STRUCTURAL CHARACTERISTICS OF THE UPPER HIMALAYA CRYSTALLINE SEQUENCE，IN NYALAM AREA，SOUTHERN TIBET AND GEOLOGICAL IMPLICATIONS

The South Tibet Detachment System (STDS) is a low-dipping normal fault that separates the Upper Himalaya Crystalline Sequence (UHCS) below from the Tethyan Sedimentary Sequence (TSS) above．The STDS structure and related deformation are critical to understand the evolution of the Himalaya collision zone and its mechanism．This paper focuses on the kinematics of a segment of the STDS the Nyalam Detachment shear zone (NDsz) and below within the UHCS．By using the Wallis method and Rxz/β method to study the kinematic vorticity within the UHCS．The results show that:(a) the kinematic Wm vorticity by Wallis method is 0.55-0.76 within the NDsz indicating pure shear components of 64-45% and the kinematic vorticity below the NDsz is 0.48-0.64 indicating pure shear components of 68-56%；(b) the kinematic Wm vorticity by Rxz/β method below the NDsz is 0.72-0.80 indicating pure shear components of 51-42%；(c) an estimated stretch of 25-39% parallel to the flow plane (detachment) and a corresponding 20-28% shortening perpendicular to the flow plane．By synthesizing kinematic vorticity analysis in Nyalam and other regions of southern Tibet，we show that:(1) measurements by the Wallis method probably underestimate the simple shear component；(2) from below the South Tibetan Detachment shear zone (STDsz) to the Main Central Thrust Zone (MCTZ) the pure shear component estimates does not show obvious change suggesting either that the Himalayan Crystalline Sequence (HCS) below the STDsz was affected by a homogenous shear deformation with less than ~50% of simple shear or，more probably，that the measurements do not accurately reflect the finite deformation of the rocks; (3) Within the STDsz the component of simple shear appears higher (~70%) than in the HCS below (less than 55%) in better accordance with field observations．