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

Geologic mapping along the Budhi-Gandaki River in central Nepal reveals 6 significant structures: (1) South Tibetan Detachment system (2) Main Central thrust (3) Ramgarh thrust (4) Lesser Himalayan duplex including the Trishuli thrust (5) Main Boundary thrust and (6) Main Frontal thrust system. A balanced cross-section between the South Tibetan Detachment system and Main Frontal thrust reveals that the region has a minimum total shortening of 76% or 420 km. The breakdown of the accommodation of shortening on each thrust is as follows: Main Central thrust - 115 km Ramgarh thrust - 120 km Lesser Himalayan duplex including the Trishuli thrust - 156 km Main Boundary thrust - 10 km Main Frontal thrust system - 19 km.In order to validate the balanced cross-section, a reconstruction program was used to forward model the system. By moving faults with appropriate amounts of displacement over a reasonable configuration of undeformed stratigraphy from the hinterland to foreland, the deformation of the Himalayan thrust belt along the Budhi-Gandaki River cross-section is reproduced. The forward modeling program moves hanging wall rock over stationary footwall rock using each individual fault identified in the balanced cross-section. Hanging wall rock deforms as it is thrust over footwall structures. Using forward modeling, the cross-section has a shortening estimate of 412 km or 75%. The two shortening estimates are virtually identical indicating the balanced cross-section along the Budi-Gandaki River is viable and admissible.