Abstract

Geothermobarometric, micro- and macro-structural data indicate that peak metamorphic pressure and temperature of the Greater Himalayan Sequence (GHS) of the Garhwal Region of India increase dramatically across the Main Central Thrust (MCT). Metamorphic pressure and temperature increase from ~5 kbar and ~550 ºC in the Lesser Himalayan Crystalline Sequence (LHCS) in the footwall to ~14 kbar and ~850 ºC at ~3 km above the MCT in the hanging wall (GHS). Pressures decrease slightly upsection to ~8 kbar and temperatures remain nearly constant at ~850 ºC to the structurally overlying South Tibetan Detachment (STD). The LHCS exhibits a high temperature-depth gradient (30 ºC/km) whereas the lower GHS has a much lower temperature-depth gradient (18 ºC/km) that increases to ~28 ºC/km near the STD. The pressure-temperature pattern is consistent with conduction of heat from the hotter (initially deeper) GHS into the colder (initially shallower) footwall of LHCS and conductive cooling of the hotter hanging wall of GHS along the STD. Numerical "channel flow" models predict a pressure-temperature pattern for the exhumation of the GHS similar to what is observed in the Garhwal Region of India. However, observed pressures (~10-14 kbar) are higher than predicted in the models (~10-12). The higher pressure of the GHS is likely due to the greater exhumation from displacement along the Munsiari Thrust (MT). In other words, the GHS in the Eastern Garhwal region provides a deeper view of the channel material than elsewhere in the range. The temperature-depth ratios of the Eastern Garhwal region also exhibit a very different pattern of conductive heating and cooling of the LHCS and GHS respectively, than elsewhere in the range. Ductile features within the GHS exhibit sheath fold geometries, indicative of high degrees of ductile flow. Overprinting the ductile structure are two populations of extensional conjugate fractures oriented both parallel and perpendicular to the orogen. These fractures crosscut major tectonic boundaries in the region such as the MCT and STD, and are found throughout the LHCS, GHS, and Tethyan Sedimentary Sequences (TSS). The crosscutting of these brittle structures across the major tectonic boundaries in the area indicate that the various tectonolithic sequences were exhumed during widespread extensional deformation as one coherent block.

Degree

College and Department

Physical and Mathematical Sciences; Geological Sciences

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