Geometry and kinematics of the Darjeeling-Sikkim Himalaya, India: Implications for the evolution of the Himalayan fold-thrust belt

The Darjeeling-Sikkim Himalaya lies in a zone where the convergence vector between the Indian and Eurasian plates is almost perpendicular to the Himalayan deformation front. Current outcrop pattern preserves high-grade hanging wall rocks of the Main Central thrust (MCT) and the Pelling thrust (PT) within 5km of the mountain front. U-Pb SHRIMP analysis on zircon grains from MCT sheet record two ²⁰⁶Pb/²³⁸U age groups of ca. 2575 and ca. 943Ma and from the PT 1835±13Ma. The Lesser Himalayan duplex in the footwall of the MCT is composed of two duplex systems; a structurally higher hinterland dipping duplex, and a lower duplex system that varies from hinterland dipping in the north to an antiformal stack in the middle and foreland dipping duplex in the south. The foreland-dipping horses and reactivation of the roof thrust resulted in large translation of the overlying MCT and Pelling thrust sheets. The duplex also resulted in a plunge culmination forming a broad N-S trending anticline. A transport-parallel, restorable, balanced cross section reveals a minimum shortening of ~450km (~81%) south of the South Tibetan Detachment system, and an average long-term shortening rate ~20mm/yr. Although the study area lies in a zone of high convergence angle with the deformation front, the surface structures do not record the greatest shortening of the orogen. The Shillong plateau and a tranverse zone lying southeast of the study area have accommodated a part of the convergence-related shortening, thereby under-representing the shortening in the Sikkim Himalayan fold-thrust belt. Lateral variations in the original width of the Lesser Himalayan basin may have played a role in partitioning the shortening in the fold-thrust belt.