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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS017 Oral Presentation 2369
New heat flow data in the Indian Shield: implications for mantle
heterogeneity beneath different terranes
Dr. Sukanta Roy
National Geophysical Research Institute, India Tectonophysics IASPEI
R. Srinivasan
Thirty-five new heat flow values have been determined in the greenstone-granite-gneiss and gneissgranulite
provinces of the southern Indian shield. The new sites include 11 in the largely tonalitic
Western Dharwar Craton (WDC) where the rocks are in the age range of 3.3 to 2.6 Ga, 9 in the 2.5 Ga
Closepet Granite (CG) batholith, and 15 in the Late Archaean and Pan-African gneiss-granulite terrain.
Together with the data obtained earlier from measurements at 42 sites in the 2.6 to 2.5 Ga largely
granodioritic Eastern Dharwar Craton (EDC), 4 sites in the WDC, and 21 sites in the gneiss-granulite
province to their south, the present data set contributes to improved characterization of the thermal
state of the southern Indian Precambrian 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 previously reported 28-45 mW m-2, heat flow range for the gneiss-granulite terrain of south
India, 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 batholithic unit can be explained by variations in
radiogenic heat production at different crustal levels. Heat production generally decreases with depth in
plutons although not necessarily in a systematic way. (iv) Mantle heat flow estimates in the Dharwar
greenstone-granite gneiss province vary as follows: 11 mW m-2 beneath the region of the oldest,
unmodified tonalitic crust of the WDC; 12 -14 mW m-2 in the Closepet Granite terrain, and 12-19 mW
m-2 in the EDC. While these estimates of mantle heat flow are consistent with the range observed in
other Precambrian provinces (e.g., Canadian, African and Baltic shields), the region beneath the Late
Archaean granulites south of the EDC and WDC but north of the Palghat-Cauvery lineament zone
exhibits distinctly higher estimates in the range 23-32 mW m-2. Crustal thermal models derived on the
basis of heat flow and heat production datasets demonstrate that the variations in heat flow within the
greenstone-granite-gneiss province of the Dharwar craton are explained by variations in heat production
of upper crustal rocks. On the other hand, the granulitic terrain of the Dharwar craton exhibits a similar
heat flow regime but a distinctly lower heat production in the upper crust relative to the greenstonegranite-gneiss
terrain, which results in a higher component of mantle heat flow. Contrary to previous
hypotheses, the Precambrian terrain of southern India cannot be represented by a single thermal model
covering all the geological sub-provinces, but represents a mosaic of terrains with varying infra- and
sub-crustal thermal regimes.
Keywords: heat flow, heat production, crustal thermal structure