Understanding the mechanism of shallow crustal fluid-rock interaction in the Deccan Trap basement rocks and its significance in the Koyna-Warna Seismogenic region, Western India

Abstract

The present study investigates the fluid's interaction with the severely fractured granitoids basement rocks underlying the Deccan Volcanic Province in Western India and its potential contribution to the recurring seismicity in the Koyna-Warna Seismogenic Region, a hotspot of artificial reservoir-triggered-seismicity. The presence of chlorite, epidote, calcite and illite, along the pre-existing faults and fractures, has been identified by the detailed petrologic investigation at mesoscopic and microscopic scales along with X-ray diffraction. This indicates the fluid-rock interaction along these mechanically weak planes and the subsequent propylitic grade of hydrothermal alteration. The microscopic appearance of biotitic remnants within neoformed chlorite indicates the transformation of biotite into chlorite and epidote due to fluid interaction which is further supported by the mass balance calculations. Additionally, the geochemistry also shows that K₂O released during biotite dissolution explains the formation of illite at a particular depth, whereas plagioclase dissolution justifies the production of albite and partially calcite. Furthermore, chlorite, when formed in such highly stressed fault zones, shows ripplocations like intracrystalline deformation in response to the differential behaviour of the hydrogen bonds connecting the talc-like T-O-T layer with the brucite-like sheet and thus accommodates strain that promotes fault creep. Although epidote found here should favour the fault slip, the relatively higher abundance of chlorite suppresses this contrasting impact of epidote. Thus, biotite chloritization due to fluid-rock interaction along faults and fractures facilitates steady fault creep which may be one of the plausible explanations for recurring seismicity in the study area for the last 50 years.