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We have investigated 78 core samples of basaltic trap and Neoarchean basement in the laboratory from a 981 m deep scientific borehole KBH-05, in the Koyna−Warna seismic zone, in order to characterize their thermal and physical properties and present a probable crustal thermal model. The Koyna−Warna, located in the Deccan Volcanic Province (DVP), is globally one of the most prominent Reservoir Triggered Seismicity (RTS) regions. Thermal conductivity, density, and porosity vary widely (1.0-1.7 Wm -1 K -1 , 2400-3000 kg m -3 , 0.2-10%) for the basalt due to their lithological heterogeneities, i.e., massive/amygdaloidal/vesicular. In comparison, the basement, which dominantly consists of gneiss/migmatite gneiss (granodiorite to tonalite to quartz monzodiorite in composition) and amphibolite, have shown a wider range in thermal conductivity (2.2-3.4 Wm -1 K -1 ) but constricted range in density and porosity (2600-2800 kg m -3 , 0.01-0.15%).Based on gamma and sonic logs, as well as radioelements (Th, U, K), the 499 m thick basaltic trap can be divided into two thick layers (325, 174 m) and five sub-layers, which can be correlated with different basaltic formations. Similarly, the underlying basement can also be divided into two layers (93, 384 m). The upper basement layer has two times higher concentrations of Th and U than the lower layer, with heat production of 2.0, 0.8 μWm -3 . Further, the study provides a robust temperature estimate of 165-250 o C at 10 km depth, considerably higher than reported earlier for the DVP, and reveals that in addition to thermal parameters, RTS also plays an important role in the seismogenesis of the region.

Summary A thermal conductivity profile through the upper crustal column is an essential ingredient in any thermal modelling. Granitoid is one of the major constituents of the upper crust in the Archaean cratons. However, granitoid have a wide range in composition and yet, data on their thermal conductivity at elevated temperatures is very limited. At present, a single value is commonly used to characterize the decrease in thermal conductivity with temperature for the upper crust. We are reporting thermal conductivity measured at 25° C, 50° C, and thereafter at 50° C intervals up to 300° C on thirty-four granitoid samples of four compositionally different types. The samples are alkali granite, biotite granite, granodiorite, and metasomatised granodiorite from one of the Archaean cratons of the Indian shield, known as Bundelkhand Craton. Before studying the samples at elevated temperatures, these have been studied for their physical, petrological, and geochemical characteristics. By 300° C, the thermal conductivity decreases on the average by 28 to 31 per cent for metasomatised granodiorite, alkali granite, and biotite granite, and in stark contrast by 16 per cent for granodiorite. Expressing the thermal conductivity variation with temperature as λT = λRT (1 + bT)–1, two distinct temperature coefficient (b) values have been found, 1.1 х 10–3 to 2.2 х 10–3 K–1 for alkali feldspar granite to monzogranite and 0.4 х 10–3 to 1.2 х 10–3 K–1 for granodiorite to tonalite to quartz diorite. One of the implications of this outcome is illustrated by applying these two distinct temperature coefficients for the upper crust for a 1-D generic model with a surface heat flow and appropriate radiogenic heat production of the crustal column in arriving at crustal temperature-depth profiles. The temperature differences at the base of a 40-km crust vary as much as 90° C. Further, the temperature coefficient can be expressed as b = 0.71 x λRT–0.63 for the alkali feldspar granite to monzogranite, whereas b = 0.83 x λRT–1.26 for the granodiorite to tonalite to quartz diorite, which will be useful in determining the temperature coefficient of various types of granitoid from thermal conductivity at room temperature (λRT).

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Summary In spite of the fact that rhyolite rocks constitute a vital part of the key tectonic environments, such as continental rift-arc systems and oceanic islands, the data on the thermal and physical properties are scarce which hinders the exact thermal modelling of these regions. Here, we have investigated the thermal conductivity from room temperature (25 ° C) to elevated temperatures (up to 300 ° C) for eleven massive rhyolite samples, collected from the greenstone belt of the Bundelkhand Craton, central India. The petrographical, geochemical (major oxide and trace elements), and physical (density and porosity) properties have been studied to characterize the samples before measurement of thermal conductivity at elevated temperatures. Geochemical results indicate that these rhyolites are high-K (K2O: 3.57 to 5.35 wt. per cent), calc-alkaline in nature with enriched REE signatures {(La/Yb)N : 9.4 to 22.3, (Gd/Yb)N : 1.2 to 1.9} and are similar to FI-type Archaean rhyolites. The density of these rhyolites depicts a narrow range between 2590 and 2690 kg m−3, with an average of 2637 kg m−3, and negligible porosity. Their thermal conductivity at room temperature varies between 2.5 and 3.3 W m−1 K−1, with an average of 2.8 W m−1 K−1; the decrease in thermal conductivity from room temperature to 300 oC ranges between 16 and 32 per cent, with an average of 23 per cent; and the temperature coefficient of thermal conductivity b, in the expression λT = λRT (1 + bT)−1, varies between 0.7 × 10−3 to 1.7 × 10−3 K−1 with an average of 1.1 × 10−3 K−1. Our study suggests that the massive rhyolites have an almost similar density as their intrusive equivalent like Bundelkhand granitoids, but their thermal properties, such as thermal conductivity at room temperature (λRT), decrease in thermal conductivity with temperatures (Δλ), and the temperature coefficient of thermal conductivity (b), lie between the two extreme variety of the granitoids, that is (i) alkali feldspar granite to monzogranite, and (ii) granodiorite to tonalite to quartz diorite. We suggest that the temperature coefficient of the massive rhyolite can be expressed as b = 0.81 × λRT–1.21, which will be useful in determining the thermal conductivity of such rhyolites at an elevated temperature from their thermal conductivity at room temperature (λRT). Thermal and physical parameters reported for rhyolites will provide an important constraint in various geophysical and thermo-mechanical modelling for the rhyolitic terrains.