Fluid–rock interaction in the basement granitoids: A plausible answer to recurring seismicity

Abstract

The Koyna–Warna Seismogenic Region in the western part of the Indian Subcontinent has been recognized as one of the most significant sites of Reservoir–Triggered–Seismicity (RTS) during the last five decades. The basement granitoids, overlain by the porous and vesicular Deccan Trap basalt, contain numerous interconnecting fractures which act as the ascending and descending pathways of fluid flow. As a result of this fluid flow along fractures, the host rock has been subjected to significant chemical alteration along with the subsequent formation of some new minerals at the expense of a few other pre–existing mineral phases. Mesoscopic observations followed by Optical microscopy in the core samples of the basement rocks upto 1.5 km depth retrieved from the borehole KBH1 near Rasati (about 4.7 km from the Koyna Dam) have revealed the presence of chlorite and the precipitation of calcite, whereas the bulk mineralogical XRD has reaffirmed the presence of chlorite, calcite along with illite at a certain depth. This entire secondary mineral assemblage resembles the propylitic kind of hydrothermal alteration at temperatures < 350°C under acid–to–neutral solution conditions and also indicates water channelization up to the deeper level in the basement granitoids (>1.5 km). In addition, the presence of the hydrophilic clay minerals along fault and fracture zones may be responsible for triggering the seismicity in the Koyna Seismogenic Region as their absorption of water reduces the shear strength of faults and their low frictional strength accelerates the fault weakening process causing the generation of slip surfaces. Thus, in addition to several seismotectonic features, fault geometry and existing stress pattern, the clay mineralisation along the pre–existing faults and fractures of the basement rocks may also be a factor behind the recurring seismicity in this region.