Low thermal Peclet number intraplate orogeny in central Australia

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Abstract

The late Phanerozoic Alice Springs Orogen in central Australia is an archetypal intraplate orogen characterised by a dense, granulitic core exhumed from beneath a carapace comprising a highly radiogenic granitic mid-upper crust and sediments deposited in a shallow intracratonic basin. Exhumation occurred in large part along a crustal penetrative fault system, the Redbank Shear Zone, producing one of the largest gravity anomalies (∼150 mgal) known from the continental interiors. The lithospheric strength implied by the preservation of this anomaly for more than 300 Myr raises the intriguing conundrum of what localised the intraplate deformation in the first place. Available biostratigraphic and thermochronologic data imply bulk convergence rates of less than 1 mm/yr for the orogen as a whole, several orders of magnitude lower than typical of plate margin orogens. The thermal and mechanical evolution of intraplate orogens deformed at such low thermal Peclet numbers differs in fundamental ways from plate margin orogens. In particular, at such low thermal Peclet numbers the conductive response to exhumation of heat sources cools the mid to deep crust during progressive orogenic activity. This is consistent with the hypothesis that the density structure and associated gravity anomalies may have been locked-in by virtue of the strength acquired during the orogenic process provided that the lithospheric strength changes associated with a reduction in average crustal temperature of 20–30°C are of the same order as the forces that drive intraplate deformation.

Introduction

Plate margin orogens typically evolve at rates commensurate with the subduction of oceanic lithosphere attached to one of the two interacting plates. Since subduction forms an important component of the natural convective pattern in the Earth’s interior it is necessarily characterised by velocities (greater than about 1 cm/yr) that prohibit significant conductive heating of the subducting slab during its descent into the deeper mantle. Indeed, the thermal density defect of the subducting slab provides a principal source of stress driving mantle flow and the motion of surface plates [1]. The connection between plate margin orogens and subduction implies that they also typically evolve at high thermal Peclet numbers (PeT>10, Fig. 1), with their thermal evolution dominated by the advection of material through the orogen [2]. Material advection through orogenic belts tends to cool the deeper parts of the lithosphere, where the flow is dominantly downwards (mantle downwelling), and heat the upper parts of the orogen, where flow is typically towards the surface due to erosion [2]. When flow in the crust stagnates, as happens when the advection of the buoyant crustal material into the orogen exceeds its removal by erosion, significant heating accrues from the accumulation of a thickened radiogenic crust [3]. These factors tend to promote crustal heating and weakening with respect to typical continental lithosphere, making plate margin orogens susceptible to tectonic collapse once the forces driving orogenesis are relaxed [4].

Unlike most plate boundary settings, orogens that form in intraplate settings in response to transmission of stress from distant plate boundaries are not required to deform at rates commensurate with subduction. For very low convergence rates (i.e. low PeT<1) the thermal effects of material advection in the crust are subordinate to conductive heat loss (Fig. 1). Moreover, low convergence rates reduce the possibility of material accumulation in the orogen implying limited potential for the development of a thickened radiogenic crust. The thermal evolution of this kind of small, low-PeT orogen will be dominated by the conductive response to the redistribution of radiogenic heat sources induced by the material flux through the orogen. In particular, the slow exhumation of radiogenic crust along structures that root beneath the heat producing parts of the crust must lead to crustal-scale cooling, allowing the possibility of progressive orogenic strengthening. Such low-PeT orogens have limited potential for collapse once the driving forces for convergence relax and should be capable of preserving non isostatically balanced density structures formed early in orogenic construction.

The Alice Springs Orogen (ASO) in Central Australia (Fig. 2, Fig. 3) is an archetypal intraplate orogen formed in the late Phanerozoic [5], [6]. One of the most intriguing aspects of the ASO relates to its gravity signature ([7], Fig. 3a) that includes some of the largest anomalies (∼150 mgal) known from the continental interiors. These extraordinary anomalies clearly relate to the orogenic architecture developed during the construction of the orogen [5], and their long-term (>300 Myr) preservation implies virtually no readjustment of the orogenic architecture since the end of convergence, consistent with an exceptionally strong crust. Curiously, this raises a conundrum of why intraplate deformation was localised in such a strong part of the continent. One possibility is that tectonic stresses were greatly amplified in the central Australian region, compared to surrounding regions, to the extent that they were capable of deforming relatively rigid lithosphere. In this case the principal source of stress must almost certainly relate to a dynamic mantle process directly beneath central Australia, rather than distant plate boundary sources. Arguing against this hypothesis is the fact that Alice Springs aged deformation is now known to have occurred over vast regions of the Australian continent ([8]; Fig. 3). An alternative possibility is that the deformation was localised in a relatively weak region that, by virtue of the orogenic process, has radically changed its mechanical properties [9]. The proposed mechanism relates to the way deformation and associated denudation has redistributed crustal radiogenic heat sources, thereby leading to changes in lithospheric thermal regimes. This mechanism is only likely to provide a viable mechanism for locking-in the extraordinary gravity anomalies if the thermal structure of the orogen was able to adapt to the redistribution of heat sources during the deformation [10]; as would apply if it were deformed at a low PeT (PeT<1). This paper reviews evidence for the time-integrated deformation rates associated with the ASO, suggesting that it did indeed evolve as a low-PeT orogen, and briefly explores the implications of this hypothesis for the notions of lithospheric strength.

Section snippets

The ASO

The ASO formed part of a widespread zone of intraplate deformation in the central part of the Australian continent in the late Phanerozic ([8], Fig. 2, Fig. 3). Deformation was most spectacularly developed in the vicinity of Alice Springs in a region that was, up until this time, covered by an extensive Neoproterozoic to Early Phanerozoic basin termed the Centralian Superbasin [11]. Within the ASO, Paleao-Mesoproterozoic metamorphic and granitic basement complexes have been exhumed from beneath

Thermal evolution of the Redbank Shear Zone

As noted above, a significant fraction of the ASO shortening was accommodated on the Redbank Shear Zone which forms a crustal-scale ramp (Figs. 2) offsetting the Moho by ∼10 km. The horizontal displacement of the Redbank Shear Zone and its associated splays, such as the Ormiston Thrust, during the ASO has been estimated at ∼20 km [15]. Given this estimate, the time-averaged slip rates on the Redbank Shear Zone lie between ∼0.15 and 0.33 mm/yr depending on whether (1) slip accumulated throughout

Discussion

The preservation of the extraordinary Central Australian gravity anomalies implies significant lithospheric strength and seems at odds with the localisation of intraplate deformation in this region. The notion that significant thermal structuring of the crust accompanied the development of these gravity anomalies is supported by the distribution of radiogenic heat sources in the current erosion surface (Fig. 3). The calculations summarised in this paper suggest that the redistribution of heat

Acknowledgements

This work has benefited enormously from the discussion with Martin Hand and Peter Haines. This research has been funded by the Australian Research Council. Greg Houseman and Christian Teyssier are thanked for their reviews of the manuscript.[BW]

References (23)

  • S.P Mathur

    Relation of Bouguer anomalies to crustal structure in southwestern and central Australia

    BMR J. Austr. Geol. Geophys.

    (1976)
  • Cited by (0)

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