Heat flow and heat generation in the Archaean Dharwar cratons and implications for the Southern Indian Shield geotherm and lithospheric thickness

Abstract

We report terrestrial heat flow density measurements from 11 new sites in the Archaean Eastern and Western Dharwar cratons of the Southern Indian Shield. The new values, together with already published data, have yielded a mean heat flow of 40 ± 3.4 (s.d.) mW m −2 ( n = 11) for the Eastern Dharwar craton, which is a predominantly gneiss-granite terrain. The mean heat flow in the Archaean supracrustal belts of Western Dharwar craton, which is a granite-greenstone terrain, is 31 ± 4.1 (s.d.) mW m −2 ( n = 4). The observed difference in surface heat flow between the two cratons is ascribed to differences in radiogenic heat production in upper crustal layers, inferred to be related to different crust-forming processes. It has been argued that low heat production appears to be an inherent characteristic of the crust similar to that of the Western Dharwar craton, which evolved during the early Archaean and suffered depletion of the lithophile elements of its lower crust prior to the late Archaean. Radiogenic heat production measurements were carried out on borehole core samples for one heat flow site (Sortur), and data have been compiled from the literature on three other sites and on a variety of rocks of the Southern Indian Shield. Heat flow and radiogenic heat production for Sortur, when combined with the published data, define a linear relation ( r = 0.97) with a slope of 11.5 km and an intercept of 23 mw m −2. The thickness of the thermal lithosphere beneath the Dharwar cratons is over 200 km, thereby indicating, beneath the cratons, the existence of deep continental roots-a concept which is also supported by teleseismic P-wave travel time data. The geothermal data has clearly demonstrated that the Archaean Dharwar cratons of the Indian landmass are associated with low mean surface heat flow (37.7 mW m −2), reduced heat flow, Moho heat flux (12–17 mW m −2), and a cool and thick lithosphere—all characteristics of many Archaean-early Proterozoic terrains of other shields. It is inferred that the thermal structure of the South Indian lithosphere is identical with that of other cratons.