Tectonic architecture of the Paleogene belt and adjoining lithostratigraphic units in Parwanoo- Subathu sector of the Himachal Himalaya, India

摘要

The north-northwest- to south-southeast-trending Paleogene belt between the Krol Thrust/Main Boundary Thrust (MBT) in the north and the Bilaspur Thrust/Main Boundary Fault (MBF) in the south comprises the Paleocene to Lower Miocene marine to fluvial rocks of the Sirmur Group, in Parwanoo-Subathu area of the Himachal Himalaya, India. The Sirmur Group of rocks is divisible into the Subathu, Dagshai and Kasauli formations in chronological order. The northerly-dipping Krol Thrust demarcates the footwall rocks of the Paleogene belt from the hanging wall pre-Tertiary lithostratigraphic units except outliers of the Paleocene to Lower Eocene age, the shallow marine Kakara Formation equivalent to lower Subathu Formation. The Paleogene rocks overlie the various pre-Tertiary/Proterozoic rocks of the Simla Group, Baliana Group, Krol Group and Shali/Deoban Group with an unconformable contact having thin layer of laterite. The Paleogene rocks override the Middle Miocene to Middle Pleistocene fluvial molasse deposits of the Siwalik Group in the south along the northerly-dipping MBF. Besides regional/megascopic thrusts/faults, many mesoscopic faults/thrusts have traversed the Paleogene belt. Three phases of folds in pre-Tertiary rocks, two phases of folds in the Paleogene rocks of the Sirmur Group and fault/thrust-related folds in the rocks of the Siwalik Group have been observed in the area. The overall geometry and NW-SE to NNW-SSE orientation of the thrusts, folds and b-c extensional joints within the Paleogene and Siwalik belts suggest southward translation of the Himalayan rock mass as thrust sheets under almost NE-SW-trending sub-horizontal maximum compressive stress (σ1). This paper describes the structural elements of the Paleogene belt and their geological relationships with the adjoining pre-Tertiary and Neogene lithostratigraphic units.