylonite zone beneath the overlying lower Paleozoic sediments. Geothermobarometry on the mylonitic gneiss records a peak temperature of ~730°C and a pressure of ~8 kbar. Previously published results from Rongbuk (N of Everest) yield P/T estimates of 630°C 4.6 kbar whereas new geothermobarometry from the Kharta area (NE of Everest) yield 680-720°C and 6-7 kbar for samples collected within the upper 2 km of the GHS. The structure of the STDS also shows significant variability along these three transects. The Gondasampa and Rongbuk transects preserve a similar sequence of rock types in a 150-300 m-thick mylonite zone that grades into migmatitic gneiss. Across this zone, deformation mechanisms progress from a distributed shear zone of high-grade gneiss, through a narrowing mylonite zone into a discrete detachment. In contrast, a 1000 m-thick gradation between high-grade gneiss through calc-mylonites and into sedimentary rocks marks the STDS in the Kharta area. Variation in P-T reached by garnet bearing samples within, and 100's of meters below, the mylonite zone are attributed to variable extrusion and exhumation along the STDS. We interpret the 1000 m-thick mylonite zone of the Kharta area as representing the earlier stages of exhumation along the STDS whereas the STDS as recorded in Rongbuk and Gondasampa preserve the later stages of exhumation that progressed into a narrow zone and brittle detachment. Additionally, we emphasize that exhumation of the GHS was accommodated by deformation that was partitioned into a relatively narrow shear zone (STDS) along the upper margin of the GHS.