Fluvial sedimentary response to late Quaternary climate and tectonics at
the Himalayan Frontal Thrust, central Nepal
Abstract
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 ten 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 sediments 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 mm/a on
the MFT. Three major transitions between fluvio-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 strengthened
monsoon led to increased river discharge and advance of coarse
bedload-dominant braided channels, whereas 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.