Kinematic analyses of orogen-parallel L-tectonites from Pelling-Munsiari thrust of Sikkim Himalayan fold thrust belt: Insights from multiple, incremental strain markers

Abstract

Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (Wk) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (∼56–96%) that progressively became simple-shear dominated (∼29–54%), as recorded by the oblique fabric and subgrains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen.The L-tectonites record ∼30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.