STRAIN, DEFORMATION TEMPERATURES AND VORTICITY OF FLOW AT THE TOP OF THE HIGH HIMALAYAN SLAB, EVEREST MASSIF, TIBET

Greenschist to sillimanite grade quartz-rich metasedimentary rocks exposed at the top of the High Himalyan Slab in the Rongbuk to North Col region of the Everest massif are characterized by Type I and II cross-girdle quartz c-axis fabrics indicating approximate plain strain conditions. Fabric opening angles progressively increase with depth beneath the overlying Lhotse detachment and indicate increasing deformation temperatures of 525-625 ± 25 °C at depths of 300-600 m beneath the detachment. Deformation temperatures of ca. 450 °C are indicated by fabric opening angles in epidote-amphibolite facies mylonites located closer to the overlying detachment. A top down-to-the north (normal) shear sense is indicated by the asymmetry of microstructures and c-axis fabrics, but the degree of asymmetry is low at distances greater than 400 m beneath the detachment, and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine, indicating that deformation occurred at close to peak metamorphic temperatures. These "quenched" fabrics/microstructures indicate rapid exhumation in agreement with previous isotopic dating studies.

Mean kinematic vorticity numbers (Wm) were independently calculated by three different analytical methods. Calculated Wm values range between 0.73 - 0.98, and indicate that although a simple shear component is generally dominant, particularly in greenschist facies mylonites above the detachment, there is also a major component of pure shear in samples located at 400-600 m beneath the detachment (pure and simple shear make equal contributions at Wk=0.71). Assuming plane strain deformation, and taking into account Wm values indicated by the different vorticity methods, stretches of 10-40% parallel to the flow plane/transport direction are calculated, and correspond to shortening estimates of 10-30% perpendicular to the flow plane.