In the Karnali valley of west Nepal, detailed mapping, thermobarometry, quartz-petrofabrics, vorticity analysis, and thermochronology delineate three tectonometamorphic domains separated by structural and metamorphic discontinuities. The lowest domain, the Lesser Himalayan sequence, is weakly metamorphosed and preserves evidence of primary sedimentary features and a polydeformational history. The Greater Himalayan sequence (GHS) is pervasively sheared and metamorphosed and overlies the Lesser Himalayan sequence along the Main Central thrust. The Greater Himalayan sequence is sub-divided into two tectonometamorphic domains that display contrasting metamorphic histories. The lower portion of the Greater Himalayan sequence contains garnet- to kyanite-grade rocks whose peak metamorphic assemblages developed during top-to-the-south directed shear and a metamorphic pressure gradient that increases up structural section. The upper portion of the Greater Himalayan sequence contains kyanite and sillimanite-grade migmatites that preserve polymetamorphic assemblages and a metamorphic pressure gradient that decreases up structural section. The upper and lower portions of the Greater Himalayan sequence are separated by a metamorphic discontinuity that roughly coincides with the bottom of the lowest migmatite unit. Vorticity estimates indicate roughly equal contributions of pure and simple shear during deformation of the upper and lower portions of the GHS. Quartz petrofabrics suggest deformation temperatures are equivalent to peak metamorphic temperatures in the lower Greater Himalayan sequence. These observations are consistent with channel flow tectonic models whereby the upper portion of the Greater Himalayan sequence is ductily extruded to the south while ductily accreting the subjacent lower portion of the Greater Himalayan sequence across a metamorphic discontinuity. 40Ar/39Ar thermochronology indicates Miocene homogeneous cooling of the Greater Himalayan sequence. Cooling rates of the GHS and the homogeneous cooling profile suggest east-west extensional exhumation followed peak-metamorphism and south-directed shearing and supports the hypothesis of the southeast propagation of the Gurla-Mandhata-Humla fault system into the Karnali valley. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2010-09-20 09:23:07.103

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.

Field observations of the Greater Himalayan Sequence in Eastern Nepal demonstrate a ductile, highly strained package of metamorphic rocks that show extensive evidence of crustal anatexis throughout. These can be distinguished from the Lesser Himalayan sequence below by a distinct reduction in metamorphic grade, an inverted metamorphic sequence and a high strain zone corresponding to the Main Central Thrust. Metamorphic studies are combined with geochronology to demonstrate a protracted period of crustal melting followed by rapid decompression from 18.7 Ma to 15.6 Ma. A metamorphic decompression rate is quantified at c.2mm/yr during this period. This is interpreted to represent exhumation of the Greater Himalayan Sequence by a process of ductile, channelised flow from the mid-crust beneath Tibet. Below a prominent band of kyanite gneiss, previously used to locate the Main Central Thrust, but here mapped within the Greater Himalayan Sequence, partial melting is still exhibited. Here monazites are dated at 10.6 Ma. In the Lesser Himalaya below, allanites record a similar 10.1 Ma event. This implies that following channel flow during the mid-Miocene, the channel widened in the lower-Miocene to incorporate a greater structural thickness. Following these two periods of exhumation and ductile extrusion, separated in time and space, Fission Track studies indicate that much slower, erosion driven exhumation proceeded, at <1 mm/yr. This rate increases slightly in the Pliocene, most likely in response to Northern Hemisphere glaciation; no difference in exhumation is seen across the Greater Himalayan Sequence with respect to the different, earlier, phases of ductile channel flow related exhumation. These results demonstrate the episodic nature of channel flow in the Himalaya and reconcile arguments about the position of the MCT in Eastern Nepal.