Complex Lesser Himalayan tectonics in the Darjeeling-Sikkim Himalaya: New perspectives from a fold-duplex model

This study provides a comprehensive account of the modes of crustal shortening in the Lesser Himalayan Sequence (LHS) of the Darjeeling-Sikkim Himalaya (DSH). The distributed ductile deformation episodes are integrated with the localized thrusting events in the LHS. The LHS records four major episodes of buckle folding in distributed ductile deformations. From field-based structural correlations, it is demonstrated that multiple orders of third-generation orogen-parallel (F3) and late-stage orogen-perpendicular (F4) folds have resulted in complex interference patterns, varying from plane non-cylindrical (Type 1) to non-plane non-cylindrical (Type 2). Continued N-S shortening in the DSH produced a crustal-scale thrust with ramp-flat geometry, locally known as the Daling Thrust (DT) under the influence of a mechanically weak coal-shale-bearing Gondwana layer, which can be compared with the present-day Main Himalayan Thrust (MHT). The thrust ramp eventually shifted towards the foreland during the India-Asia collision. A fold-duplex model is proposed to explain the potential mechanism of forelandward basal ramp migration. Our model suggests that the southern shallower Main Himalayan Thrust (MHT) flat is susceptible to creeping aseismically due to the influence of coal-shale rheology. In contrast, the mid-crustal ramp and the presently active frontal splay faults (e.g., Main Frontal Thrust, MFT) are potentially seismogenic. These findings have important implications for the interpretation of the MHT seismic cycles.