New Heat Flow Data in the Indian Shield

Thirty-five new heat flow values have been determined for the Indian shield. The new sites include 11 in Mid Archaean largely tonalitic Western Dharwar Craton (WDC), 9 in the Closepet Granite (CG) batholith, and 15 in the gneiss-granulite terrain of southern India. Together with the data set obtained earlier from the Late Archaean largely granodioritic Eastern Dharwar Craton (EDC) and the granulite terrain to the south of it, the present data set contributes to a complete characterization of the thermal state of southern Indian crust. Results emerging from the study are as follows: (i) the WDC is characterized by low heat flow (range 29-32 mW m-2) relative to the EDC (range 25-51 mW m-2). (ii) The previous heat flow range for the gneiss- granulite terrain of south India, 28-45 mW m-2, stands revised in the light of new heat flow value of 58 mW m-2 determined in the Kerala Khondalite Belt. (iii) Measurements at two different crustal levels within a single pluton, the Closepet Granite, demonstrate that heat flow variations in a tectonic province can be explained by variations in radiogenic heat production of rocks constituting the upper crust. Heat production generally decreases with depth in plutons although not necessarily in a systematic way. (iv) The new data set reveals differences in crustal thermal structure between individual geologic sub-provinces comprising the southern Indian Precambrian terrain. Mantle heat flow estimates vary from ~11 mW m-2 beneath the region of the oldest, undeformed tonalitic crust of the WDC to ~23 mW m-2 beneath the region of the Late Archaean post-accretion granulites. The mantle heat flow range in the Closepet granite is 12-14 mW m-2, which is consistent with previous estimates from the EDC. In conclusion, the variations in heat flow within the greenstone-granite-gneiss terrains of the Dhawar craton are explained by variations in heat production of upper crustal rocks. However, the granulitic terrain of the Dharwar craton exhibits a higher component of mantle heat flow relative to the greenstone-granite-gneiss terrain, in addition to significant differences in heat production of upper crustal rocks. Contrary to previous hypotheses, the Precambrian terrain of southern India cannot be represented by a single model covering all the geological sub-provinces, but represents a mosaic of terrains with varying subcrustal thermal regimes.