Sedimentary thickness variations and deformation intensity during basin inversion in the Flinders Ranges, South Australia
Introduction
The modern view of orogenic belts as manifestations of relative motion across active plate boundaries implies, at the largest scale, that deformation intensity is related to proximity to plate boundaries. However, it is well known that both ancient and modern orogenic belts show significant regional variations in both style and intensity of deformation that are not readily explicable in terms of proximity to plate boundaries. One of the most spectacular examples of this in the modern Earth is the partitioning of active deformation evident around the Tarim Basin in Central Asia (e.g. Neil and Houseman, 1997). While such regional variations in deformation intensity (and style) are likely to reflect variations in the mechanical response of the orogen, the specific controls that mediate the mechanical response of the continental lithosphere remain poorly understood. This is particularly true of deformations involving basement reactivation in intracratonic settings (e.g. Rogers, 1995).
The Adelaide fold belt in South Australia (Fig. 1) consists of a Neo-Proterozoic to Cambrian sedimentary (cover) succession deformed along with its Meso-Proterozoic basement in the late Cambrian–early Ordovician (∼500–490 Ma) Delamerian Orogeny. Like many fold belts, it shows systematic regional variations in style and intensity of deformation. At the largest scale, the fold belt can be divided into three distinct zones characterised by different styles and intensities of deformation (Marshak and Flottmann, 1996);
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a southern zone, including the southern Adelaide fold belt (with orogenic shortening of 30–50%) and the Nackarra Arc (where orogenic shortening strains average >6%—see Fig. 1b);
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a central zone (the central Flinders zone), characterised by shortening <5% (Fig. 1b); and
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a northern zone (the northern Flinders zone) characterised by intermediate deformation intensity with shortening averaging ∼11% (Fig. 1b).
Late Cambrian palaeogeographical reconstructions place the active continental margin to the southeast of the preserved fragments of the fold belt (Coney et al., 1990), with the central and northern parts of the fold belt bounded by the older, relatively undeformed cratonic blocks of the Gawler Craton (to the west) and the Curnamona Craton (to the east). In parts of the fold belt, the deformation has involved the basement, which is now exposed as a series of inliers in the north (the Painter and Babbage Inliers), in the east (the Willyama Inliers) and in the south (the Houghton and Myponga Inliers). Elsewhere the deformation has detached the cover sequences from the underlying basement (e.g. the Nackarra Arc).
Paul et al. (1998) have shown that the first order variations in the deformation intensity and the extent of basement reactivation in this central and northern part of the fold belt correlate with spatial variations in stratigraphic thickness (Fig. 2), with much of the shortening strain localised on reactivated growth faults. Together with the demonstrably intracratonic setting for the central and northern parts of the fold belt, this raises a number of important questions, including:
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why is significant Delamerian deformation only found where there is a significant thickness (≳5 km) of Neo-Proterozoic–Cambrian sediment; and
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what specific controls have localised the (relatively) more intense deformation in the northern parts of the fold belt?
The purpose of this paper is to further explore the role stratigraphic thickness plays in mediating the intensity of deformation using quantitative numerical models which couple the thermal and mechanical state of the deforming orogen at the time of onset of deformation. Specifically, we analyse the modern day heat flow regimes to develop constraints on thermo-mechanical models designed to evaluate the parameters controlling the style and distribution of Delamerian deformation in the northern and central Flinders Ranges. We begin with a brief summary of the sediment thickness distribution in the northern and central Flinders Ranges. This is followed by a discussion of thermal regimes during the terminal stages of basin development using constraints from modern heat flow as well as inferences drawn from metamorphic assemblages developed during the Delamerian orogeny.
Section snippets
Stratigraphic thickness variations in the central and northern Flinders ranges
The Adelaide fold belt deforms a thick succession of sediments deposited in a series of rift-sag basins during the Neo-Proterozoic and Cambrian (Preiss, 1987; Jenkins, 1990; Jenkins and Sandiford, 1992). These deformed and in part metamorphosed sediments are now mostly confined to a region of elevated topography forming the Flinders and Mount Lofty Ranges (Fig. 2); the modern topography reflects the reactivation of the fold belt during the late Tertiary. During the Delamerian Orogeny, the
Thermal considerations
The coincident patterns of sediment thickness and deformation intensity during ‘basin inversion’ suggest the possibility that deformation may have been localised by some process intrinsic to the development of the basin. Two possibilities spring to mind. Firstly, the basin inversion (as well as basin formation) may have been localised by fundamental and long-lived regional variations in lithospheric strength. Alternatively, the development of the basin itself may have altered the mechanical
Thermal controls on strength distribution in the continental crust
An important outcome of our earlier discussion is the insight that the development of the sedimentary pile prior to the Delamerian Orogen resulted in significant but variable burial of a radioactive basement. The thermal parameters relevant to this hot basement are reflected in the modern heat flow field and in the measured heat production rates of exposed basement rocks in and around the fold belt. These parameters can be used to reconstruct plausible thermal regimes attendant with basin
Conclusions
The general correspondence between Delamerian deformation intensity and anomalous heat flow (averaging 90 mWm−2) in the Flinders Ranges, suggests that thermal weakening of the crust has played a significant role in localising deformation in the northern and central, intracratonic parts of the Adelaide fold belt. However, the somewhat more intimate correspondence between the deformation intensity and thickness of the sedimentary sequence, both at the basin-wide and intrabasinal scale, suggests a
Acknowledgements
We thank Geoff Fraser and Greg Houseman for commenting on the manuscript, and Martin Hand for many discussions concerning the origin and implications of high heat flow in the Australian continent.
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