GEOMORPHIC EVIDENCE FOR ACTIVE STRUCTURES NEAR THE MAIN CENTRAL THRUST: BURHI GANDAKI AND TRISULI RIVERS, CENTRAL NEPAL

The Himalayas of central Nepal are characterized by a sharp topographic break between the Higher and Lower Himalaya. This "Himalayan topographic front"-herein referred to as the HTF-is especially prominent in the Burhi Gandaki and Trisuli drainages, where it follows a nearly linear, east-west trend approximately 20 km south of the mapped trace of the Main Central Thrust (MCT). Geomorphic and structural observations suggest that the HTF is the surface expression of either a buried or surface-breaking thrust-sense shear zone, which may have been active as recently as the Pleistocene. In both the Burhi Gandaki and Trisuli drainages, the HTF is characterized by breaks in channel steepness values, hillslope frequency distributions, and valley morphology. Channel steepness (K_s) values in these drainages, normalized to a concavity of 0.45, increase from a mean of 145 m^0.9 below the HTF to a mean of 411 m^0.9 above the HTF over a distance of 20-30 km, consistent with a narrowly distributed increase in uplift rates from south to north. Hillslopes calculated from nearest-neighbor 90-m pixels have a median value of 21.2 degrees below the HTF, compared with 28.8 degrees between the HTF and the MCT, and 27.7 degrees above the MCT. Fluvial valley floors are characterized by wide, alluviated morphologies below the HTF and steeper, narrower valleys above. The HTF also truncates thick (80+ meter) Pleistocene fill terraces and associated terra rosa soils in both the Burhi Gandaki and Trisuli rivers, suggesting accelerated removal of these materials above the HTF since Pleistocene time. Finally, the presence of aligned drainages in tributaries to the Trisuli suggests a narrow zone of lithologic weakness, which is approximately co-linear with two surface-breaking shear zones identified in the field. These observations strongly suggest that the classical model of foreland propagating thrusts may be incomplete for the Himalayan system. Instead, motion on the Main Boundary and Main Frontal Thrusts may be contemporaneous with movement along thrust zones further to the north. Additional thermochronologic, structural, and geomorphic studies are currently underway to test this hypothesis.