CHRONOMETRIC EVIDENCES OF QUATERNARY EXTENSION ALONG TRANS-HIMADARI FAULT IN CENTRAL HIMALAYA, INDIA

The tectonic activity of Trans-Himadri fault (THF, equivalent of the South Tibetan Detachment System) during the late Miocene (20-11 Ma) has been documented. However in the absence of chronologically constrained field evidences, activation of THF during the Quaternary has been debated. In this contribution new evidences suggesting tectonics activity during the Quaternary are presented. Presence of knick zones, perched glacial valleys, deformed lacustrine sediments, incised outwash gravels and scree cones in two lacustrine (Garbayang and Goting) and one glaciated basins (Goriganga) in the vicinity of THF, conjuctively suggest episodic tectonic activity during the Quaternary. The older tectonic events resulted in the formation of knick zones (deep gorges) and perched glacial valleys associated with the footwall of THF. Dating of terminal moraines in Gangotri basin suggest that the maximum extent of valley glaciation in the region occurred at ~64 ka. (Sharma and Owen, 1995), the Presence of moraines in Garbyang at identical elevations and at the base of lacustrine sediments abuting against the THF places the timing of gorge incision to > 64ka. Deformed lacustrine sediments located on the subsiding hanging wall provide evidence of younger tectonic events. Optical dating of lacustrine sediments and outwash gravel suggests three major tectonic events during 22-17 ka, 14-13 ka and <11 ka. Linear extrapolation of the ages places onset of lacustrine sedimentation at ~65ka. This is consistent with the mimimum age of the underlying moraines. Based on the genetic relationship of seismicity between MCT and THF, the evidences suggest an ongoing activity on the THF that has been synchronous with movements along the MCT. The present results support the steady-state model of Himalayan orogeney.