Regional geochemistry and continental heat flow: implications for the origin of the South Australian heat flow anomaly

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Abstract

Existing measurements from South Australia define a broad (>250 km wide) zone of anomalously high surface heat flow (92±10 mW m−2). This zone is centred on the western margin of the Adelaide Fold Belt (Neoproterozoic to early Phanerozoic cover floored by Palaeoproterozoic to Mesoproterozoic basement), where it borders the eastern Gawler Craton and Stuart Shelf (Palaeoproterozoic–Mesoproterozoic). To the west, in the western Gawler Craton (Archaean to Palaeoproterozoic), heat flow averages ∼54 mW m−2 while to the east in the Willyama Inliers (Palaeoproterozoic) heat flow averages ∼75 mW m−2. We use a regional geochemical dataset comprising >2500 analyses to show that the anomalous heat flow zone correlates with exceptional surface heat production values, mainly hosted in Palaeoproterozoic to Mesoproterozoic granites. The median heat production of Precambrian ‘basement’ rocks increases from <3 μW m−3 west of the anomalous zone to ∼6 μW m−3 within the anomalous zone. In the highest known part of the heat flow anomaly, Mesoproterozoic gneisses and granites of the Mount Painter Province in the northern Adelaide Fold Belt yield an area-integrated mean heat production of 9.9 μW m−3. These data suggest that the anomalous heat flow reflects an unusual enrichment in U and Th in this part of the Proterozoic crust, with the total complement of these elements some 2–3 times greater than would be expected for Proterozoic crust on the basis of the global heat flow database. This extraordinary enrichment has played an important role in modulating the thermal regime of the crust in this region, and particularly its response to tectonic activity.

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]

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    Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3010, Australia.

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