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 similar to 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.
