THE TECTONOSTRATIGRAPHY AND STRUCTURE OF THE EASTERN NEPAL HIMALAYA

Six years of geological research in eastern Nepal has resulted in a new geological map of the eastern Nepal Himalaya which includes the region stretching from the Sikkhim border in the east to the Kathmandu Valley in the west, and from the summits of the Higher Himalaya in the north to the Ganges Plain in the south. This research has permitted die determination of the tectonostratigraphy and structure of one section of the central Himalayan arc. South of the Tibetan Plateau the eastern Nepal Himalaya can be divided into three distinct, thrust-bound tectonic packages: (1) the Higher Himalayan thrust sheet composed of the Higher Himalayan Crystallines, (2) the Lesser Himalayan thrust sheet composed of the Lesser Himalayan Series, and (3) the Sub-Himalayan imbricate zone composed of sedimentary rocks belonging to the Siwalik Group. The Higher Himalayan thrust sheet of eastern Nepal has been thrust over the Lesser Himalayan Metasediments a minimum of 140 km, and possibly as much as 175-210 km, along the Main Central Thrust (MCT). The Lesser Himalayan thrust sheet is overlain by the MCT and is underlain by the Main Detachment Fault (MDF) and the Main Boundary Thrust (MBT). Out-of-sequence thrust faults in the hanging wall of the MBT have breached and offset the presently inactive MCT. The Sub-Himalayan imbricate zone is an emergent imbricate fan bounded by the MBT to the north and the Main Frontal Thrust (MFT) to the south and is underlain by the MDF which lies at a depth of between 5 km and 7 km. A balanced cross section constructed across the Higher, Lesser, and Sub-Himalaya of eastern Nepal shows that the eastern Nepal Himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal, north-south tectonic shortening since the initiation of the MCT. The Lesser and Sub-Himalaya have absorbed 70 km of north-south shortening by thrusting along the basal MDF, of which the Sub-Himalayan imbricate zone has accommodated 25 km, the Sun Kosi Thrust has accommodated about 10 lan, and the MBT has accommodated the remaining 35 km of shortening. Since the initiation of the MCT between 15 Ma and 25 Ma shortening across the eastern Nepal Himalaya has occurred at an average rate of 8.4-18.6 mm per year. The structural geometry of the eastern Nepal Himalaya suggests an overall "piggyback" sequence of thrusting, with motion transferred from the MCT to the underlying MDF and its emergent splay thrust, the MBT, and with the MBT rotated to its present steep orientation by imbricate thrusting within the Sub-Himalaya.