Duplex and along-strike structural variation: A case study from Sikkim Himalayan fold thrust belt

摘要

The Sikkim Lesser Himalayan fold thrust belt records along-strike structural variations over 15 km. The lower Lesser Himalayan (Rangit) duplex (LHD) is blind, and the overlying Pelling and Ramgarh thrust sheets are more intensely folded and preserved in eastern Sikkim than in western Sikkim. A retrodeformable, regional balanced cross-section reveals a minimum shortening of similar to 403 km (similar to 80%) with an average long-term shortening rate of similar to 18 mm/yr in the eastern transect. Comparison with published data indicates that eastern Sikkim records similar to 47 km lesser shortening. In western Sikkim, an similar to 2km high footwall Main Himalayan thrust (MHT) ramp below the Rangit duplex caused higher structural culmination exposing the duplex. In contrast, an similar to 0.35 km high ramp below the Rangit duplex in eastern Sikkim did not create sufficient culmination to expose the duplex, causing along-strike discontinuous duplex exposure. The LHD with a blind component, and having fewer (11) but gentler dipping (40) horses transferred more displacement to the roof thrust in eastern Sikkim than the twelve, steeper dipping (60) horses in western Sikkim. Lateral variations in location and height of footwall MHT ramp, initial width of the Lesser Himalayan basin, and presence of a lateral ramp explain the structural variation in Sikkim.