THRUST TECTONICS, CRUSTAL SHORTENING, AND THE STRUCTURE OF THE FAR-EASTERN NEPAL HIMALAYA

Balanced and restored structural sections across the far-eastern Nepal Himalaya have been constructed in order to determine the structure and evolution of the Himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the Main Central Thrust (MCT). The far-eastern Nepal Himalaya is comprised of three distinct, thrust-bound tectonic packages; the Higher Himalayan (Crystalline) thrust sheet, the Lesser Himalayan (Metasediment) thrust package, and the Sub-Himalayan imbricate fan. The Higher Himalayan Crystallines, consisting of kyanite- and sillimanite-bearing gneisses intruded by the Miocene (?) Jannu leucogranites, have been thrust over the Lesser Himalayan Metasediments along the MCT for a distance of 140 km to 175 km. The Lesser Himalayan Metasediments are a 12 km thick unit consisting primarily of phyllites, metaquartzites, and mylonitic augen gneisses in which garnet, biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels. The Lesser Himalayan (Metasediment) thrust package is underlain by a decollement, the Main Detachment Fault (MDF), which lies at a calculated depth of between 6 and 10 km underneath the Mahabharat Lekh, and at a calculated depth of 20 to 25 km north of the Tamar Khola Dome. The Tamar Khola Dome overlies a footwall ramp along the MDF where the MDF cuts upsection through the Lesser Himalayan Metasediments. The Lesser Himalayan thrust package probably has an internal structure aproximating a hinterland-dipping duplex, with the MCT and the MDF corresponding to the roof and floor thrusts, respectively. Both the Tamar Khola Thrust, an out-of-sequence breach thrust, and the Main Boundary Thrust (MBT) are splay thrusts off of the MDF. The Sub-Himalaya, consisting of nonmetamorphosed sedimentary rocks, displays an emergent imbricate fan geometry and is underlain by the southern continuation of the MDF which lies at a depth of 5.5 km to 6 km beneath the Siwalik Hills. Folding and thrusting within the Lesser Himalayan thrust package and the Sub-Himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening. Total north-south shortening across the Higher, Lesser, and Sub-Himalaya of far-eastern Nepal, south of the Tibetan Plateau, has been of the order of 185 km to 245 km and has occurred at an average rate of 7.4 mm to 15.3 mm per year since the initiation of the MCT between 16 and 25 Ma.