Kinematic Evolution and Exhumation History of the South Tibetan Detachment System, Everest Massif, Tibet

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

Within the central-eastem sector of the Himalayan orogen the highest grade metamorphic rocks are exposed in the High Himalayan slab, a 20-30 km thick northward-dipping wedge of deep crustal rocks metamorphosed at 14-56 km depth at 35-15 Ma. The slab is bounded along the base by the south-vergent Main Central Thrust (MCT), and along the top by the South Tibetan Detachment System (STDS) of north-vergent normal faults which separate the metamorphic and anatectic core of the Himalaya from unmetamorphosed rocks of the Tibetan plateau. Beginning in Early Miocene time, southward extrusion of the High Himalayan slab has had a profound influence on the geologic and geomorphic evolution of the Himalaya, and the highest topography and deepest erosion corresponds with the upper part of the extruding wedge. The regional scale geometries of the thrust and non-nal faults bounding the slab are now reasonably well known, and much work has been done on documenting shear sense indicators along the upper and lower surfaces of the slab and constraining the early stage PTt paths of rocks within the slab. However, critically important gaps remain in our understanding of both the kinematics (vorticity) of flow and relationships between flow and progressive exhumation within the extruding wedge. Deten-nining the spatial and temporal relationships between timing and magnitude of displacement along the wedge-bounding faults and the kinematics of flow within the evolving wedge are crucial to understanding of the crustal thickening, exhumation, and erosional history of the orogen. For example, is the interior of the slab dominated by pure shear deformation and bounded by stretching faults as suggested in one recently published extrusion model, or is flow throughout the slab dominated by simple shear as suggested in other models. Identification of a pure shear component is critically important because operation of a significant pure shear component would result in: 1) thinning and dip-parallel extension of the slab itself, 2) relative to strict simple shear, an increase in both strain rates and extr-usion/exhumation rates. Testing of extrusion models requires that spatial and temporal distributions of kinematic (vorticity) domains be mapped out across the slab, and also requires a close integration between kinematic and PTt analyses in order to constrain progressive deformation and exhumation paths. Only one published quantitative vorticity analysis has been made along a basal section of the High Himalayan slab, and no such studies exist for the upper-middle sections of the slab. The PI proposes to undertake an integrated study of the kinematic evolution and exhumation history of the High Himalayan slab along a N-S traverse across the slab, and has chosen the Everest region for this study. In the Everest region the N-S transect across the slab is some 60-80 km in length and, given the detailed fieldwork required for the study, it would be impossible to complete the entire transect across the slab in the 2-3 years of a standard NSF-funded project. The PI therefore proposes to break the transect into two separately funded stages starting in this Proposal at the northern end of the transect with rocks lying in the immediate footwall to the STDS that are exposed in the Rongbuk and Kangshung valleys on the north and east sides of Mt. Everest respectively. For the vorticity part of the project he will employ a range of different analytical techniques allowing him to cross-check between results. The reconnaissance studies in the Rongbuk area, using three different analytical techniques, demonstrate that mean kinematic vorticity numbers (Wm) range between 0.73-0.98. These data indicate that although a simple shear component is generally dominant, particularly in samples adjacent to the STDS, there is also a major component of pure shear in samples located at 400-600 in beneath the detachment (pure and simple shear make equal contributions to flow at Wk=0.75). Closely spaced sampling sites are essential, however, for identifying potential step functions in the kinematic vorticity number that may exist with depth beneath the STDS. The almost continuous exposure in the region is ideally suited for this work, and will allow the PI to sample to depths of 3-4000 in beneath the STDS. Temporal variations in vorticity will be correlated with deformation temperatures using microstructural and petrofabric criteria, and these will in turn be linked to exhumation paths determined by then-nobarometry and excimer laser 4OAr/39Ar microprobe analyses.