TOPO-OZ: Insights into the various modes of intraplate deformation in the Australian continent
Introduction
Australia is the lowest and flattest of all continents, consistent with relatively low-levels of tectonic activity to be expected for a continent that has remained remote from active plate boundaries throughout the Cainozoic (Veevers, 1984). However, low-level tectonic activity is indicated by widespread earthquake activity (Leonard, 2008), and seismic moment release rates are elevated relative to many other stable continental regions (Braun et al., 2009, Johnston et al., 1994). A rich palaeo-seismic record of surface fault breaks and Quaternary faults (Crone et al., 2003, Crone et al., 1997, Quigley et al., 2006) raises intriguing questions about the longer-term record of tectonic activity in the continent. As part of the Indo-Australian plate (IAP), Australia is also the fastest moving continent having drifted northwards ~ 3000 km over the last 45 myr (Fig. 1). In so doing it has passed over a complex deep mantle density structure as revealed partly by a ~ 60 m differential in the geoid field across the continent, as well as the unusual ocean bathymetry of the Southern Ocean in the vicinity of the Australian–Antarctic discordance — or AAD (Gurnis et al., 1998). The dynamic topographic effects of such differential motion between the plate and the underlying mantle are significant, and most clearly evident in a fundamental asymmetry in the morphology and stratigraphic relations of its margins (Sandiford, 2007).
A further attribute of geomorphic significance is that Australia is amongst the most arid of continents. Aridity has affected much of the interior part of the continent through the late Cainozoic (Bowler, 1976), providing it with a remarkable geomorphic ‘memory’. The exquisite detail preserved in extremely old landforms, such as the Eocene beach barrier systems and associated lagoons on the eastern Nullarbor Plain (Fig. 2), imply surface processes have been remarkably ineffective. This preservation of ancient palaeo-shorelines and lake systems, in particular, allows for unambiguous reconstruction of subtle deformations of the landscape, the memory of which would have been greatly obscured or obliterated in environments characterised by more active surface processes.
The last few years have seen a resurgence of interest in the tectonic geomorphologic record of the Australian continent (Celerier et al., 2005, Quigley et al., 2007a, Quigley et al., 2007b, Sandiford, 2003a, Sandiford, 2003b, Sandiford, 2007, Sandiford et al., 2004). Parallel studies have established a basic framework for understanding key neotectonic ‘drivers’ such as the in situ stress state (Coblentz et al., 1998, Hillis and Reynolds, 2000, Reynolds et al., 2002, Sandiford et al., 1995, Sandiford et al., 2004) mantle structure (Debayle et al., 2005, Kennett et al., 2004) and thermal regime (McLaren et al., 2003, Neumann et al., 2000). In this paper we review the main insights gained from these studies. We begin with a brief summary of the current geodynamic state of the Australian continent, including our current knowledge of the in situ stress and geoid fields and constraints on deformation rates provided by the seismic record. We then explore the neotectonic record at a variety of scales. This record in interpreted in terms of three distinct modes of deformation each with a characteristic temporal and spatial scale. At the shortest wavelength (order 101–102 km) active faulting is demonstrably shaping the landscape in several parts of the continent and can be related to propagation of stress from distant plate boundaries. At intermediate wavelengths (order 102–103 km) low amplitude undulations have produced distinctive patterning of continental relief most clearly associated with modern and palaeo-lake systems. Finally, at the longest wavelength (> 103 km) a pattern of continental tilting can be related to dynamic topography associated with its northward passage from the geoid low, dynamic topography high now lying south of the continent, to a geoid high, dynamic topographic low centred over the subduction zones of South East Asia and Melanesia.
Our prime concern is on the topographic record of the Australian continent in the context of the evolution of the IAP. The IAP was born of a complex sequence of correlated events that saw the fusion of the Indian and Australian plates around 45 million years ago (Fig. 1). The critical element of this was the termination of spreading in the north-central Indian Ocean that accompanied deceleration of the Indian plate during the initial stages of Himlayan collision (Patriat and Achache, 1984), and the associated northward acceleration of the Australian Plate (Gaina and Muller, 2007). Prior to 50 million years ago Australia formed the core of a much more slowly moving plate largely surrounded mid-ocean ridges. As such, its palaeo-geographic setting was more reminiscent of the present-day Africa, in the sense of having a higher degree of symmetry in the plate boundaries compared to today (Fig. 1b). With termination of spreading in the north central Indian Ocean and the Tasman Sea to the east of Australia, Australia became one of two continental crustal fragments (the other being India) fused into a fast moving plate with strongly asymmetric boundary configurations (mid-ocean ridges to the south, convergent margins to the north). The onset of the modern compressional intraplate stress field (see below) most probably dates to this transition, with the pre-Eocene continent likely to have been subject to mild extensional stress regimes, in analogous fashion to the African continent (Coblentz and Sandiford, 1994, Sandiford and Coblentz, 1994, Sandiford et al., 1995).
Understanding just how the Australian landscape has been modified by, and records, subtle tectonic processes should inform how to isolate comparable signals in other slower moving, faster eroding settings such as Europe (Cloetingh et al., 2005). With international programs such as TOPO-EUROPE now focussing interdisciplinary work on all manner of phenomenon related to the topographic evolution of the continents, our purpose here is simply to briefly summarise our recent insights from Australia that link the active deformation to the contemporary geodynamic setting and various geophysical parameters, such as in situ stress, seismic strain rate, geoid and dynamic topographic fields.
Section snippets
The in situ stress field
Focal mechanism solutions provide the most comprehensive source of information relevant to the in situ stress state at the plate scale as shown in Fig. 3, and these data are augmented by other indicators in and around the continent, most notably bore-hole breakout data (Hillis and Reynolds, 2000). Fig. 4 shows the computed stress field based on minimising misfits with in situ stress indicators (Reynolds et al., 2002). The distributed seismicity in the central Indian Ocean calls into question
The seismic strain rate field
To estimate seismic strain it is important to assess the degree of completeness of the relevant seismic catalog at various magnitudes. The completeness levels of the Australian earthquake catalog have recently been reviewed by Leonard (2008). Completeness levels are ML = 3.5–4, 1980+, ML = 4–5, 1970+, ML = 5–5.5, 1960+ and ML = 5.5+, 1910+ (Fig. 5). Seismicity shows significant spatial variability, suggesting corresponding variability in seismic strain rates (Braun et al., 2009, Leonard, 2008). This
The geoid and dynamic topography fields
Density anomalies that source the long-wavelength geoid also drive mantle flow and thus contribute to the dynamic topographic field (Richards and Hager, 1984, Richards and Hager, 1988, Richards and Hager, 1989). In this context it has long been recognised that zones of plate convergence are characterised by geoid highs and dynamic topographic lows. Thus continents like Australia drifting towards zones of convergence should be expected to experience dynamic subsidence (Lithgow-Bertelloni and
The Neogene and Quaternary faulting record
The clearest evidence for active tectonic deformation in the Australian landscape is found in the fault-related landscapes around upland systems in southeast part of the continent (Sandiford, 2003b). The most extensively documented faulting record occurs in the Flinders and Mount Lofty Ranges of South Australia (Bourman and Lindsay, 1989, Quigley et al., 2006), and in southern Victoria, in upland systems such as the Otway Ranges bordering the southern coastline (Sandiford, 2003a).
The Flinders
Continental-scale tilting
As summarised by Sandiford (2007), significant variations in the Cainozoic vertical movements around Australia implicate a profound dynamic topographic signal. Differential motion, including both absolute uplift and subsidence, is evidenced by a north–south asymmetry in the elevation of marine sediments, stratal relations (vis-a-vis onlap versus offlap) and the width of the present-day continental shelves (Fig. 8).
The southern margin is characterised by an extensive onshore Cainozoic marine
Deformation at the intermediate wavelength
Arguably the most enigmatic signal of Cainozoic deformation is provided at intermediate wavelengths (100–1000 km) where a number of 100–200 m, landscape “undulations” have developed, and in some cases disappeared, on timescales of 1–10 myr. For example, (Celerier et al., 2005) noted that the pattern of deformation of the Flinders Ranges and the surrounding basins such as the Torrens and Frome Basins includes an undulation of several hundred metres amplitude over about 200 km between the basins
Discussion
The geomorphic record of Australia provides several unique insights into the subtle dynamics of plate interiors. The first relates to the propagation of stresses from distance plate boundaries. As part of a complex plate, it is now well understood that plate boundary activity contributes to the in situ stress field within the plate (Cloetingh and Wortel, 1986, Coblentz et al., 1995, Coblentz et al., 1998, Reynolds et al., 2002). Because the active continental orogenic systems bordering the
Acknowledgements
Our ideas on the evolution of the Australian topographic field and its relation to the Indo-Australian plate have benefited from discussions with numerous people over the years including Jim Bowler, Jean Braun, Dan Cark, David Coblentz, David Egholm, Andrew Gleadow, Mike Gurnis, Richard Hillis, Barry Kohn, Dietmar Mueller, Paolo Vasconcelas and Malcolm Wallace. The manuscript has been improved by comments from David Coblentz, Sierd Cloetingh and two anonymous reviewers. This research was
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