Australia: The Land Where Time Began
Antarctica - Before and After Gondwana
Rifting that took place in the Mesozoic led to the formation of sedimentary basins along the margins of southern Australian and the conjugate Antarctica margins. A number of alternative models have been proposed for the configuration of Australia and Antarctica prior to rifting. In some reconstructions Australia is positioned further to the east, relative to Antarctica that is in a fixed position, than other proposed configurations. There are implications for the assessment of different reconstruction models by the use of continuity of geological terranes, surface-mapped shear zones, and geophysical signatures, such as magnetic anomalies, between Australia and Antarctica.
The authors1 tested a range of scenarios for the full fit configuration of Australia and Antarctica. They investigated how different reconstruction models were able to reconcile different geological and geophysical signatures from the conjugate plates, in light of the palinspastic reconstructions of the continental crust that was extended within each margin. Their findings indicated that a model that matches the Leeuwin Fracture Zone in the margin of Australia with the Vincennes Fracture Zone in the Antarctic margin, reconciles structures of Proterozoic age that had previously been correlated between the 2 continents based on the geological similarity, including rocks from the Albany-Fraser Orogeny, and the Kalinjala mylonite zone and Mertz Shear Zones. The authors1 also found that this model reconciles the constraints from palinspastic reconstruction of Mesozoic extension better than models suggesting that, relative to Antarctica, Australia is further east or west, though within Gondwana. The postulated alignment between the Darling Fault in Western Australia and the Denman Glacier in western Wilkes Land, Antarctica, is not supported by this model. It is suggested by the preferred full-fit reconstruction model, together with other evidence from the early breakup history between the Australian and Antarctica plates, that overall the opening direction was broadly from NNW-SSE, though included in this are phases of N-S and NW-SE-directed extension.
Prior to the rifting and breakup of Gondwana in the Cretaceous Australia was attached to Antarctica. The older geological domains in these 2 continents have been the subject of many studies that attempted to establish the links between the structures that border the conjugate margins on each side of the Southern Ocean, such as (Stump et al., 1986; Foster & Gleadow, 1992; Fitzsimons, 2003; Goodge & Fanning, 2010; Flottmann &Oliver, 1994; Gibson et al., 2011; Cayley, 2011; Veevers, 2012). Correlations within the framework of schematic reconstruction sketches some of these studies have drawn correlations. The configuration of the continents based on Euler Poles of rotation from a single model that has been chosen for the full-fit configuration of Australia and Antarctica have been considered by others. There have been a number of alternative models proposed for the full-fit configuration and history and direction of relative plate motions of early seafloor spreading during the rifting apart of the continents. The relative position of Australia and Antarctica within Gondwana varies widely between these models. The result is lead to large differences in the lateral position of Australia relative to Antarctica within Gondwana. The result is that there are major implications with regard to the alignment of the geological and geophysical 'piercing points' that have been identified in each continent.
In the study by the authors1 to investigate this issue they tested a number of different reconstructions for the full-fit configuration of Australia and Antarctica, constructed on the basis of evidence of only rifting in the Cretaceous and early seafloor spreading (Williams et al., 2011). In this paper the authors1 examine the correlation of onshore geological and geophysical structures in the context of the various reconstruction models.
Full-fit reconstructions - Australia and Antarctica - for the full article see source 1 below.
The Naturaliste Model resulted in the greatest discrepancies between the basement geology of Australia and Antarctica of all the models tested. The results produced by the preferred model produces an alignment that is strikingly consistent for the Kalinjala Shear Zone and the Mertz Shear Zone, and is also consistent with the interpreted continuation of the Albany-Fraser Orogenic Belt into Antarctica. The proposed alignment of the Darling Fault with the Denman Glacier, an alignment in the Tasmania-Victoria Land Sector, that is better represented by the use of the poles of rotation of Powell, 1988). A plate configuration that is intermediate between the 2 models could yield a better match to the onshore geology.
Palinspastic restoration of the extended continental margins was used to test each of these models (Williams et al., 2011) based on crustal thickness grids that had been derived from gravity inversion (Kusznir, 2008), the results indicating that a good fit to the unstretched continental outlines can be achieved within a reconstruction that aligns the Leeuwin Fracture Zone and the Vincennes Fracture Zone at full-fit. The alignment of the Naturaliste model leaves a significant gap between South Australia and eastern Wilkes Land. A model with initial NE-SW rifting (Powell et al., 1988), results in significant overlap in the unstretched continental boundaries in this region as well as further to the east. The amount of closure between Australia and Antarctica, based on the restoration of crustal thickness data, appears to be significantly overestimated, in the model proposed by Royer & Sandwell (1989). In light of the geological reconstructions that have been tested in this study, the full-fit pole of rotation that has been derived by Williams et al. (2011), based on palinspastic reconstruction, appears to also reconcile the onshore geological structures for each part of the conjugate margin system.
According to the authors1 the units that have been discussed in this paper, particularly those in Western Australia and South Australia, are much older than the breakup of Gondwana. For these alignments to help in the determination of the most robust reconstruction of Australia and Antarctica shortly prior to rifting in the Late Jurassic-Early Cretaceous, the boundary along which Australia separated from Antarctica in the Mesozoic may have acted as a zone within which intraplate tearing occurred during the Palaeozoic and earlier (Veevers, 2000). It has been argued that lateral motions across this tear during the Palaeozoic could have offset older structures before the breakup of Gondwana. Sinistral (Flottmann and Oliver, 1994) and dextral ( Cayley, 2011) shear senses have been invoked, and for any such displacements magnitudes are poorly constrained. It was assumed by the authors1 for their comparisons that ittle lateral displacement between Australia and Antarctica has taken place throughout much of the Neoproterozoic and Palaeozoic.
Implications for southern marginal basins of Australia
There are important implications for the study of sedimentary basin that developed within the conjugate continental margins in the determination of a model for full-fit configuration of Australia and Antarctica. Significantly different directions for the overall relative motion of the plates during rifting have been implied by the candidate reconstruction models, with an overall NE-SW initial opening, while a NW-SE overall opening is predicted by the Naturaliste model, which is highly oblique to the Tasmania-Northern Victoria Land margin sector. For the Williams et al. (2011) full-fit reconstruction pole results in an overall NNW-SSE displacement. A large overlap between Tasmania and Cape Adare is avoided by such a model, as an overlap arises if the initial direction of rifting is assumed to follow the trend of Leeuwin and Vincennes Fracture Zones (Tikku & Cande, 1999,2000).
These opening directions of overall plate motions over a period covering the expanse of time from the initiation of rifting, about 160 Ma, up to the earliest seafloor anomalies in the Bight Basin at about 83 Ma, many tens of millions of years later. Therefore, variability of relative plate movements during this timespan would not be surprising. The full-fit pole (Williams et al., 2011) has been used (Whittaker et al., submitted), together with an additional pole of rotation at about 100 Ma, to produce a set of reconstructions that reconcile better the observations from the Kerguelen-Broken Ridge section of the plate boundary of Australia-Antarctica to the west of the continental rift zone. The change in plate movements at 100 Ma corresponds to a regional reorganisation of plates (Veevers, 2000).
The direction of opening for basins on the southern margin was initially N-S, which changed to a more NW-SE motion after 100 Ma that persisted until the movement accelerated and a change of relative motion to N-S at about 49 Ma occurred. The change of plate motions that occurred at 100 Ma corresponds to an increased rate of subsidence (Totterdell et al., 2000) and, in the Otway Basin and Sorell Basin, relative motions that were more oblique.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|