Structural Evolution of the Himalayan Thrust Belt, West Nepal

Himalayan foreland basin strata were recently documented to crop out in a structural window in the central portion of the Himalayan thrust belt. Presently, structural interpretations of the thrust belt do not explain in detail how these strata were incorporated into the thrust belt and why they are not widely exposed throughout the Himalaya. My research provides answers to these two issues. The structural window is located in the Lesser Himalaya of western Nepal and exposes rocks which lie structurally beneath the Main Central thrust (MCT) and Ramgarh thrust (RT) sheets. The thrust sheet consists of Proterozoic metamorphic rocks. Below the thrust sheet, footwall rocks exposed in the window are unmetamorphosed sedimentary rocks which consist of the early foreland basin strata (the Suntar and Swat formations) and the pre-foreland basin strata (the Melpani and Lakharpata formations). These unmetamorphosed rocks are present in the foreland basin beneath Siwalik group, ~100km to the south (towards the foreland). Two geologic maps were created; One covering the structural window and another covering the thrust belt from the High Himalaya to the MFT (Main Frontal Thrust). Several cross-sections were constructed from both maps. Structural reconstruction of these cross-sections reveal the following: 1) ~ 75 km-long hanging wall flat extends northward from its surface trace to the southern margin of the Lesser Himalayan duplex; 2) The geometry of the Jarjarkot klippe is narrower and structurally deeper than klippe to the west. The geometry of the northern flank of the klippe results from stacking of duplex horses, while the geometry of the southern flank results from slip over a ramp in the footwall of the MBT, 3) The early foreland basin strata in the window are modeled to have originated at the front of the thrust belt, and subsequently buried by the Ramgarh and MCT thrust sheets, and 4) Exposure of these strata results from growth of the duplex which brought them to a higher structural level than most parts of the thrust belt. This structural model explains why the only other exposure of foreland basin strata within the thrust belt is documented within a duplex on the north side of the Dadeldhura klippe. Moreover, this model predicts that foreland basin strata are likely to be exposed wherever duplexes exist.