THE GEOMETRY AND IMPLICATION OF A FORELAND DIPPING DUPLEX, THE RANGIT DUPLEX, DARJEELING-SIKKIM HIMALAYAS, INDIA

In the Darjeeling-Sikkim Himalayas (DSH), the Main Central thrust sheets (MCT1 & MCT2) carry high grade crystalline rocks ~ 80 km farther southward than in the western Himalayas. Sub-MCT structures exposed in the Rangit window (in Sikkim) suggest possible mechanics for the large southward translation of the MCT & Ramgarh sheets. The window extends over a north-south distance of ~ 30 km and exposes the Lesser Himalayan sequence (LHS). The LHS includes greenschist facies metasedimentary sequences of Precambrian Daling and Buxa Formations together with slightly metamorphosed Gondwana Formation. The window rocks are surrounded by overlying Precambrian Buxa and Daling Formations of the Rangit-Tendong thrust sheet (equivalent to the Ramgarh sheet of the western Himalayas and the North Kalijhora sheet in the southern DSH). Within the window, the LHS sequence is repeated in a series of horses forming the Rangit duplex. Cleavage-bedding relationships and stratigraphic younging directions indicate that the northern horses are upward-facing and hinterland (NW) - dipping with cleavage steeper than bedding, while the southern horses are upward facing and foreland (S)-dipping with cleavage gentler than bedding. The southernmost slice is overturned (downward facing) with cleavage steeper than bedding, both dipping NW. Shear sense indicators suggest top to the foreland shear in all the horses. Microscopic evidence indicates early pressure solution and minor plastic deformation, and late stage cataclasis. Early LPS strain axial ratios in the transport (XZ) plane, measured using the Fry technique on sandstones, vary from 1.18 to 1.09. There is no systematic variation in strain within the duplex, although strain generally increases toward the base of individual horses. The presence of a Lesser Himalayan foreland dipping duplex and the large displacements associated with individual imbricate faults are consistent with the large southward translation of the MCT sheets observed in the DSH.