LAUSR

Abstract We compare and contrast the structural evolution of two successive internal thrusts, the Main Central thrust (MCT) and the Pelling-Munsiari thrust (PT), from the crystalline core of the Sikkim Himalaya in the context of progressive deformation involving a footwall Lesser Himalayan duplex; the PT is its roof thrust. The mylonitic foliations in both the shear zones are overprinted by successive cleavages. The youngest cleavage from the PT zone continues within the hangingwall MCT sheet that records one additional cleavage possibly indicating time-transgressive cleavage development. The cleavages become steeper while their intensity decreases structurally higher up within individual thrust sheets. The Rs-values and angular shear strain decrease from the mylonite zones to structurally higher up within the sheets. Both the shear zones are Type II with decelerating strain paths. Rs-θ′ relationships and microstructures indicate thrust-parallel stretch is greater than thrust-perpendicular component. Both the shear zones record strain partitioning with lesser competent mylonite domains recording higher Rxz, and a greater proportion of simple-shear than the protomylonite domains. The shear zones recorded the growth of the duplex during progressive deformation that contributed to pure-shear dominated general-shear, higher flattening strain and greater translation on its roof thrust, the PT, than the overlying MCT.