Upper crustal shortening and forward modeling of the Himalayan thrust belt along the Budhi-Gandaki River, central Nepal

摘要

A balanced cross-section along the Budhi-Gandaki River in central Nepal between the Main Central thrust, including displacement on that fault, and the Main Frontal thrust reveals a minimum total shortening of 400 km. Minimum displacement on major orogen-scale structures include 116 km on the Main Central thrust, 110 km on the Ramgarh thrust, 95 km on the Trishuli thrust, and 56 km in the Lesser Himalayan duplex. The balanced cross-section was also incrementally forward modeled assuming a generally forward-breaking sequence of thrusting, where early faults and hanging-wall structures are passively carried from the hinterland toward the foreland. The approximate correspondence of the forward modeled result to observe present day geometries suggest that the section interpretation is viable and admissible. In the balanced cross-section, the Trishuli thrust is the roof thrust for the Lesser Himalayan duplex. The forward model and reconstruction emphasize that the Lesser Himalayan duplex grew by incorporating rock from the footwall and transferring it to the hanging wall along the Main Himalayan thrust. As the duplex developed, the Lesser Himalayan ramp migrated southward. The movement of Lesser Himalayan thrust sheets over the ramp pushed the Lesser Himalayan rock and the overburdens of the Greater and Tibetan Himalayan rock toward the erosional surface. This vertical structural movement caused by footwall collapse and duplexing, in combination with erosion, exhumed the Lesser Himalaya.