Regional geochemistry and continental heat flow: implications for the origin of the South Australian heat flow anomaly
Introduction
The thermal structure of continental interiors is fundamental to their long-term tectonic and geochemical evolution. Our understanding of thermal regimes within the continental crust has been greatly influenced by surface heat flow measurements. These measurements are particularly important because they yield information concerning the thermal structure of the lithosphere and constrain the vertical distribution of heat sources [1], [2], [3]. Further, heat flow data provide a unique insight into the geochemistry of the crust because they constrain the depth integrated abundance of heat-producing elements [4], [5]. In the recent past, considerable effort has been expended towards understanding global heat flow averages, and the source distributions that contribute to them (e.g. [3], [6], [7], [8]). While such global averages are undoubtedly important, their significance should be evaluated with regard to the following points. Firstly, the so-called global heat flow dataset is strongly biased by measurements made in North America, Europe and southern Africa, with the heat flow field from other continental regions virtually unknown. Secondly, given that many important geological processes are temperature dependent, the natural spatial variation in thermal parameters is of greater relevance than global averages. In this regard, regions of elevated heat flow are fundamental to our understanding of the thermal structure of the continental crust.
This paper concerns itself with a region of elevated heat flow in South Australia. Existing heat flow measurements in this region suggest either anomalous mantle activity or that radiogenic crustal sources contribute more than twice what would be expected on the basis of global heat flow averages. In this region, crustal growth occurred mainly from the Palaeoproterozoic through to the early Mesoproterozoic [9]. Heat flow measurements are often of poor quality or display broad scatter, and as with all such measurements there is a need to evaluate their plausibility. A primary purpose of this paper is to determine the validity of these heat flow values in light of inferences about mantle thermal regimes and surface heat production parameters derived from regional geochemical and geophysical datasets. These data suggest that the anomalous heat flow reflects extraordinary concentrations of heat-producing elements in the crust. In Section 6, we briefly explore the origin and implications of this exceptionally enriched crust.
Section snippets
Some preliminary remarks concerning global heat flow
Surface heat flow is a measure of the combined heat flow from the convective mantle, radiogenic heat from the decay of U, Th and K within the lithosphere and transient perturbations associated with tectonic, magmatic, hydrologic and/or climatic activity. Over 10 000 global continental heat flow measurements [7] have been used to constrain the chemical and thermal structure of the lithosphere. However, it should be stressed that 90% of these measurements are from three continents; Europe, North
Geological setting
South Australia is dominated by two Archaean–Proterozoic cratonic terranes separated by the Adelaide Fold Belt (Fig. 3). Unless otherwise stated, all geochronological data are from Drexel et al. [19] and Daly et al. [20]. The western region of South Australia comprises the Gawler Craton, a stable crystalline basement terrane composed of Archaean to Mesoproterozoic magmatic and metasedimentary rocks. The northern and western boundaries of the Gawler Craton are covered by the sedimentary
Surface heat flow in South Australia
Existing heat flow values from 22 different locations within South Australia (Table 1) increase from west to east, with a distinct rise in values evident between the western and eastern Gawler Craton (Fig. 4). The zone of elevated heat flow, which we term the South Australian heat flow anomaly (SAHFA), overlaps the boundary between the eastern Gawler Craton and the Adelaide Fold Belt, including the Stuart Shelf (Fig. 3). Heat flow measurements at the Olympic Dam Cu–U–Au-REE deposit on the
Heat production in the SAHFA
Given that the concentration of heat-producing elements in the SAHFA is anomalously high, it is appropriate to identify the source of the elevated heat production distribution. For this purpose, heat production values for the main Proterozoic lithologies in this region have been compiled (Table 2). Calculated heat production values for metasedimentary units within this transect are consistent with accepted lithological means and show little variation across provinces, suggesting that they do
Discussion
In the previous sections, we have shown that the high heat flow of the SAHFA is largely the result of elevated crustal heat production, with inferences about the seismic velocity of the upper mantle supporting the notion that mantle heat flow is less than 30 mW m−2. This suggests that the crust in this region contributes on average 60–75 mW m−2 to the surface heat flow, which is 2–3 times what would be expected on the basis of terranes of comparable or younger age in other continents. Such a
Acknowledgements
We thank Primary Industries of South Australia (PIRSA) for access to the South Australia Geoscientific GIS Dataset and Kathy Stewart for many useful discussions on the geochemistry of Proterozoic granites in South Australia. Hans Jurgen Förster, Simon Turner and Ross Taylor are thanked for helpful reviews.[AH]
References (46)
- et al.
Thermal structure, thickness and composition of continental lithosphere
Chem. Geol.
(1998) - et al.
Heat production in an Archean crustal profile and implications for heat flow and mobilization of heat-producing elements
Earth Planet. Sci. Lett.
(1987) - et al.
Thermal regime of the continental lithosphere
J. Geodyn.
(1984) - et al.
Diversion of heat by Archean cratons: a model for southern Africa
Earth Planet. Sci. Lett.
(1987) - et al.
Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary Block, South Australia, and implications for mineral exploration
Lithos
(1996) - et al.
The thermal structure and thickness of continental roots
Lithos
(1999) - et al.
High geothermal gradient metamorphism during thermal subsidence
Earth Planet. Sci. Lett.
(1998) - et al.
Sm–Nd evidence for the provenance of sediments from the Adelaide Fold Belt and southeastern Australia with implications for episodic crustal addition
Geochim. Cosmochim. Acta
(1993) - et al.
On the variation of continental heat flow with age and the thermal evolution of the continents
J. Geophys. Res.
(1980) - P. Morgan, The thermal structure and thermal evolution of the continental lithosphere, in: H.N. Pollack, V.R. Murthy...
Heat flow and the chemical composition of continental crust
J. Geol.
The heat flow through oceanic and continental crust and the heat loss of the earth
Rev. Geophys. Space Phys.
Heat flow from the earth’s interior: Analysis of the global data set
Rev. Geophys.
A global analysis of heat flow from Precambrian terranes: implications for the thermal structure of Archaean and Proterozoic lithosphere
J. Geophys. Res.
Sm–Nd constraints on the evolution of Precambrian crust in the Australian continent
AGU Geodyn. Ser.
The Vredefort radioelement profile extended to supracrustal strata at Carletonville, with implications for continental heat flow
J. Geophys. Res.
Heat production and thermal conductivity of rocks from the Pikwitonei–Sachigo continental cross section, central Manitoba: implications for the thermal structure of Archean crust
Can. J. Earth Sci.
The vertical distribution of radiogenic heat production in the Precambrian crust of Norway and Sweden: geothermal implications
Geophys. Res. Lett.
Distribution of heat-producing elements in the upper and middle crust of southern and west central Arizona: evidence from core complexes
J. Geophys. Res.
Continental heat flow–age relationships
EOS Trans. AGU
Cited by (136)
Fluid-assisted intra-plate seismicity at the edge of the Gawler Craton, South Australia
2024, Physics of the Earth and Planetary InteriorsRelationship between radiogenic heat production in granitic rocks and emplacement age
2023, Energy Geoscience
- 1
Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3010, Australia.