A balanced cross section across the Himalayan frontal fold-thrust belt, Subathu area, Himachal Pradesh, India: thrust sequence, structural evolution and shortening

摘要

The structural evolution of the frontal fold-thrust belt in the NW Himalayas, has been deciphered through the construction of a balanced cross section, which incorporates all the Tertiary and the Precambrian sedimentary rock units belonging to the Sub-Himalaya Zone and the Lesser Himalaya Zone, respectively. Towards the foreland the structural geometry is rather simple with widely spaced thrust ramps and simple fault-bend folds in the hangingwall. In the central sector, the structural geometry seen at the surface is essentially controlled by a thrust system that can be approximately described as a buried hinterland-dipping duplex. The structural geometry becomes extremely complex in the northeastern sector (i.e. towards the hinterland) owing to out-of-sequence thrusting, low ramp spacing leading to dislocation of older thrusts and related structures by younger thrusts, folded thrusts, and breached horses. Stacked-up horses, occupied by Lesser Himalaya Zone formations, dominate the structural geometry in this sector. An in-sequence thrusting event followed by out-of-sequence thrusting in an approximately break-back style best describe the structural evolution of the fold-thrust belt in this area. A total slip of about 96 km occurred along the detachment and the shortening partitioned within the fold-thrust belt is about 72 km or about 71%.