Pleistocene rapid uplift of the Himalayan frontal ranges recorded in the Kathmandu and Siwalik basins

Core-drilling project carried out in the southern margin of the Kathmandu Basin revealed that muddy debris flow deposits dammed up the Proto-Bagmati river to form the Paleo-Kathmandu Lake during the Jaramillo subchron from 1.07 to 0.97 Ma. Subsequent deposition of the alluvial fanglomerate, derived from the uplifting Mahabharat Range to the south, raised the dam deepening the lake-water. After 1 Ma, in the southern part of the basin, palaeo-current directions changed from southward to northward and deposition of gneissose and granitic detritus are replaced by meta-sediments derived from the Mahabharat Range. During the same time, at about 1 Ma, the boulder conglomerates were deposited on top of the Siwalik Group as piggy-back basins in front of an intra-basinal high, along the Main Dung Thrust in Nepal and NW India. Onset of movement of the Main Dung Thrust is dated back to 3 to 2.4 Ma [Mugnier, J.L., Huyge, P., Leturmy, P., Jouanne, F., 2003, Episodicity and rates of thrust-sheet motion in the Himalayas, Western Nepal. Am. Assoc. Petrol. Geol., Mem. 82, 1-24]. The Main Frontal Thrust is most active at present suggesting that imbricated structure of the Siwalik Group was formed by convergence of the Indian plate during the last 3 myr. The accretionary wedge of the Siwalik Group, stacked beneath the Main Boundary Thrust, might have started to jack up the frontal range of the Lesser Himalaya since 1 Ma. Coeval uplift and erosion of the frontal range of the Lesser Himalaya and the intra-basinal high in the Siwalik since 1 Ma are possible causes of an abrupt increase in both sedimentation rate and grain size of detrital quartz, and changes in composition of clay minerals, recorded in the sediments of the Bengal Deep Sea Fan at 0.9 Ma.