EXPLORING THE LATERAL CONTINUITY OF HIMALAYAN EVENTS VIA LESSER HIMALAYAN THERMOBAROMETRIC CONSTRAINTS

The Himalayas are composed of roughly parallel, crustal-scale fault systems separating similar lithologies. The Indus Tsangpo Suture juxtaposes Indian shelf sediments (Tethys Formation) against Asian rocks; the South Tibetan Detachment separates the Tethys from Greater Himalayan gneisses. The Greater Himalaya are thrust over the Lesser Himalaya along the Main Central Thrust (MCT). Farther south, the Main Boundary Thrust (MBT) separates the Lesser Himalaya from Neogene molasse, the Siwalik Formation. South of the MBT, the Main Frontal Thrust (MFT) separates the Siwalik from the Indo-Gangetic Plains. Two chains of granites intrude the Tethys and upper structural levels of the Greater Himalaya. The synchronous nature of the emplacement of these granites and metamorphic events recorded by the Greater Himalaya support the idea that models for Himalayan convergence at one location are readily exportable to the entire range. To test the idea that rocks directly beneath the MCT experienced similar metamorphic conditions along strike, we obtained P-T data from Lesser Himalayan garnet-bearing assemblages in NW India and compared them to those reported ~600 km east in central Nepal. In central Nepal, the rocks follow progressive pattern of inverted metamorphism, recording 600-700°C and 7-10 kbar directly beneath the MCT and 550-600°C and 5-6 kbar at deeper structural levels. Lesser Himalayan samples in NW India do not show this progressive inverted metamorphism, instead recording 530±25°C and 5.6±0.8 kbar at the MCT, with X-ray element garnet maps of structurally deeper samples showing significant diffusive zoning and polymetamorphism. These results, along with observations that Lesser Himalayan monazites crystallized in the Pliocene in NW India and central Nepal, but only by the Late Miocene in eastern Nepal and Sikkim, suggest that the boundary conditions controlling the evolution of the MCT since the Early Miocene may have changed along strike.