The geometry and kinematics of the Main Boundary Thrust and related neotectonics in the Darjiling Himalayan fold-and-thrust belt, West Bengal, India

Abstract

The Main Boundary Thrust (MBT) is well-exposed in a section of the Tista River valley in the Darjiling Himalayan fold-and-thrust belt. The fault trace trends E–W across the N–S flowing Tista and thrusts Gondwana (Permian) age sandstones over Lower Siwalik (Miocene) age sandstones. The MBT has been folded into a fault propagation synform–antiform pair by tectonic activity along a younger, South Kalijhora Thrust (SKT) in its footwall that transports Lower Siwalik rocks over Middle Siwalik sandstones; the SKT cuts through the fault propagation structure preserving the MBT fault–propagation fold pair in its hanging wall. The motion along the SKT rotated the bedding and the limbs of the folded MBT in its hanging wall. Footwall imbrication in front of the SKT further rotated the folded MBT. Footwall imbrication continued to about 30 km south of the study area in the Middle Siwalik and younger section. The region has been subsequently reactivated as evident from uplifted gravel beds and strath terraces along the folded trace of the MBT fault zone and also in the footwall of the SKT. The reactivation resulted in the formation of connective splays off the existing faults, probably forming horses of a connecting splay duplex in the sub-surface within the Siwalik section. The motion along these splays passively folded overlying beds, and formed uplifted gravel beds and strath terraces along the local drainage. The rotations resulting from footwall imbrication and reactivated splays probably caused the southern limb of the fault propagation synform to become overturned. The duplexing also raised the river bed and arrested the flow of the River Tista causing the river to ‘pond’ and deposit its load, resulting in the formation of a bar upstream of the MBT trace. The tectonic reactivation of the area points to taper-building activities in a subcritical wedge probably in response to monsoon-induced excessive erosion in the middle of the Darjiling–Sikkim–Tibet Himalayan thrust wedge.