Heat flow researches in India: results and perspectives

Abstract

Data on geothermal gradients representative of regional crustal conditions have been aquired through temperature measurements in 183 boreholes and 2 deep mines. Combing with thermal conductivity data on rocks over relevant depth sections, heat flow could be evaluated at many localities in India covering Precambrian provinces, Gondwana basins, and the Tertiary Cambay basin. Certain general characteristics emerge. The Archaean Dharwar greenstone-granite-gneiss province (DP) and the adjoining Southern Granulite province (SGP) have heat flow ranging from 25 to 50 mWm^-2. This low heat flow level is found to hold over the Deccan Volcanic Province in the region south of the Son-Narmada-Tapti lineament zone, and in the southwestern fringes of the Cuddapah basin, both of which have the DP basement. The Proterozoic segments covered so far have revealed heat flow higher than 50 mW m^-2; 59 to 63 along the Singbhum Thrust Zone, 51 to 62 in the Bastar craton, and 56 to 96 in the Aravalli province. The Gondwana basins have a generally high but variable heat flow, 46 to 107 mW m^-2. The Tertiary Cambay basin has high heat flow of 75 to 96 mW nv2 in the northern part, and a lower heat flow, 55-67 mW m^-2, in the southern part. Crustal thermal structure has been derived for the DP and the northern block (NB) of the SGP (i.e., north of the Palghat-Cauvery lineament) based on heat flow and models of crustal heat production envisaged using radiogenic heat production data of major crustal litho-units. Mantle heat flow estimates for the DP are in the range 11 to 19 mWm^-2, similar to other Precambrian provinces of the world studied so far. It is significantly higher, 25-30 mW m^-2, in the NB of the SGP, in conformity with crustal and sub-crustal velocity structure emerging from tomographic studies and an underlying LILE-enriched mantle inferred from geochemical and isotopic data. Temperature estimates at the Moho range from 285° to 410°C in the DP and 580° to 660°C in the NB of the SGP. Heat flow determined from a specially drilled borehole at the site of the M_W 6.2 1993 Latur earthquake (Killari), is 43 mW m^-2. Estimated crustal temperatures imply a brittle crust up to a depth of ~30 km. The estimated centroid depth is low, 2.6 km. An event with magnitude M_W>6 occurring within the upper part of a relatively 'cold'.