Insight on recent tectonic deformation in the Himalayas of central Nepal provided by fluvial terrace geometries

Topography is a direct manifestation of the coupling of tectonic and surface processes and this connection between rapid erosion and high uplift rates is most readily evident in the frontal High Himalayas– an area that provides an excellent opportunity to study the progressive evolution geomorphic features in response to the interplay of these processes. An abrupt topographic break between the low-relief Lesser Himalaya and the high-relief Greater Himalaya has received significant attention, but the processes that govern its evolution remains debated. While it is commonly accepted that active tectonics are required to produce the topographic break, it remains debated whether it is driver by a blind mid-crustal ramp or discrete thrust faulting that daylights at the mountain front. Evidence for out-of-sequence thrusting has been documented along the orogen at similar elevations as the topographic break, suggesting active surface faulting could play a major role in generating and sustaining this marked topographic break. In central Nepal, where the topographic break is most pronounced, thermobarometric data indicate pronounced differences in maximum pressure temperature estimates (>300°C, >4 kbar) experienced by juxtaposed Greater Himalayan units. Consequentially, this structure likely plays a major role in accommodating shortening within the orogen, which is expected to build significant topography. In the Annapurna region, this boundary lacks a thick mylonitic shear zone, suggesting that it may have experienced recent brittle activity. This study investigates neotectonic offsetting and warping of fluvial terraces that record recent thrust activity within the past tens of thousands of years. Newly available two-meter resolution digital elevation data coupled with field observations, provide an unprecedented opportunity for identifying neotectonic deformation of fluvial terrace geometries across the topographic break. We present terrace tread data from the Seti river drainage in central Nepal. An important limitation, however, is that these digital elevation data allow for detailed imaging of terrace tread deposits, rather than bedrock strath terraces, and thus are also influenced by sedimentation processes. We present preliminary interpretations based on first-order changes in terrace tread geometries over kilometers distance, ensuring that evidence is recorded across multiple terrace levels, and in some cases supported by additional bedrock data. Seti River terrace tread profiles suggest divergence upstream of the topographic break, which can be caused by differential uplift or changes in sediment flux. Multiple terrace levels also appear folded near the structural position of the Chamrong thrust, mapped in the neighboring Modi Khola drainage. At this location, we also report evidence of pervasive brittle bedrock deformation. The combination of these features suggests possible tectonic deformation at multiple locations along the Seti River that are consistent with active brittle out-of-sequence thrusting along the mountain front. We plan to combine these data with UAV models of strath terrace geometries to clarify these preliminary interpretations.