MID-CRUSTAL FLOW, STRAIN PARTITIONING AND EXHUMATION IN THE CENTRAL HIMALAYA, TIBET/NEPAL

Two detachment systems are exposed in the central high Himalaya. The South Tibetan detachment system (STDS) is a low-angle, north-dipping shear zone that juxtaposes low-grade Tibetan Sedimentary Series rocks in the hanging wall against the anatectic core of the Himalayas (Greater Himalayan series; GHS) in the footwall. Early melt-present deformation in the footwall of the STDS accommodated large-scale flow of the low-viscosity middle-crust. Decompression-related anatexis and emplacement of granites at the structurally highest positions of the GHS (17-12 Ma) marks the final stages of south-directed extrusion of low-viscosity mid-crust. Exhumation of the ~30-km-thick GHS was accommodated by a relatively narrow solid-state mylonite zone that progressed into a brittle detachment. Mean kinematic vorticity estimates (rigid porphyroclasts) combined with deformation temperatures from Gondasampa and Ra Chu, Tibet record a progression in deformation from ultra-mylonitic leucogranite (~350-450ºC; 58-67% pure shear) to mylonitic marble (>300ºC; 41-51% pure shear) to moderately foliated marble (~200-300ºC; 35-46% pure shear). Cessation of movement on the STDS was followed by anatexis in the lower portion of the middle crust (~11-12 Ma; Ama Drime Massif) during orogen-parallel extension. Shear zone nucleation in the upper portion of middle crust (~8-12 Ma; Ama Drime and Nyönno Ri detachments) exhumed rocks from the deepest structural position in the central Himalaya. Kinematic and vorticity analyses combined with deformation temperature estimates from the Ama Drime detachment indicate: 1) top-to-the-west displacement was consistent between 400-650ºC; 2) Mean kinematic vorticity estimates range between ~0.50–0.70 (~50–66% pure shear) using rigid porphyroclasts and ~0.70–0.9 (~27–50% pure shear) using oblique grain shape fabrics. Mid-crustal flow and exhumation share several characteristics during south-directed and orogen-parallel flow. Melt-weakened rocks accommodated large-scale lateral flow within a wide region of distributed shear. During exhumation strain was partitioned into a relatively narrow zone of solid-state fabric development that records a transition from pure-to simple-shear-dominated flow prior to the development of brittle detachments during final stages of exhumation.