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Abstract

In an orogeny, shear zones accommodate a considerable amount of strain by means of both ductile and brittle deformation through ductile creeping and earthquakes, respectively. Several studies, largely based on the V_P/V_S ratios and EM surveys speculate the role of fluid/brines/melt in present-day earthquakes in shear zones. However, direct investigations into the function of the fluid in an active orogenic belt are not yet feasible. In light of this, we have chosen shear zone outcrops in the Himalayan orogenic belt, South Almora Thrust (SAT) zone, Kumaun Himalaya, India, that have preserved signatures of both paleo-earthquakes and fluid activity for a comprehensive study. We have used veins, striated faults, vein microstructures, and fluid inclusions for deciphering fluid-tectonic stress interplay in the SAT. Both implosion and explosion inclusions in veins show a rapid change in stress when the fluid is trapped in the veins. Fluid inclusion microthermometry implies a considerable fluid pressure fluctuation. Significant variations in the absolute paleostress value, estimated from 3D Mohr circles reconstructions, indicative of changes in the tectonic stress in the SAT. The fluctuating ambient stress, variability in fluid pressure, and evidence of both the implosion and explosion textures are collectively interpreted to have developed during seismic cycles. Bracketing of geochronological data suggests that the recorded fossil earthquakes are Late Oligocene-Early Miocene (21–25 Ma) events. In a nutshell, this study reports evidence of fossil earthquakes and quantifies (semi-quantitative approach) the effect of seismic cycles, and successfully demonstrates the role of metamorphic fluid in earthquakes.