“Fault dynamics and paleoseismic evidence in the Darjeeling-Sikkim Himalayan foothills: A study of fault-controlled landscapes”

Abstract

The Darjeeling-Sikkim Himalayan foothills exhibit a dynamic interaction of regional thrusts, back thrusts, and transverse faults, facilitating strain adjustments. The back-thrust system is characterized by the hinterland-sloping Thaljhora, Nagrakata, and Baradighi scarps, while the transverse faults are controlling river courses, including the Neora, Murti, Jaldhaka, Gathia, and Jiti. This region, known as the Jaldhaka recess, shows evidence of active deformation.

Analyses of slope and longitudinal profiles of rivers and fan surfaces suggest folding in Thaljhora and Baradighi surfaces, with major rivers forming prominent knickpoints as they cross scarps related to back thrusts. A parallel river network further indicates transverse fault-controlled drainage. A trench excavation perpendicular to the Thaljhora scarp, supplemented by natural outcrops, revealed sediment deformations linked to a southerly dipping back thrust fault. Soft Sediment Deformation Structures, which we consider as seismites, indicate moderate-magnitude earthquakes intermittently activated by Thaljhora back thrust.

The radiocarbon age of deformed carbonaceous clays suggests a significant uplift along the Thaljhora thrust between ∼31,000 and ∼ 28,000 CalBP. Another activation phase occurred around ∼21,000 CalBP, forming the Baradighi scarp, with reactivation at ∼9000 CalBP. The Nagrakata scarp likely resulted from rotational movement and strain adjustments among the Thaljhora backthrust and the transverse Jaldhaka and Gathia faults. Thus, evidence from river profiles, trench excavations, and dating techniques (radiocarbon and OSL) confirms late Pleistocene seismic activity, with at least two significant earthquakes (Mw 6–7) influencing fault movements and landscape evolution.