Penetrative strain variations across Sikkim Himalayan fold thrust belt (FTB) and its contribution to the FTB shortening budget

The Himalayan FTB accommodates ~492-818 km of convergence-related minimum shortening by the folded thrust systems and fault-related folds. A significant component of shortening is also accommodated by penetrative strain that is not generally incorporated in the shortening estimates from regional balanced cross-sections. In this study, we estimate penetrative strain from thrust sheets of the Sikkim Himalayan FTB and integrate these results with regional balanced cross-sections to address how shortening gets partitioned across scales during orogeny. In the Sikkim Himalaya, the folded thrusts of the Main Central thrust (MCT), the Pelling-Munsiari thrust (PT), the Lesser Himalayan duplex, the Ramgarh thrust (RT), the Main Boundary thrust (MBT), and the Main Frontal thrust (MFT) have accommodated ~403-450 km of minimum shortening. The estimated minimum displacements are higher in the internal thrusts of the MCT (~91-101 km), the PT (~86-98 km), and the RT (~58-65 km), while it decreases in the frontal thrusts (~2-32 km). We quantified 2D and 3D penetrative strain from deformed quartz grains from these thrust sheets following Fry, R_f-φ method, and the best-fit 3-D ellipsoid Mathematica program. The tectonic strain (R_s) decreases from the internal thrust sheets (~1.6-2.21) to the frontal thrust sheets (~1.39-1.58). The R_s values within individual thrust sheets increase from structurally higher up toward the base of the thrust sheet, i.e., within the shear zones indicating strain localization. Finite strain ellipses are folded along with the thrust sheets indicating that the penetrative strain developed prior to folding of the thrust sheets by footwall imbrication. To understand strain partitioning across scales, we estimated shortening strain from individual thrust sheets from regional balanced cross-sections and penetrative strain percentage from corresponding thrust sheets at the grain-scale. Although the displacement on individual thrusts vary systematically and significantly from the internal to the frontal thrusts, shortening strain remained remarkably similar in the internal and frontal thrust sheets. Penetrative strain percentage of individual thrust sheets remain lower than shortening strain, and it progressively decreases from the internal to the frontal thrust sheets.