REVEALING THE LINK BETWEEN TECTONICS, TOPOGRAPHY AND EXHUMATION OF THE SIWALIK FORELAND BASIN, NEPAL, HIMALAYA: AN INTEGRATION OF QUANTITATIVE GEOMORPHIC ANALYSIS AND DETRITAL THERMOCHRONOLOGY

The Himalayas, one of the most active collisional orogenic belts with the highest topography, is characterized by a thick succession of Miocene Siwalik sedimentary rocks deposited in the Himalayan Foreland Basin. The Siwalik rocks are bounded in the south and north by two east-west trending thrust fault systems, the Main Frontal Thrust and the Main Boundary Thrust, respectively. Originating from the Himalayan hinterland, a number of major rivers flow southward across these faults and expose ~2 km thick Siwalik rocks. Here, we integrated quantitative geomorphology of the Surai Khola River network, and laser ablation detrital thermochronology of the Siwalik rocks exposed along this river to understand the interplay between tectonic and topographic development with that of hinterland exhumation. Using quantitative stream power-law function with 30 m resolution ASTER DEM, we extracted and analyzed river long profiles from 23 rivers within the Siwalik foreland basin in southwestern Nepal. Geomorphic anomalies in these profiles such as knickpoints and abrupt changes in slope and flow directions are identified, and spatial variation of steepness indices are mapped. Our geomorphic analysis result shows presence of distinct knickpoints separated by variable steepness indices where the knickpoint density is higher in areas with larger number of contractional faults. The spatial distribution of these transient knickpoints also separate regions of high and low uplifts followed by fault trends. We also isolated ~200 euhedral to subhedral zircon grains from 65 to 500 µm fractions of the Siwalik sandstones for our ongoing laser ablation detrital zircon (U-Th)/He thermochronologic dating. This technique, being the most recent advancement in the field of thermochronology, depends on the bombardment of high intensity photons with mineral grains, which results in vaporization and ablation. Results of this integrated study would reveal the timing of exhumation of the Himalayas and contribute towards the understanding of the tectonic deformation and adjustment of the high topographic landscape of the Himalayas.