THE SOUTH TIBETAN DETACHMENT SYSTEM IN THE RONGBUK VALLEY, EVEREST REGION - DEFORMATION FEATURES AND GEOLOGICAL IMPLICATIONS

Structural and microstructural investigations into the South Tibetan D etachment System in the Rongbuk valley (southern Tibet) pointed to the presence of a complex extensional setting with the development of two major fault zones with movement in a top-to-NE direction. The lower o ne is an at least 1000 m wide ductile shear zone that brings into cont act the overlying North Col Formation, subject to low grade metamorphi sm, and the high grade micaschist and gneiss of the Higher Himalayan C rystallines, characterized by polymetamorphism, from Barrovian to low- pressure/high-temperature conditions. The upper low-angle fault zone i s characterized by a metric thick level of cataclasites and brings int o contact the slightly metamorphosed Ordovician limestones and the Nor th Col Formation. The lower ductile shear zone is thought to accommoda te at least 5 km of vertical displacement, whereas the upper low-angle normal fault zone is thought to accommodate at least 1600 m of vertic al displacement. The lower ductile shear zone is thus to be considered the main extensional feature of the STDS in the Rongbuk valley. (C) 1 998 Elsevier Science Ltd. All rights reserved.