Labani Ray

Geothermal investigations in the Upper Vindhyan sedimentary rocks of Shivpuri area, central India

Heat flow has been determined by combining temperature measurements in 7 boreholes with thermal conductivity measurements in the Upper Vindhyan sedimentary rocks of Shivpuri area, central India. The boreholes are distributed at 5 sites within an area of 15 × 10 km2; their depths range from 174 to 268 m. Geothermal gradients estimated from borehole temperature profiles vary from 8.0–12.7 mK m−1 in the sandstone-rich formations to 25.5–27.5 mK m−1 in the shale-rich formations, consistent with the contrast in thermal conductivities of the two rock types. Heat flow in the area ranges between 45 and 61 mW m−2, with a mean of 52±6 mW m−2. The heat flow values are similar to the >50 mW m−2 heat flow observed in other parts of the northern Indian shield. The heat flow determinations represent the steady-state heat flow because, the thermal transients associated with the initial rifting, convergence and sedimentation in the basin as well as the more recent Deccan volcanism that affected the region to the south of the basin would have decayed, and therefore, the heat flow is in equilibrium with the radiogenic heat production of the crust and the heat flow from the mantle. The present study reports the heat flow measurements from the western part of the Vindhyan basin and provides heat flow information for the Bundhelkhand craton for the first time. Radioelement (Th, U and K) abundances have been measured both in the sedimentary rocks exposed in the area as well as in the underlying basement granite-gneiss of Bundelkhand massif exposed in the adjacent area. Radioactive heat production, estimated from those abundances, indicate mean values of 0.3 μW m−3 for sandstone with inter-bands of shale and siltstone, 0.25 μW m−3 for sandstone with inter-bands siltstone, 0.6 μW m−3 for quartzose sandstone, and 2.7 μW m−3 for the basement granitoids. With a total sedimentary thickness not exceeding a few hundred metres in the area, the heat production of the sedimentary cover would be insignificant. The radioactive heat contribution from the basement granitoids in the upper crust is expected to be large, and together with the heat flow component from the mantle, would control the crustal thermal structure in the region.

Heat flow, heat production, and crustal temperatures in the Archaean Bundelkhand craton, north‐central India: Implications for thermal regime beneath the Indian shield

Heat flow and heat production datasets constrain the crustal thermal structure in the 2.5-3.5 Ga Bundelkhand craton, the oldest cratonic core in northern Indian shield, for the first time and allow comparisons with the southern Indian shield. Temperature measurements carried out in 10 boreholes at five sites in the craton, combined with systematic thermal conductivity measurements on major rock types yield low heat flow in the range 32-41 mW m-2, which is distinct from the generally high heat flow reported from other parts of the northern Indian shield. Radioelemental measurements on 243 samples of drill cores and outcrops reveal both large variability and high average heat production for the Neo-Archaean to Palaeo-Proterozoic granites (4.0±2.1 (SD) μWm-3) relative to the Meso-Archaean TTG gneisses (2.0±1.0 (SD) μWm-3). On the basis of new heat flow and heat production datasets combined with available geological and geophysical information, a set of steady-state, heat flow - crustal heat production models representative of varying crustal scenarios in the craton are envisaged. Mantle heat flow and Moho temperatures are found to be in the range 12-22 mW m-2 and 290-420 oC respectively, not much different from those reported for the similar age Dharwar craton in southern India. This study reveals similar mantle thermal regimes across the northern and southern parts of the Indian shield, in spite of varying surface heat flow regimes, implying that much of the intra-province and inter-province variations in the Indian shield are explained by variations in upper crustal heat production.