Fluvial Architecture and Tectonic Controls on Siwalik Stratigraphy in the Trijuga Valley, Eastern Nepal Himalaya

The Himalayan orogen originated from the collision of the Indian and Eurasian plates, and the Nepal Himalaya occupies about 800 km of the central arc. The Sub-Himalaya, or Siwalik Group, is the southernmost and youngest fold-thrust belt of the orogen; it is bounded by the Main Frontal Thrust to the south and the Main Boundary Thrust to the north. subsidiary thrusts duplicate Siwalik strata and frame longitudinal Dun valleys such as the Trijuga Valley in eastern Nepal, where a ~5 Myr record of foreland-basin deposition by Himalayan Mountain rivers is exceptionally well preserved. Through route-scale mapping, measured sections, and thin-section petrography, we resolve a threefold succession-Lower, Middle, and Upper Siwalik-dissected by the Kamala Tawa and Marine Khola thrusts. Up-section facies transitions from fine-grained flood-plain sandstones to multistorey channel belts and finally matrix-supported conglomerates track a progressive increase in channel competence and bed-load rates. Sandstone beds thicken from 2-5 m in the Lower Siwalik to more than 15 m in the Middle Siwalik, while clast sizes jump from coarse sand to boulders in the Upper Siwalik, signaling stepwise rises in peak unit discharge. Petrographic spectra shift from feldspar-rich sub-arkose to quartz-dominated sub-litharenite, and a pronounced biotite spike in the Middle Siwalik records accelerated erosion of rapidly uplifting phyllitic and gneissic catchments. These architectural and compositional trends correspond to sequential activation of the bounding thrusts, showing that tectonically driven headwater expansion steepened river gradients and boosted coarse-grained sediment supply. The resulting positive feedback between uplift, sediment flux, and channel mobility enhanced foreland aggradation and aided the preservation of high-relief topography. By linking stratigraphic architecture, grain-size evolution, and petrographic provenance to thrust kinematics and monsoon variability, this study illuminates how fluvial dynamics and sediment routing govern long-term landscape evolution in actively deforming mountain belts.