LATERAL VARIATION IN STRUCTURAL ARCHITECTURE OF THE SIKKIM HIMALAYAN FOLD THRUST BELT: INSIGHTS FROM A BLIND LESSER HIMALAYAN DUPLEX

Along-strike variation in structural architecture is observed in major fold thrust belts (FTB), at various scales. Studies addressing such lateral variation provide critical insights into the various factors that play important roles in the evolution of a FTB.

We focus this study on the Sikkim Lesser Himalayan FTB where there is a significant lateral variation in its structural geometry over a distance of ~15 km. The structurally lower Lesser Himalayan (Rangit) duplex is not exposed east of the Teesta valley in eastern Sikkim. The overlying thrust sheets of the Rangit duplex, i.e., the Pelling and the Ramgarh thrusts are more intensely folded in eastern Sikkim than in western Sikkim. At a first-order, such along-strike structural variation indicates strain partitioning at the scale of constituent thrust sheets. We construct a transport-parallel, retrodeformable regional balanced cross-section, and estimate a minimum orogenic shortening of ~410 km (~80%) in the eastern Sikkim Himalayan FTB, south of the South Tibetan Detachment system. The long-term shortening rate is ~18mm/year. The shortening rate varied spatially and temporally along the transect. Based on the balanced cross-section and bedding-cleavage relationships, the Rangit duplex is constrained as a foreland dipping blind duplex with three horses along the eastern Sikkim transect. Kinematic reconstructions suggest that the roof thrust of the blind Rangit duplex is the Late Ramgarh thrust, while the Pelling thrust is the roof thrust of the upper Lesser Himalayan duplex. 3D finite strain analyses from major thrusts reveal that the early plastic strain was modified by footwall imbrication, while the foreland thrusts track the early layer parallel shortening strain. Thrust sheets with greatest elongation strain also record the highest penetrative strain. Lateral variation in the initial width of the Lesser Himalayan basin, along with a presence of a lateral ramp directly east of the studied transect, can possibly explain the along-strike structural variation in the Sikkim Himalayan FTB.