Imprints of Himalayan thrust tectonics on the Quaternary piedmont sediments of the Neora–Jaldhaka Valley, Darjeeling–Sikkim Sub-Himalayas, India

Abstract

Geological mapping in the Neora–Jaldhaka Valley of the Darjeeling–Sikkim Sub-Himalayan foothill region reveals the presence of discontinuous strands of tectonically active Quaternary thrust faults. The zones of active faulting coincide with occurrences of deeply incised piedmont deposits and river terraces along an E–W trending corridor. In order of younging, the successive Quaternary thrust faults delineated are the Gorubathan, Matiali and Chalsa thrusts. Unequivocal geological evidence of surface-rupture faulting, however, is noted only along the Gorubathan thrust where the Precambrian Daling Group of rocks is thrust over the Quaternary piedmont sediments along a low northerly-dipping plane. The presence of deformational features in the Quaternary sediments such as anticlines, bending moment faults and back-tilting of both the piedmont and the incising river terraces along active fault scarps are interpreted to signify the presence of north-dipping blind thrusts below the Matiali and Chalsa scarps. Radiocarbon dates of organic-rich clay components indicate that the Gorubathan thrust post-dated 33,875 ± 550 Ybp and the lower bound age of the Chalsa thrust is 22,030 ± 130 Ybp. Successive foreland-ward development of these active strands and the increasing spacing between the successive thrust traces are interpreted to be due to foreland-propagating blind thrusts of the Himalayan FTB (Fold and Thrust Belt) in the Sub-Himalayan terrain. Evidences of backthrust movement are also presented along the Thaljhora North fault. The segmented and variable geometry of these thrusts imply that the MFT (Main Frontal Thrust) is still in its early stage of growth and the lateral linkages between the active fault-strands are yet to be established.