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

摘要

Throughout the Himalaya, the fethyan 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 (D-2) north-verging back folds that oppose the vergence of the orogen. The folds are met printed and cross-cut by the c, 22 Ma South Tibetan detachment system and the Manaslu leucogranite. New Structural mapping ill Hidden valley provides further constraints oil the character of D-2: megascopically, large asymmetric north-verging folds define D-2 microscopically, it is characterized by,in axial-planar cleavage. Balanced cross-sections and bed-length restoration of F, folds indicate a minimum of 32 38%, shortening and 180% thickening during D-2 these data indicate that north-verging folds played a significant role in pre-Miocene crustal thickening of the Himalayan Superstructure. The formation of those 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.