RECOGNIZING INDIAN BASEMENT FAULTS BASED ON ALONG-STRIKE DIACHRONOUS METAMORPHISM IN THE HIMALAYAN METAMORPHIC CORE OF FAR WEST NEPAL

The geometry of the basal detachment of the Himalaya, the Main Himalayan Thrust (MHT), significantly controls the evolution of the overlying Himalayan Orogen. The MHT geometry can be modified along-strike by the reactivation of pre-orogenic faults rooted in the underthrusted Indian plate during orogenesis. The location and reactivation episodes of Indian basement faults are ascertained using the metamorphic record of the overlying Himalayan metamorphic core as a proxy for the evolution of the MHT geometry through time.

The MHT in far west Nepal contains a present-day MHT lateral ramp that coincides with the northward projection of an Indian basement fault, the Great Boundary Fault. Far west Nepal is consequently an ideal location to link the evolution of the Himalayan metamorphic core with the development of the MHT lateral ramp and reactivation of the Great Boundary Fault.

New pressure-temperature-time-deformation data along the Seti Khola in far west Nepal is compared with the published tectonometamorphic evolution along the adjacent Karnali river valley, on the eastern side of the modern MHT lateral ramp. Quartz <c> axis crystallographic preferred orientation results in addition to field observations are used to compare equivalent structural levels of the Seti Khola samples with the Karnali valley results. Prograde metamorphism along the Seti Khola initiated at ca. 39 Ma, as recorded by monazite petrochronology. Peak metamorphic conditions of 645-745°C and 0.85-1.1 GPa were reached at 28-22 Ma along the Seti Khola, 10-14 Myr prior to the Karnali valley, indicating segmentation of the Himalayan metamorphic core across the MHT lateral ramp. It is postulated that such segmentation is the result of the Himalayan metamorphic core thrusting over differing ramp-flat geometries on either side of the MHT lateral ramp. The segmentation and change in MHT geometry are interpreted to be caused by the reactivation of the underthrusted Great Boundary Fault during the Oligocene to earliest Miocene. The comparison of tectonometamorphic histories along-strike in far west Nepal helps to define the MHT geometry changes through time and highlights the necessity to consider the pre-orogenic structural features of the plates involved when evaluating orogenic evolution.