Neotectonic impact on drainage development in the Eastern Himalayan foreland basin

Abstract

The present study intends to divulge the complex drainage evolution in connection to neotectonic controls focusing the Jaldhaka and Raidak River interfluve on the eastern Himalayan Foreland Basin (HFB). It involves analysing the surface and sub-surface changes in the fluvial system's evolution with field and geospatial evidence bearing geomorphic markers, sediment analogy and channel morphological properties. The channel orientation, planimetry and the channel longitudinal process anomaly were attributed solely from the geospatial perspective as surface evidence of neotectonic controls. The results unveil an intense neotectonic control on channel development–from structurally guided channel avulsion that shaped the presently active course of the Torsa River to deformation-guided channel morphological changes. Moreover, relatively higher deflection angles, intense channel sinuosity and tight meandering bends forming along the structural elements symptomizes the structure-controlled drainage development in recent times. Furthermore, sediment analogy from deep well logs and shallow subsurface trenches was analysed to obtain subsidence-driven fluvial sequences and geochronology of sedimentary deformation in channel beds and floodplains. The Optically Stimulated Luminescence (with fading correction) technique-based determination of shallow sub-surface sediment's age indicates occurrences of palaeoseismic events between 1.6 ka (±0.1 ka) and 1.35 ka (±0.2 ka) as well as between 1.35 ka (±0.2 ka) and 1.14 ka (±0.1 ka) had resulted in channel bed sagging while stress impacted the formation of syn-sedimentary folding. The palaeoseismic events that led to relative subsidence are evident in upward sediment stacking (channel fill) sequences being observed at multiple trenches. The deep sub-surface staking of channel floor deposition lenses is also evident of basin subsidence that influenced channel oscillations of reoccupation type.