Role of fluid in strain softening within the Main Central thrust in Sikkim: The origin of quartz-rich mylonites

Abstract

The presence of fluids promotes strain softening, and profoundly affects the evolution of shear zones. The Main Central thrust (MCT) is a major shear zone that accommodated at least 90 km of shortening, and played a significant role in Cenozoic evolution of the Himalaya. Surprisingly, no information exists on the role of fluid in evolution of the MCT. This study integrates mineralogical, and geochemical analyses with mass-balance calculations to explore the role of fluids in evolution of the MCT shear zone from four exposures, which span ~40 km distance along the transport direction in Sikkim, eastern India. Our analyses reveal significant fluid-induced strain softening in the investigated exposures, attested by retrogression of biotite to chlorite, and feldspar to muscovite, and quartz. The retrogressed feldspar grains are surrounded by recrystallized quartz, and the muscovite, and chlorite produced due to retrogression define the mylonite foliations, which indicate fluid activity was syn-tectonic. We propose a three-stage conceptual model to analyze the role of fluid in the evolution of the MCT in Sikkim: First, the shear zone nucleated along pre-existing structurally weak, and compositionally distinct zones. Following which, the proto shear zone developed in closed-system conditions, and deformation localized by dynamic recrystallization, and dislocation creep resulting in formation of ~200–1000 m-thick protomylonites. Then, fluids infiltrated the shear zone, and induced retrogression, and element mobility. In the hinterland-most exposures, significant fluid-induced retrogression transformed the calc-silicate and paragneiss protoliths to mylonites resembling micaceous quartzites. In the frontal-most, and structurally intermediate exposures, the muscovite and chlorite grains underwent further dissolution, leaving behind a quartz-rich residue; thus, transforming tonalite orthogneiss, and paragneiss protoliths to mylonites resembling monomineralic quartzites. The deformation front narrowed through time, and became progressively localized within the ~120-410 m-thick strain-softened mylonites at the shear zone core, whereas, deformation ceased or continued slowly in the ~100-880 m-thick protomylonites at the margins. The study improves our understanding of the strain softening mechanisms operative within the MCT, and the origin of quartz-rich mylonites.