Musings on the origin of the in situ stress field in southeast Australia
Southeast Australia is characterised by an unusual stress regime in as much as the maximum horizontal stress (SHmax) is aligned approximately perpendicular to the plate motion vector. A neotectonic record of reverse faulting across southeast Australia, concords with the orientation of the compressional in situ stress field inferred from earthquake source mechanisms and bore-hole breakouts. The neotectonic record of more or less continuous activity extends back to the late Miocene (10-6 Ma) when it was marked by substantial, regional-scale tilting, uplift and erosion, now best preserved by regional unconformities in offshore basins. The onset of the modern-day tectonic regime in this interval implicates an important role played by plate-boundary forces, related to changes in the relative plate velocities of the Australian and Pacific plate. Widespread intraplate compression extending back to, but not earlier than, the early Eocene relates to the asymmetric configuration of plate boundaries that followed the amalgamation of the Indian and Australian plates.
While it is widely recognised that the in situ stress field in most continents results from the interaction of plate boundary forces with intraplate variations in the density and rheological properties of the lithosphere (Zoback et al., 1989), the way in which these various factors conspire to produce the observed stress regimes in continental interiors remains poorly constrained. At least for the recent geological past, the configuration of plate boundaries and their interactions can be relatively well constrained. Importantly, plate boundary configurations evolve on much shorter timescales than do intraplate material properties. Consequently, the neotectonic record of continental interiors can potentially provide profound insights into the relative contributions of various plate boundary and intraplate sources of stress. In this contribution we focus on the neotectonic and Neogene stratigraphic record in southeast Australia with reference to the origin of the stress field in this part of the Indo-Australian plate (eg. Hillis & Reynolds, 2000).
The in situ stress field in the Indo-Australian plate contrasts with the other fast moving, "compressional" plates such as the North American and South American plates where there is a general alignment of the axis of principal horizontal compression (SHmax) with plate velocity (Zoback et al., 1989; Zoback, 1992; Richardson, 1992). The main feature of the Indo-Australian intraplate stress field is the broad arcuate trend in SHmax from N-S in India through E-W in the central Indian Ocean to western margin of the continent to NE-SW in northern Australia (Figure 1, Coblentz et al., 1998; Hillis & Reynolds, 2000). This trend implicates the importance of collisional torques in the Himalaya and New Guinea in balancing the driving torques associated with subduction and ocean lithosphere cooling (Coblentz et al., 1995, 1998; Sandiford et al., 1995). However, the E-W to SE-NW SHmax trend in the southeast part of the continent (Figure 2) cannot easily be related to any such balance, and its origin has remained enigmatic. Two distinct hypotheses have been proposed for this component of the stress field. Coblentz et al. (1995, 1998) suggested that it relates to interactions along the Pacific-Australian plate boundary and, more specifically, to the torques associated with the generation of the South Alps of New Zealand. In contrast, Zhang et al. (1996) have shown that broad E-W compression in eastern Australia may result from the density structure associated with the development of the eastern Australian margin, which exhibits a classic rift related escarpment between the Eastern Highlands and a narrow coastal plain bordering the Tasman Sea (Figure 2). While the crustal density structure beneath the Eastern Highlands must undoubtedly influence the stress regime in this part of the world, it is unlikely to account for the general SE-NW SHmax orientation in southern Victoria and the offshore basins further south (including the Gippsland, Bass and Otway Basins, Figure 2), since there is no coastal escarpment along much of this coast. The notion that the in situ stress field in southeast Australia relates to the Pacific-Australian plate interactions that have built the southern Alps in New Zealand is testable inasmuch as the Southern Alps have been constructed since the late Miocene due to a change in the relative velocities of the Pacific and Indo Australian (eg., Walcott, 1998). Prior to this, the Pacific Đ Australian plate boundary was characterised by transtensional to strike slip motion. Thus if the broad compression in southeast Australia indicated by the contemporary, low-level seismicity arises from Pacific Plate interactions, any associated neotectonic response should have commenced in the terminal Miocene.
Figure 1. Pattern of in situ stress (SHmax) in the Indo-Australian plate (after Zoback et al, 1989). RP = ridge push, SP = slab pull, CC = continental collision, NZ = New Zealand, NG = New Guinea.
Our objectives are twofold. Firstly, we show that despite its remote position with respect to active plate boundaries the southeast part of the Australian continent (Figure 2) contains a surprisingly rich record of late Neogene tectonic activity. Secondly, we use constraints on the timing, orientation and amplitude of this neotectonic activity to assess the role of Pacific-Australia plate boundary sources of stress in the evolution of the unusual pattern of in situ stress in southeast Australia.
The in situ stress field in southeast Australia
The in situ stress field in southeast Australia (Figures 1 & 2) is constrained by both earthquake focal mechanisms and borehole breakouts (Denham & Windsor, 1991; Hillis & Reynolds, 2000). The southeast is one of the most seismically active parts of the Australian continent (Figure 2), with a broad distribution of earthquakes up to ~ ML 6.4 across a zone ~ 1000 km in width from the eastern seaboard to the Gawler Craton in the west. Distinct concentrations in seismic activity occur in the Mount Lofty-Flinders Ranges-eastern Gawler Craton region of South Australia, and in the belt trending from the west coast of Tasmania, through south-central Victoria, northeast through the Eastern Highlands to southern New South Wales (Figure 2). The intensity of seismic activity in these zones contrasts the intervening Murray Basin and the cratons to the west. Strike-slip and reverse focal mechanisms for earthquakes in the Flinders Ranges yield a principal horizontal compression (SHmax) of 83±30ˇ (Greenhalgh et al., 1994; Hillis & Reynolds, 2000). In the Eastern Highlands of Victoria, reverse focal mechanisms define a SE-NW azimuth for SHmax (Gibson et al., 1981). Hillis & Reynolds (2000) summarise borehole breakout data from two basins along the southeast margin where the data are considered sufficient to define a significant trend. In the Otway Basin, the azimuth of SHmax derived from breakouts is 136ˇ±15ˇ, while in the Gippsland Basin near the southeast corner of the continent breakouts yields a SHmax of 130ˇ±20ˇ.
Figure 2. Distribution of seismicity (Geoscience Australia earthquake database), topography and SHmax in situ stress trends (after Hillis & Reynolds, 2000) in southeast Australia continent. OR = Otway Range, PPB = Port Phillip Bay, MLR = Mount Lofty Range.
The late Neogene faulting record in southeast Australia
The late Neogene record of southeast Australia contains abundant evidence for faulting. The intensity of this faulting shows marked spatial variation correlating, to a large extent, with the distribution of seismicity (Sandiford, in press). The most extensive faulting record occurs in the Flinders and Mount Lofty Ranges of South Australia, and in southern Victoria, in upland systems such as the Otway Range bordering the southern coastline.
The Flinders and Mount Lofty Ranges in South Australia are bounded by N-S to NE-SW trending fault scarps, with morphology of the Mount Lofty Ranges in particular providing dramatic testimony to role of active faulting and formation of tilt blocks in shaping the large-scale landscape (Figure 3a). Exposures of the main range-bounding faults bounding characteristically reflect steep reverse motion with a hanging-wall of Proterozoic or Cambrian metamorphosed basement above a footwall comprising Quaternary conglomerates shed from the developing upland systems in the last ~1 Ma, and/or deformed late Palaeogene to early Neogene sedimentary successions deposited in the basins that now flank the uplands (Figure 3b). Fault-slip kinematics are consistent with structures having formed in response to reverse stress regime with SHmax trending between 080 and 125ˇE (Figure 3a) similar to the in situ stress field. Slip rates on the major range-bounding faults have been estimated at between 20-100 m/myr. The cumulative post-Miocene displacement on the fault network that forms the western front of the Mount Lofty Ranges is estimated to be ~ 240 m (Sandiford, in press).
On the northern flanks of the Otway Range, the remnants of an early Pliocene strandline systems rise ~120 metres over a series of ENE trending faults and monoclines to elevations of ~ 250 metres (Figure 4a & b). These strandlines, and correlatives in the Murray Basin, were developed during regression of from an early Pliocene sea stand high approximately 65 m above present day sea level (Brown & Stephenson, 1991) and thus imply ~ 200 m of uplift post early to mid Pliocene, due to contemporaneous faulting. Volcanism associated with valley incision dates the faulting to 1-2 Ma (Sandiford, in press), in response to a stress regime with SHmax inferred to trend ~150ˇ (Figure 4a).
Stratigraphic constraints on late Neogene tectonism in southeast Australia
The Oligocene to mid-Miocene sections in southeast Australian basins are dominated by cool-water carbonates reflecting the limited siliciclastic supply to the continental shelves and low continental erosion rates. Changes in the depositional regime at around the Mio-Pliocene boundary are indicated an influx of siliciclastic sediments that, in most instances, unconformably overlie the older carbonates (Figure 3c). This unconformity is best developed in near-shore and onshore positions, where the angularity is typically less than 5 degrees but locally up to 90 degrees. In the St Vincent Basin, bordering the western Mount Lofty Range front, rotation on the Para Fault tilt block alone has excised ~ 100 m of section prior to the Pliocene (Figures 3a & 3c). In the Victorian basins, the Miocene and older sediments underlying the unconformity are commonly folded on NE-SW to ENE-WSW axes (Figure 4c, Dickinson et al. 2002), reflecting generation under a stress regimes with similar orientation as the in situ stress field. These relationships unequivocally implicate deformation, uplift and erosion of Miocene sediments prior to the Pliocene, although erosion of missing section has been augmented by contemporaneous eustatic sea level changes (Carter, 1978; Roy et al., 2000). The timing of deformation is best constrained at localities where the least erosion of the underlying succession has occurred. In the Otway and Port Phillip Basins, the age of the youngest underlying Miocene succession is approximately10 Ma while the age of the overlying Pliocene section at these and other localities is approximately 5 Ma (Dickinson et al., 2002). The onshore non-marine succession of the Gippsland Basin, Oligocene and Miocene brown coals underlie the Mio-Pliocene unconformity. The very low siliciclastic content of the brown coals implies very low continental erosion rates during the Oligocene and Miocene. In contrast, an influx of quartz-rich gravels above the Mio-Pliocene unconformity reflects an amalgam of tectonic events across the basin, as well climatic changes that enhanced erosion rates (Bolger, 1991).
Figure 3. (a) Topography of the Mount Lofty Ranges showing the youthful, fault bounded landscapes. The main range bounding faults are the Para (PF), Eden-Burnside (E-BF), Clarendon (CF), Willunga (WF), Bremer (BF), Palmer (PaF) and Milendella (MF) faults. (b) Outcrop of the Milendella Fault (location shown in Fig. 3a). Cambrian metasedimentary sequences are thrust above Quaternary outwash gravels (~780 ka) and deformed, overturned Miocene limestone (~ 20 Ma, Bourman & Lindsay, 1988). (c) N-S cross section through township of Adelaide after Selby and Lindsay (1982) (location shown in Fig. 3a). The Miocene-Pliocene unconformity implicates significant titling of the Para Fault block in the late Miocene. Mio = Miocene, Plio = Pliocene.
Stratigraphic relationships in southern Victoria imply significant generation of topographic relief in the terminal Miocene. For example, around the Otway Ranges, as much as 600 - 1000 m of section has been removed from beneath the terminal Miocene unconformity (Dickinson et al., 2001, 2002). In comparison with the ~ 200 m of relief generation subsequent to the deposition of the Pliocene strandline system (Figure 4), this suggests a peak in tectonic activity in the late Miocene.
The Miocene-Pliocene unconformity is generally absent from more seaward locations in the offshore basins, where large growth anticlines hosting giant oil fields initiated by inversion commencing in the early Eocene (Brown, 1986; Johnstone et al., 2001) amplified by several hundred meters in the late Miocene (Dickinson et al., 2001). In the offshore Otway Basin, around the Miocene-Pliocene boundary the position of the shelf edge and submarine canyon systems display a dramatic shift seaward at this time (Leach & Wallace, 2001) in response to regional uplift. Inversion in the Gippsland Basin commenced at ~ 52 Ma, following a period of mild extension along NW-SE trending growth faults with NE-SW trending relay systems that linked the growth faults providing the locus for inversion (Johnstone et al ., 2001).
Figure 4. (a) Topography of the Otway Range, SW Victoria, showing deformed Pliocene strandline systems. (b) Topographic profiles along strandline (X-Y in Fig. 4a) rise over 100 m across a set of fault traces. (c) Shallow offshore seismic section south of the Otway Range, showing Miocene-Pliocene unconformity and folding of the underlying Miocene section along a NE-trending axis.
The observations discussed above point to a widespread, low intensity neotectonic activity throughout southeast Australia, consistent with the in situ stress field as determined from active seismicity and bore-hole breakouts. This neotectonic response, which has substantially modified the landscape through the Quaternary and late Pliocene in a number of regions (most notably in the Mount Lofty and Otway Ranges), can be traced back to between10 and 5 Ma, where it resulted in substantial, regional-scale tilting, now best preserved by regional unconformities in proximal offshore basins. These observations suggest SE Australia preserves a neotectonic record of substantial displacement in the terminal Miocene (Dickinson et al., 2001, 2002), followed by ongoing deformation at somewhat lower bulk strain rates.
In terms of the our understanding of the in situ stress field in southeast Australia, the neotectonic record of deformation since the terminal Miocene implicates a substantial role for plate boundary forces associated with Australian-Pacific plate interactions, at least in terms of establishing the critical stress magnitudes for faulting. At present, the only data pertaining to the nature of the palaeo-stress regime prior to the late Miocene come from offshore basins (eg., Johnstone et al., 2001) suggesting a major reorganisation of the stress field, associated with a transition from extensional to compressional stress regimes, in the early Eocene. As noted by Sandiford et al. (1995) a critical change in plate configuration occurred in the early Eocene, when spreading in the north central Indian Ocean and Tasman Sea ceased and the Australian and Indian plates amalgamated. Prior to this, the Australian plate was slow moving and bounded to a large degree by mid-ocean ridges. As shown by Sandiford & Coblentz (1994) and Coblentz & Sandiford (1994), such plate boundary configurations naturally engender extensional stress regimes. The asymmetric disposition of plate boundaries following this amalgamation, more closely resembles the modern ŇcompressionalÓ plates such as the North America and South America plates. The reorganisation of the stress field in southeast Australia at this time further corroborates the notion that the intraplate stress field is to large degree controlled by plate boundary effects.
The notion that the neotectonic record of southeast Australia reflects a change in the relative velocities of the Australian and Pacific plates has a number of important implications. Firstly, it provides observational support for the results of numerical models (eg. Coblentz et al., 1995, 1998) that suggest plate boundary interactions propagate stresses 1000Ős of km across the interior of plates. Secondly, it implies that the tectonic record of mild deformation in continental interiors can provide subtle insights into far field plate boundary interactions. As shown here, intraplate deformation with amplitudes as low 102 metres can potentially augment the record of plate margin interaction, usually interpreted from regions much closer to the active plate boundary. Finally, it suggests that the most substantial strain increments in intraplate settings relate to change in intraplate stress regimes from one state to another. This is perhaps best understood in terms of an initial phase of accelerated deformation as pre-existing structures rearrange themselves with regard to the new stress field. While this result may have been anticipated, this is non-trivial, in as much as it suggests that the episodic nature of the tectonic record in intraplate environments may have more to do with the mechanical response of the lithosphere to changes in tectonic forcing than it does to the magnitude of the forcing.
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