Tectonic and climatic controls on fill terrace deposition and preservation in a large Himalayan river catchment

Terraces have been studied across the globe to reconstruct past tectonic and climatic change. Himalayan rivers have some of the highest sediment loads globally, transporting a total of c. 10³ Mt of sediment annually to ocean basins. Significant quantities of alluvial sediment in the Himalaya are stored in fill terraces, which have been identified across the range. These terraces record past tectonic and climatic events such as earthquakes, glacial lake outburst floods, landslides, changes in water and sediment flux during glacial-interglacial cycles, or rapid tectonic incision into valley deposits from an increase in rock uplift. Our knowledge of the geographical extent of Himalayan terraces is currently incomplete and often focused on reaches of individual rivers. This is partly a consequence of low preservation due to erosional processes within a rapidly uplifting mountain range, or past limitations in field access and remote sensing techniques.

Using an automatic method for identifying river floodplains and terraces, we identify terraces along every major river within the Gandaki catchment of central western Nepal. We explore the spatial pattern and extent of terraces along each river within the catchment. We link terrace deposition and preservation to tectonic drivers by analysing the relationship between terrace exposures and channel steepness and major structural boundaries along the river profile. We find that terrace preservation within the Gandaki catchment is largely focused within a catchment-wide tectonic window between the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT), suggesting tectonics to be the primary control. Terrace long profiles are a commonly used method to distinguish between drivers of the terrace formation, deformation, and preservation in active mountain ranges and can provide insights into past events by comparing terrace elevation and slope with the modern channel. We further explore the differences in the deposition and preservation histories of terraces within the Gandaki catchment by automatically constructing terrace long profiles for each major river. Terrace profiles between the MBT and MCT are converted into normalised slope plots, comparing the downstream slopes of terraces to the slope of the modern channels. Terraces are then binned vertically, enabling the analysis of terrace profiles at varying heights above the channel. Through the observation of automatically generated terrace slopes, and supported by previous interpretations of terrace profiles, we find that terraces slope more steeply than the modern channel within tectonic accommodation which may reflect sediment oversupply into transport-limited systems, or steepened deposition from debris flows. Downstream towards the MBT and the Mahabharat range, we observe back-tilted terraces reflecting active tectonic deformation. Upstream and near the MCT, back-tilted terrace slopes may record tectonic deformation and past ponding events.