RATES OF SHORTENING IN THE HIMALAYAN FOLD-THRUST BELT, WESTERN NEPAL

New orogen scale balanced cross-sections from western Nepal reveal that the rate of shortening since ~25 Ma is similar to the present rate of convergence. These three new cross-sections span 200 km across strike and provide an integrated view of the Himalayan fold-thrust belt using the modern concepts of thrust belt geology. Key structures that accommodate shortening are the hybrid hinterland dipping-antiformal duplex which incorporates Lesser Himalayan rocks north of the Dadeldhura crystalline klippe, the Ramgarh thrust sheet, and rocks of the Greater Himalayan thrust sheet. The Lesser Himalayan duplex contains a minimum of seven thrust sheets and a maximum of thirteen thrust sheets across strike from far western to mid western Nepal. The northernmost thrust sheet of the Lesser Himalayan duplex is the Ramgarh thrust sheet which extends southward for up to 125 km. Rocks of the Greater Himalayan thrust sheet crop out in the Dadeldhura crystalline klippe in the southern part of the fold-thrust belt. These structural features are continuous along strike from the Indian border eastward across the Nepalese Himalaya.

The balanced cross-sections reveal the following shortening estimates from the Main Central thrust sheet to the Main Frontal thrust, which includes restoration of the Main Central thrust sheet to the basal decollement: Api, 418 km (78%); Chainpur, 403 km (78%); Simikot, 534 km (76%). Because meso-scale faults and folds or penetrative strain have not been included, these are minimum shortening estimates. When these values are added to shortening estimates in the Greater Himalayan and Tibetan Himalayan portions of the fold-thrust belt, the cross sections imply that over 900 km of shortening is recorded between the Indus suture and the Main Frontal thrust. These shortening estimates from the Main Central thrust to the Main Frontal thrust in western Nepal translates into a rate of shortening between the Indian continent and Asia since ~25 Ma of approximately: Api, 17 mm/yr; Chainpur, 16 mm/yr; Simikot, 21 mm/yr. These rates are comparable to the present rate of convergence in the central Nepalese fold-thrust belt of ~17 mm/yr (Bilham et al., 1997; Larsen et al., 1998).