Pre-Miocene deformation of the Himalayan superstructure, Hidden valley, central Nepal

摘要

Throughout the Himalaya, the Tethyan sedimentary sequence forms the detached carapace, or superstructure, to exhumed mid-crustal rocks. In central Nepal, low metamorphic-grade Cambrian to Jurassic rocks of this carapace preserve five phases of deformation, structurally dominated by second phase (D₂) north-verging back folds that oppose the vergence of the orogen. The folds are overprinted and cross-cut by the c. 22 Ma South Tibetan detachment system and the Manaslu leucogranite. New structural mapping in Hidden valley provides further constraints on the character of D₂: megascopically, large asymmetric north-verging folds define D₂; microscopically, it is characterized by an axial-planar cleavage. Balanced cross-sections and bed-length restoration of F₂ folds indicate a minimum of 32–38% shortening and 180% thickening during D₂. These data indicate that north-verging folds played a significant role in pre-Miocene crustal thickening of the Himalayan superstructure. The formation of these folds is compatible with wedge extrusion or channel flow of the underlying mid-crustal rocks, whereby their geometry reflects early coupling between the upper and middle crust.