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

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 ∼403 km (∼80%) with an average long-term shortening rate of ∼18 mm/yr in the eastern transect. Comparison with published data indicates that eastern Sikkim records ∼47 km lesser shortening. In western Sikkim, an ∼2 km high footwall Main Himalayan thrust (MHT) ramp below the Rangit duplex caused higher structural culmination exposing the duplex. In contrast, an ∼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.