STRAIN PARTITIONING ACROSS SCALES IN THE SIKKIM HIMALAYAN OROGENIC WEDGE

During orogeny convergence-related shortening is accommodated in shallow crustal rocks by folded thrust systems, fault-related folds and by penetrative strain forming fold thrust belts (FTB). Thus, integrating shortening estimates along with magnitude and orientations of strain ellipses from thrust sheets can provide insights into strain partitioning and wedge kinematics at various scales during its evolution.

The folded thrust system of the Main Central thrust (MCT), the Pelling-Munsiari thrust (PT), the Lesser Himalayan duplex (LHD), the Main Boundary thrust (MBT) and the Main Frontal thrust (MFT) have accommodated ~403 km of minimum convergence-related shortening (~80%) in the Sikkim Himalayan FTB. The roof thrust of the LHD, the PT, records the highest translation (~98 km). Following R_f-φ and best-fit 3-D ellipsoid Mathematica program, we quantified 2D and 3D finite strain from deformed quartz grains from these thrust sheets. We restricted the analysis to quartz-rich sections of gneisses, quartzites and sandstones from the various thrust sheets. We analyzed minimum of 100 grains per thin-section. The PT sheet records the highest strain (R_f ~2.21). The frontal thrust sheets, i.e., the MBT and the MFT, record lower strain (~1.43-1.68) than the hinterland thrust sheets (~1.65-2.21). In general, the R_f values within individual thrust sheets increase toward the fault zone indicating strain localization along discrete fault zones. Microstructural and strain analyses also indicate role of deformation mechanisms in strain partitioning within the thrust sheets. The finite strain ellipses are folded along with the thrust sheets indicating that the penetrative strain developed prior to the folding of the thrust sheets by footwall imbrication. The angle between the X-axis of the strain ellipse and the foliation (θ’) in the thin-section plane is low (~2-10°) for all the thrust sheets indicating that the early layer parallel shortening has been modified. Comparison of elongation strain from individual thrust sheets, as obtained from the restored balanced cross-section, and estimated R_f from the same thrust sheets indicate no direct correlation between these two types of strains recorded at two different scales. This observation reinforces that translation, rotation and strain are independent of each other.