High geothermal gradient metamorphism during thermal subsidence
Introduction
The thermal energy that drives metamorphism is ultimately related to the processes of heat loss from the interior of the earth. As such, metamorphism must be seen as a consequence of the conductive and advective heat transfer phenomena associated with lithospheric processes such as deformation, erosion and magma transport. Metamorphism at gradients in excess of about 40°C/km results in intermediate- to high-temperature, low-pressure metamorphism in the middle crust. The physical causes of such high geothermal gradient metamorphism (HGGM) have received considerable attention in recent years with much of the emphasis on the role of transient advective processes such as magma ascent, or abnormally high heat flows from the mantle (e.g. 1, 2, 3, 4, 5, 6, 7). Because the generation of magmas in the deep crust and upper mantle is associated with anomalously steep thermal gradients, it is clear that HGGM metamorphism in the shallower parts of the lithosphere may be produced by a combination of advective processes associated with the segregation and ascent of magmas within the lithosphere and enhanced heat flow from the mantle. Indeed, the spatial correlation between metamorphism and magmatism in many high temperature metamorphic belts has been used to support hypothesis 1, 6, 7. Rapid tectonic denudation of deep crust provides a further mechanism for generating HGGM, albeit transiently, and has been proposed as a mechanism for the production of some localised high-temperature metamorphic terranes in modern orogenic belts such as the Nanga Parbat massif 8, 9.
While this notion of transient advection remains the principal paradigm for HGGM, several recent thermochronologic studies have raised the possibility that other less transient mechanisms may play a significant or even dominant role. For example, evidence for extended periods of HGGM, with the elevated geothermal regimes apparently lasting many 10s of millions of years (e.g. 10, 11, 12), precludes a predominant advective mode. In several Australian HGGM terranes with prima facie evidence for magmatic advection of heat (in as much as metamorphism is spatially associated with granitic batholiths), recent geochronological studies have shown that the HGGM metamorphism postdates the emplacement of the batholiths by more than 100 Myr 12, 13, 14. These results seem to preclude metamorphism being driven primarily by magmatic activity or rapid tectonic denudation (based on the retrograde P–T paths), raising the questions about the physical cause(s) of the high geothermal gradients associated with the HGGM metamorphism. A specific difficulty relating to HGGM metamorphism in the absence of magmatism (and rapid denudation) is how steep geothermal gradients are sustained in the mid-upper crust, while temperatures in the deep crust are sufficiently low to inhibit large scale melting.
In the absence of advective heat transfer processes, metamorphism must represent the conductive response to burial of rocks with the elevated geotherms associated with HGGM reflecting either unusually elevated heat production in the crust, and/or high heat flows from the mantle. The role of high mantle heat flows has been addressed by a number of authors 2, 3. However, as alluded to above it is difficult to generate conductive thermal regimes appropriate to HGGM at mid-crustal levels by enhancing mantle heat flows without generating significant quantities of magma in the deeper crust, with the likely result that the advection of the generated magma would contribute to the observed HGGM signature. In the context of HGGM, the role of anomalous crustal heat production [15] remains poorly understood because of relatively entrenched, somewhat conservative views about the magnitude of crustal heat production in some instances and the way in which it is distributed within the lithosphere.
Sandiford and Hand [16] have shown that heat production distributions consistent with modern heat flows and measured surface heat production in a number of Australian Proterozoic HGGM terranes, can generate the conditions required for HGGM in the mid-upper crust without necessarily generating significant quantities of melt in the deep crust (provided mantle heat flows are low). Sandiford and Hand's [16] approach was essentially parametric in as much as they did not address the geological setting in which such conditions are likely to prevail. In this contribution we develop a coupled thermal-isostatic model to show that such conditions will naturally develop as a consequence of burial of a radioactive sequence (such as a granitic basement complex) during thermal subsidence following rifting. The motivation for this analysis is provided by the geological evolution of the Mount Painter province in the northern Flinders Ranges, South Australia, which we discuss in the context of this scenario. We believe that the geology of the Mount Painter region may have bearing on the origin of HGGM metamorphism in other settings in the Proterozoic of Australia, and we briefly discuss aspects of selected Australian Proterozoic terrains that may be resolved by this new view of HGGM metamorphism.
Because thermal subsidence following rifting is normally associated with crustal cooling (Fig. 1a–c), the suggestion that HGGM may result from thermal subsidence is somewhat counter-intuitive. Consequently, we spend some time developing an insight into the basic physics of the `HGGM during thermal subsidence' scenario. We begin with a short summary of the physical arguments for the thermal subsidence scenario for HGGM, before outlining a simple model of heat production that allows quantification of the scenario.
Section snippets
HGGM due to high crustal heat production and low mantle heat flow
The main problem presented by HGGM metamorphism in the absence of advective heat transfer is the question of how high geothermal gradients are sustained in the upper crust without inducing melting in the deep crust. A useful starting point for understanding this problem is provided by an analysis of steady-state thermal structure of the lithosphere. We begin by making the simplifying assumption that the lithosphere is laterally uniform and by ignoring compositional and temperature dependence on
HGGM metamorphism and thermal subsidence
While Sandiford and Hand [16] demonstrated the plausibility of HGGM metamorphism using parameter ranges consistent with constraints imposed by known surface heat flow–heat production data, they did not address the question of geologic setting that enables establishment of the appropriate heat production distributions and mantle heat flows. One way of achieving the requirements of elevated heat production at mid-crustal levels accompanied by low mantle heat flows is to bury an enriched sequence,
2-D variation in heat sources
An implicit assumption in the 1-D modelling presented in the previous section is that the distribution of heat sources at any depth is laterally invariant, at least for length scales large in comparison to the thickness of the lithosphere. This is a geologically implausible scenario for two reasons. Firstly, the extreme concentrations of heat sources needed to generate HGGM during thermal subsidence are only likely to occur as localised enrichments. Secondly (and perhaps more importantly),
Application to the Mount Painter province
In the previous sections we have shown that the burial of an anomalously radioactive crust during thermal subsidence allows the possibility of HGGM metamorphism with the provisos that: (1) the total contribution of the crust is greater than about 70 mW m−2, and (2) the characteristic horizontal length scale of the heat production anomaly is greater than about 50 km. In this section we describe the metamorphism in the Mount Painter province; a terrane that seems to fulfil the above criteria and
Discussion and conclusions
In the previous sections we have demonstrated that providing the pre-existing crust is sufficiently enriched in heat producing elements, then HGGM in the upper-mid crust will occur as a consequence of thermal subsidence. The important role played by subsidence is twofold. Firstly, because thermal subsidence promotes burial of the `hot' crust, it naturally increases the depth extent of the high geothermal gradients, potentially inducing very significant temperature increases during progressive
Acknowledgements
We thank Narelle Neumann for the compilation of the geochemical data listed in Table 2, and Mines and Energy South Australia for providing the geochemical and radiometric survey data summarised in Fig. 8, Fig. 9 and Table 1. We thank Geoff Fraser, Rebecca Jamieson and Page Chamberlain for their reviews of the manuscript. [CL]
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