Bore-hole Well-cores Provide Evidence for Crustal Heterogeneity Beneath the Koyna-Warna Reservoir Site, Deccan Volcanic Province, India

Extensive geophysical investigations are ongoing to address the causative mechanisms, which could explain the sustained seismicity in the Koyna-Warna region of the Deccan Volcanic Province (DVP), western India. Towards this effort, several exploratory bore-holes ranging up to 1500 m in depth were drilled by CSIR-NGRI around the Koyna-Warna seismic zone for investigation of shallow crustal properties and long term monitoring of seismicity. The focus of this study is the examination of lithology from four bore-well-cores around the Warna area: from west to east, KBH05 (Phansavale), KBH09 (Khadi Kolvan), KBH10 (Udgiri) and KBH08 (Paneri). High-resolution lithologs of these bore-well-cores have been generated, which reveal sub-trappean heterogeneity, especially given the limited areal extent of about three hundred square kilometers enclosing them. In this contribution, an account of lithological characteristics is provided, which were not available in the literature prior to this study. The discovery of relict greenstone volcanic rocks concealed beneath the Deccan Volcanic Province is reported, from one of the bore-well-cores as well as a thin layer of sediments in another. Fossil imprinted ash beds of substantial thickness are observed in two of the well-cores. Intrusive sheets of aplite/granite within the gneisses, seen in one of the well-cores, provides compelling evidence of a reworked basement beneath the Deccan Volcanic Province. The heterogeneity reported here is consistent with patterns of selective locales of earthquake occurrences in the Koyna-Warna zone; definitive roles for reservoir triggered seismicity needs further examination.