Crustal thermal structure of the Precambrian Terranes: An example from Indian shield

Speaker: Labani Ray
“In Precambrian provinces, perturbations to the near-surface temperature regime, particularly the transient, become minimal, thereby allowing reliable determinations of conductive heat flow. In such cases, the heat flow would be covariant only with the radiogenic heat production of the crustal column unlike the case in active areas where multiple heat components corresponding to several tectonic processes exist. If the crustal contribution, i.e. contribution due to the K, U and Th contents of the crustal rocks is quantified, it would enable in deducing the heat flow from the base of the crust, i.e. mantle heat flow. The surface heat flow pattern, the radiogenic heat production in the crust, the mantle heat flow, and the crustal temperatures, together, constitute the crustal thermal structure of the terrane.

In order to deduce heat flow and crustal thermal structure of the Indian shield, we have acquired heat flow data from measurements in 200 boreholes and heat production data from several thousands of sites. Heat flow data show low (25-50 mWm2), high (50-110 mWm2) as well as wide variations in different geological provinces.

Crustal thermal model of the different Precambrian provinces of the southern Indian shield, which consist of Dharwar greenstone-granite-gneiss province and gneiss-granulite province, have been attempted on the basis of dense heat flow coverage, extensive heat production data (for the rock types likely constituting the crust), and structural constraints (from geophysical and geological studies). Although the mantle heat flow of the Dharwar greenstone-granite-gneiss province (11-16 mW m-2) is similar to most Precambrian provinces of the world but it is significantly higher for the northern part of the gneiss-granulite province (25-30 mW m-2). The high mantle heat flow in a part of gneiss-granulite province can be ascribed to higher heat production in the underlying mantle, because of strong support to LILE enriched mantle from geochemical and isotopic studies. Crustal temperatures have been estimated by using crustal models for the distribution for the radiogenic heat production and using variation of the thermal conductivity with depth. Crustal temperatures and Moho temperature in gneiss-granulite province (550-660 C) are consistently higher than that of Dharwar greenstone-granite-gneiss province (285-410 C).

Also an attempt have been made to measure thermal diffusivity of the lower crustal rocks e.g., granulites at elevated temperatures (upto 500 C) in order to refine crustal thermal structure of the Precambrian terranes.”