Fluvial sedimentary response to late quaternary climate and tectonics at the Himalayan frontal thrust, Central Nepal

• Main Authors: Hamahashi, Mari, Hubbard, Judith A., Almeida, Rafael V., Haines, Samuel H., Owen, Lewis A., Mishra, Sanjita, Sapkota, Soma Nath
• Other Authors: Asian School of the Environment
• Format: Article
• Language: English
• Published: 2023
• Subjects: Engineering::Environmental engineering; Himalaya; Main Frontal Thrust; Boreholes; Fold-and-thrust belt; Fluvial environment; Indian monsoon
• Online Access: https://hdl.handle.net/10356/165012
• Institution: Nanyang Technological University

To investigate the subsurface structure surrounding the Main Frontal Thrust (MFT) in central Nepal, we drilled and cored sediments to depths of 45–100 m at 10 sites. Our boreholes were located along previously acquired high-resolution seismic profiles across the MFT imaging the upper 1–2 km of the subsurface, which revealed a beveled erosional surface in the hanging wall above a broad, gentle anticline, as well as growth strata in the footwall. The boreholes exhibit interlayered clays, silts, sands, and gravels, dated with optically stimulated luminescence and radiocarbon to <72.5 ± 4.3 ka, with a transition from finer to coarser sediments at ∼13.5 ± 0.1 ka. Near the fault tip, the deposits exhibit steeper dips and deformation bands. A 25-m-thick section of silt and clay above the south end of the buried anticline is interpreted as a temporary lacustrine depocenter formed due to uplift near the fault tip. Based on the distribution of marker beds and sediment ages, we interpret a shortening rate of 3.1–12.1 m/ka on the MFT. Three major transitions between fluvial-lacustrine and coarse fluvial channel facies are inferred from the boreholes, and the timings of these transitions correlate with Indian monsoonal intensity variations linked to Earth's precession. We infer that a strengthened monsoon led to increased river discharge and advance of coarse bedload-dominant braided channels, whereas a weak monsoon formed a finer-grained channel environment. These monsoonal climate variations have affected the depositional environment and river base levels in this region, influencing the formation and apparent relative uplift of nearby river terraces.