The Geometry and Kinematics of the Main Boundary Thrust Fault and Related Neotectonics in the Darjeeling. Himalayan Fold-and- Thrust Belt, West Bengal, India

The Main Boundary thrust (MBT) is well-exposed in the Kalikhola section in the Teesta valley near the Kalijhora town in the Darjeeling Himalayan fold-and-thrust belt . The fault trends east-west and cuts across the north-south flowing river Teesta and thrusts Gondwana (Permian) age sandstones over Lower Siwalik (M :iocene) age Chunabati sandstones. The MBT has been folded into a fault propagation synform-antiform pair by tectonic activity along a younger (South Kalijhora) thrust which transports Lower Siwalik Chunabati Formation rocks over Middle Siwalik Geabdat sandstones . The South Kalijhora thrust (SKT) cuts through the fault propagation structure preserving the fault-propagation fold pair in its hanging wall . The motion along the SKT rotates the bedding and the limbs of the folded MBT in its hanging wall . Footwall13; imbrication in front of the SKT and the formation of the Andherijhora thrust further rotates the folded MBT ; the southern limb of the fault propagation synform is overturned as a result of these rotations . Footwall imbrication continued till about 30 km south of the study area in the Middle Siwalik and younger section . The region has been subsequently reactivated as evident13; from uplifted gravel beds along the Kalikhola stream and sand terraces along the river Teesta .The reactivation resulted in the formation of splays off the existing faults which probably formed horses and connecting splay duplex in the sub-surface within the Siwalik section which passively folded overlying beds and uplifted sand terraces and gravel beds . The duplexing also raised the13; river bed and arrested the flow of the river Teesta causing the river to quot;pondquot; and deposit its load13; resulting in the formation of a bar upstream of the MBT . The tectonic reactivation of the area points to taper building activities in the wedge probably in response to excessive erosion