Heat flow in the Indian Peninsula—its geological and geophysical implications

Abstract

The distribution of surface heat flow in the various geological units of the Indian Peninsula is presented and discussed in the light of other related geodata. The heat flow data show a large variation (26 to 107 mW m −2), but indicate, in general, a thermal regime characteristic of stable continental areas. The Dharwar schist belts—greenstone belts, and greenstone-like geosynclinal piles—arc characterised by low heat flow. It appears very probable; (a) that favourable conditions for the generation of the Earth's crust and the melting near the surface developed at the very early stages of the Earth's evolution; and (b) that true greenstone belts, which represent the relics of the primitive basic ultrabasic crust, may have been more or less continuous once. It is our contention that their association with low surface heat flow is primarily due to their initial evolution and growth. The low heat flow values in green stone belts show a general increase in Archaean-Proterozoic granitic and gneisic terrains. The average heat flow in Archaean areas is significantly lower than in Proterozoic parts of the Indian Shield. In general, the average heat flow shows an inverse relation with the age of Precambrian orogeny. Further, the Proterozoic geosynclinal belts, in which sedimentation and mobility continued up to Upper Proterozoic, have been found to mark the highest heat flow areas of Precambrian India. Surprisingly, the Deccan Traps, the Cretaceous-Paleocene basic effusives, are associated with low heat flow values (40 mW m −2). The observed high heat flow in the northern part of a basin, in which a full sequence of Tertiary to Recent sediments overlie the Deccan Traps, has been ascribed to a shallow basic crustal Miocene-Pliocene intrusive body. The Gondwanas, which occur in intracratonic small, isolated, elongate trough-like, and semi-elliptical depressions, show a contrasting geothermal character with many zones of high surface heat flow, for which there is no clear explanation at present. Most of the Indian Peninsula is comprised of three cratons-the Dharwar, the Aravallis, and the Singhbhum. The Dharwar craton appears to be associated with the lowest surface heat flow, and the heat flow from the mantle and the lower crust is q r≈ 23 mW m −2 . This is followed by q r≈ 30 mW m −2 for the Aravalli craton and about 38 mW m −2 for the Singhbhum craton. Thus for the same surface radioactivity and heat flow, crustal temperatures are lowest in the Dharwar craton. For this craton, the temperatures at the base of the crust are similar to those in the western Australian shield, and in the Superior Province of the Canadian shield. We further suggest that U, Th and K in the top radioactive layer, probably do not follow the same concentration-depth pattern i.e., the characteristic depth, D, for these heat producing radioactive elements is most likely different and is the main cause of a wide scatter in the values of q r for various heat flow-heat production pairs within a heat flow province. We demonstrate that it is not worthwhile to study the correlation of surface heat flow to the gravity anomaly in the Indian Peninsula without removing the effects of the pronounced Indian Ocean gravity low from the gravity anomalies. Surface heat flow, q s , and crustal thickness. Zm, as obtained from DSS and earthquake data, show a positive (slightly exponential) correlation, which is compatible with crustal properties and dynamics. It is demonstrated that the correlation between q s and Zm in Precambrian shields can be positive or negative or even negligible, depending upon their past geological history.