High mantle heat flow in a Precambrian granulite province: Evidence from southern India

Abstract

[1] Twelve new heat flow values determined at nine sites and heat production estimated from radioelemental measurements at 330 sites in the southern granulite province (SGP) bring out contrasting crustal and subcrustal thermal characteristics between the SGP and the adjacent Archaean Dharwar greenstone-granite-gneiss province (DP) in south India. A twolayer granulitic crust of Late Archaean charnockites and gneisses characterizes the northern block (NB), north of the Palghat-Cauvery lineament (PCL). The heat production of the upper, 7-10 km thick, metasomatized granulitic layer ranges between 0.2 and 0.75 mW m À3 (mean 0.5 ± 0.3 (SD) mW m À3 ). This layer overlies radioelement-depleted granulites characterized by very low heat production ranging from 0.14 to 0.2 mW m À3 (mean 0.16 ± 0.07(SD) mW m À3). In a large sector of the NB, erosion of the upper metasomatized granulite layer has laid bare the depleted granulitic rocks, which represent one of the lowest heat-producing crustal sections. The mean heat flow in the NB is 36 ± 4 mW m À2 (N = 10). The southern block (SB), south of PCL, in contrast to the NB, comprises complexly interlayered charnockites, gneisses, granites, khondalites, and leptynites, which have variable and much higher levels of heat production ranging between 1.11 and 2.63 mW m À3 . The heat flow in the SB is 47 ± 8 mW m À2 (N = 3). Overall, the range of heat flow values in the SGP is within the range for the DP. Mantle heat flow in the NB, both from the lowest heat-producing sector and other areas, is deduced in the light of heat production and heat flow data, at 23-32 mW m À2 , whose values are distinctly higher than 11-16 mW m À2for the adjacent DP. The higher mantle heat flow in the NB appears to be a consequence of higher heat production in the subjacent mantle.

Tracking the thermal properties of the lower continental crust: Measured versus calculated thermal conductivity of high-grade metamorphic rocks (Southern Granulite Province, India)

“…Previous comprehensive TC studies of metamorphic rocks from the Southern Granulite Province, India (Ray, 2002;Ray et al, 2003;Roy et al, 2003Roy et al, , 2007 (Nyblade et al, 1990), which is in close correspondence with the respective data from this study. Two enderbite samples from Fennoscandia studied by Kukkonen et al (1999) yielded TC values of 2.1 and 2.3 W m 1 K 1 , respectively, which are considerably lower than those presented here for this rock type.…”

Fractal behavior in continental crustal heat production

“…Significant discrepancy can be seen between the observed and calculated data sets (Table 3) indicating that exponential model does not fit to the observed data and provides much lower values at similar depths. Similar inferences can also be drawn, if we consider the heat production of 1.8, 1.02 and 0.18 µW m −3 for the upper, middle and lower continental crust, respectively as recommended by Rudnick and Fountain (1995), and deduce the comparative values using exponential model and taking the initial upper crustal heat production of 1.8 µW m −3 .…”

“…Heat production vs. depth distribution has been studied in detail for the Closepet granitic batholithic region (Senthil Kumar and Reddy, 2004) situated in the central part of the craton which itself exposes almost 12 km deep section corresponding to palaeo-depth between 8 and 20 km. To this data, we have added, the recently acquired heat production data pertaining to sub-surface mid-crustal amphibolite-granulite facies rocks from Killari region (Maharashtra, India) belonging to similar terrain, and lower crustal granulitic heat production data from the Southern granulite terrain of south India (Ray et al, 2003).…”

Insights into the Crustal Structure and Geodynamic Evolution of the Southern Granulite Terrain, India, from Isostatic Considerations

“…Based on the modeling of receiver functions (computed from the IRIS broadband station in Sri Lanka), PATHAK et al (2006) suggest a hotter upper mantle beneath the region. The present day mantle heat flow in the SGT (23-32 mW m -2 ) is indeed distinctly higher than that of the adjacent Dharwar craton (11-16 mW m -2 ;RAY et al, 2003). Under isostatic conditions, heating of the lithosphere results in a further decrease in velocity/density through thermal expansion and apparently indicates a lighter upper mantle beneath the region.…”