Orogen-parallel L-tectonites from Pelling-Munsiari thrust of Sikkim Himalayan fold thrust belt: A kinematic study using multiple, incremental strain markers
Jyoti Prasad Das, Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata (IISERK), Mohanpur 741246, West Bengal, India
Kathakali Bhattacharyya, Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata (IISERK), Mohanpur 741246, West Bengal, India
Matty Mookerjee, Department of Geology, Sonoma State University, 1801 E. Cotati Avenue Rohnert Park, CA 94928, USA
Pritam Ghosh, Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata (IISERK), Mohanpur 741246, West Bengal, India
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 in the later stage (~25-54%), as is recorded by the oblique fabric and subgrains in the L tectonites. These rocks record a non-steady deformation; 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. Additionally, the L tectonites record the greatest pure-shear dominated flow (~47-63%), among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the regional, 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. The lateral ramp explains the kinematic evolution of the L tectonites.