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.

Abstract. The Koyna-Warna Seismogenic Region of western India has been recognized as one of the hotspots for reservoir-triggered seismicity (RTS) since 1967. The current study investigates the fluid's interaction with the severely fractured granitoid basement of this area and its potential contribution to the recurring seismicity. The presence of several secondary minerals, such as chlorite, epidote, calcite, illite, etc., along the pre-existing faults and fractures, is revealed by detailed petrologic investigation at mesoscopic and microscopic scales along with XRD analysis. This indicates the fluid-rock interaction along these mechanically weak planes and subsequent propylitic grade of hydrothermal alteration under acidic to neutral conditions (pH 5.5–7) and the temperature of above 200–220 °C up to about 350 °C. Additionally, the transformation of biotite into chlorite due to fluid interaction has been inferred from the microscopic appearance of biotitic remnant within neoformed chlorite which is further supported by the mass loss of K2O and concurrent gain of MgO and FeO, demonstrating the replacement of potassium (K) interlayer sheet by brucite-like [Mg (OH)2] layer during biotite chloritization. However, this released K2O further assists in the formation of illite resulting in the mass gain of K2O at a few certain depths, whereas the dissolution of plagioclase justifies the formation of albite and calcite as evidenced by the gain of Na2O and CaO. The present study also highlights that the recurring nature of the seismicity in this area may be related to clay mineralization along the faults and fractures due to fluid-rock interaction, such as chlorite, illite, etc., in addition to the existing fault geometry and stress build-up due to reservoir impoundment. At increasing stress condition, the anisotropic and weakly bonded, layered crystal structure of chlorite forming ripplocations may develop kink bands and increases the yield strength proportionally with rising pressure up to dehydration temperature. Such visco-elastic behaviour of chlorite may promote aseismic creep in the faults. On the other hand, epidote noticed at certain depths has a contrasting behaviour; it tends to wear at the micron or submicron‐scale asperity contacts and produce fine particles which generate unstable sliding. However, the relatively higher abundance of chlorite in the faults and fractures disrupts the epidote‐epidote contact asperities and prevents such wearing of epidote grains into fine particles. Thus, biotite chloritization in conjunction with relatively less production of epidote along pre-existing faults and fractures helps to release the accumulated stress through a series of small-scale earthquakes and results in the steady fault creep observed in this region during the past 50 years. In this context, fluid-rock interaction along the pre-existing faults and fractures at shallow depth has acted as a blessing for the Koyna-Warna Seismogenic region shielding it from relatively large magnitude earthquakes – a boon for the region.

The Koyna-Warna seismogenic region overlying the Deccan volcanic province of Western India has been experiencing the recurrence of earthquakes since 1967 soon after the impoundment of the Koyna Dam.A large number of small to medium-magnitude shallow focus earthquakes(＞100,000)have been detected in this intraplate region during the last six decades.

Since the impoundment of the Shivajisagar Reservoir behind the Koyna Dam in 1962, numerous earthquakes have been felt in the Koyna-Warna Seismogenic Region of Western India. The mesoscopic and microscopic observations on the basement granitoid core samples, recovered under the Continental Deep Drilling Program of the Ministry of Earth sciences, reveal the precipitation of calcite and the formation of clay minerals (illite and chlorite) along the fractures and faults. The presence of these secondary minerals alongside the primary minerals like quartz and feldspar is further supported by X-ray Diffraction, which also points to the fracture scale chemical alteration as a result of fluid-rock interactions. It's interesting to note that the precipitation of these hydrophilic clay minerals along faults and fractures might promote slip by raising fluid pressure and lowering the shear strength of the faults. Thus, secondary mineralization due to fluid-rock interaction may have a contribution to the release of strain in form of seismic tremors. On the other hand, the neoformation of these hydrophilic clay minerals along fault/fracture surfaces may also cause rheological incongruity, which could lower the density as well as P and S wave velocities. Besides, hydrogen atoms in clay-bound water may influence neutron capture, leading to over-optimistic estimations of neutron porosity. Additionally, our study supports past geophysical anomalies found in the KFD1 borehole and infers that the geophysical anomalies correlating to the growing fracture density and fault system of the basement rocks are caused by chemical alteration due to fluid-rock interaction and subsequent secondary mineralization. So, this research offers important new understandings of geochemical activity in the context of geophysics and serves as a bridge between geochemistry and geophysics.

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.

The composition of the Precambrian granitic basement below a few hundred to a few thousand meters of the Deccan Traps in western and central India remain poorly characterized, essentially due to lack of samples for laboratory analyses. In the present work, petrographic and geochemical characteristics of the basement granitoids from the Koyna region of western India have been studied through core samples and cuttings made available from the scientific drilling to the depth of 3014 m. The basement rock comprises dominantly of granite, granitic gneiss and tonalite. The major minerals are quartz, plagioclase and orthoclase, whereas the minor minerals include epidote, chlorite and hornblende. In the feldspar and quartz grains, signatures of brittle deformation, as well as shearing effects, are observed. Also, fractures and joints are noticed frequently throughout the basement rock with possibilities of more than one dominant stress direction during fracturing. Below 1100 m depth in the western and 1300 m depth in the eastern side of the Western Ghats escarpment, the basement cores are weakly foliated as well as sheared due to tectonic deformation. The bulk rock geochemistry of the basement granitoids shows range from tholeiitic to calc-alkaline series of magma types which mainly belong to adamellite–granodiorite–tonalite suites. The basement rocks are predominantly peraluminous in nature and have an overall predominance of sodium over potassium. Fractionated REE patterns with enriched light REE (LREE) and depleted heavy REE (HREE) along with a small negative Eu-anomaly are observed in the samples. Overall, the Precambrian basement rocks are lithologically heterogeneous and apparently genetically related by fractional crystallization and partially derived from magmas.

The Koyna region of Maharashtra located in the western part of the ～65 Myr old Deccan traps province, overlying the Neoarchean cratonic granitoid basement of peninsular India, has been experiencing recurring seismicity since 1962 after the impoundment of the Shivajisagar Reservoir behind the Koyna Dam.

Carbon isotope (delta C-13) and rare earth element (REE) concentrations in representative samples of the shallow marine Subathu Formation, explored from the Neelkanth and Dogadda sections of Northwestern Himalaya (India) were determined to infer the palaeo-environmental condition during the late Paleocene and middle Eocene. delta C-13 values show variation of similar to 5.0 parts per thousand with maximum excursion (-27.34 parts per thousand) in calcareous sandstone at the basal part and minimum (-22 parts per thousand) in red shale towards the terminal end. Total REE concentration varies (due to lithology) from 27.23 ppm to 564.35 ppm with an average of 187.60 ppm. The chondrite and PAAS normalized patterns exhibit positive Ce anomaly (0.95-4.45), enriched LREE, and depleted HREE, medium Y/Ho ratio (similar to 30-45) along with positive correlation between Y/Dy and Y/Ho ratio. In addition, calcite veins present in some shale samples indicate redox sensitive trace elements. The overall REE abundance and distribution suggests highly oxygenated environment under the shallow marine regressive phase of deposition. The depositional setting, biostratigraphical constrained age along with delta C-13 values and lower TOC suggested an intense warm period, that might be coeval with the Paleocene-Eocene Thermal Maxima event (PETM).

A brief literature review on the breccias of contrasting origin, their diagnostic signatures along with related terminologies is presented here. The importance of individual breccia types and their geological implication has also been reviewed. The present study suggests that breccia is formed either by igneous, sedimentary and tectonic processes or a combination of these. This review is mainly focused on the most common seven sub-classes, (i.e., volcanic, igneous-hydrothermal, chert, collapse, fault, impact and seismic) and the specification of the processes involved in their formation, which subsequently brings more clarity in its classification and characterization.

Jangalgali breccia unit (JBU), the oldest volcanogenic rock unit in the Himalayan foreland basin, has been studied from five different, however, stratigraphically equivalent localities of Jammu region in NW India. The field and thin section slides of JBU reveal that quartz and plagioclase are the two most common minerals and sanidine, magmatic zircon, rutile, hornblende and biotite are present as accessory phases. Petrographic signatures show that quartz and K-feldspar are set in a fine-grained cryptocrystalline glassy matrix and do not represent any preferred orientation. The euhedral hexagonal dipyramidal quartz phenocrysts, irregularly shaped inclusions having high volatile contents and no signatures of transport/reworking in phenocrysts prior to deposition are the prominent petrographic features of JBU. The mineral grains arrangement varies between tight-fitted fabric geometry to more open-chaotic packing. Overall, the field relationship, texture, mineralogy along with mineral chemistry, and presence of high gas and silica content in the host magma, support the volcanic origin of the JBU litho unit, which has wider implications towards understanding the timing of India-Asia collision as well as geodynamic evolution of the Himalayas.

A single zircon grain from rhyolitic breccia was investigated via Raman spectroscopy and SEM-Edx. Results showed that there were appreciable differences in Raman spectral features from centre to periphery along X and Y axis. The Raman spectra shows two different trends regarding the origin of the zircon crystal, a magmatic origin towards the centre and a metamorphic origin-along the periphery. The observed trends also revealed that the zircon grain metamorphosed only at one side while the other side of the crystal retained its original nature, which is an indication of conversion from magmatic to metamorphic zircon in one direction.

We report here the occurrence of rhyolite between the Neoproterozoic Sirban Limestone and Palaeogene Subathu Formation in Northwest Himalaya, India. It is 5-10 m thick, consists of phenocrysts of quartz and feldspars of different shapes and sizes distributed randomly in a glassy matrix. Zircon, rutile, biotite, tourmaline and haematite occur in minor amounts. Bipyramidal, angular and skeletal morphologies are common in quartz. Quartz shows resorption features and contains inclusions of negative crystals. The field feature, mineralogy, texture and whole-rock composition are typical of high-silica rhyolites. By virtue of its occurrence at the base of the Himalayan foreland stratigraphy, the rhyolite is important in exploring the timing of India-Eurasia primordial collision and in stratigraphic correlation studies.

Chert breccia that occurs between Precambrian Sirban Limestone and Late Paleocene-Middle Eocene Subathu Formation at Kalakot village in Jammu & Kashmir was studied for its mineralogy and texture by optical microscopy. It consists dominantly of quartz followed by feldspar, rutile, magmatic zircon, biotite, tourmaline and pyrite. Hexagonal dipyramidal quartz and quartz-eye textures are common in Kalakot chert breccia. Volatile rich melt inclusions occur within the quartz. The characteristic features of quartz and accessory minerals like rutile, magmatic zircon, biotite, pyrite and tourmaline clearly contradict the previous view of sedimentary origin of this breccia.