j ourna l homepage: www.e lsev ie r .com/ locate /gr
GR focus review
A reassessment of paleogeographic reconstructions of easternGondwana: Bringing geology back into the equation
L.T. White a,b,⁎, G.M. Gibson c, G.S. Lister b
a Southeast Asia Research Group, Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW200EX, United Kingdomb Research School of Earth Sciences, Building 61, Mills Road, The Australian National University, Canberra 0200, Australiac Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia
In recent years several tectonic reconstructions have been presented for Australia–Antarctica break-up, witheach putting the Australian plate in a different location with respect to Antarctica. These differences reflectthe different datasets and techniques employed to create a particular reconstruction. Here we show thatsome of the more recent reconstructions proposed for Australia–Antarctica break-up are inconsistent withboth our current knowledge of margin evolution as well as the inferred match in basement terranes on thetwo opposing conjugate margins. We also show how these incorrect reconstructions influence the fit of theIndian plate against Antarctica if its movement is tied to the Australian plate. Such errors can have a majorinfluence on the tectonic models of other parts of the world. In this case, we show how the position of theAustralia plate can predetermine the extent of Greater India, which is (rightly or wrongly) used by manyas a constraint in determining the timing of India–Asia, or India–Island Arc collisions during the closure ofTethys. We also discuss the timing of Australia–Antarctica break-up, and which linear magnetic featuresare a product of symmetric sea-floor spreading versus those linear magnetic features that result from riftingof a margin. The 46 Ma to 84 Ma rotational poles previously proposed for Australia–Antarctica break-up, andconfined to transitional crust and the continent–ocean transition zone, more likely formed during earlierstages of rifting rather than during symmetric sea-floor spreading of oceanic crust. So rotation poles thathave been derived frommagnetic anomalies in such regions cannot be used as input in a plate reconstruction.A new reconstruction of the Australia–Antarctica margin is therefore proposed that remains faithful to thebest available geological and geophysical data.
A plate tectonic reconstruction of the Earth is reliant on a series ofchoices. The choices refer to which data sources, reference framesand/or time-scales are adopted for a particular reconstruction(e.g. White and Lister, 2012). These choices ultimately explain whyone plate reconstruction is different from another.
In this paper, we review existing reconstructions of the Australianplate with respect to the Antarctic plate and show how it is possibleto arrive at very different conclusions regarding the position of theAustralian and Indian plates within Gondwana and after its dispersal.In particular, we show how the adoption of different datasets androtational poles influence the position of these plates in variousreconstructions. We further test which of the existing reconstructionsfor the break-up of eastern Gondwana are the most geologicallyplausible, and use this framework to develop a new reconstructionfor the evolution of this margin.
We chose to review the reconstructions of Australia–Antarcticabecause there is contention as to which of the published models isthe best representation of available geophysical and geological data(e.g. Tikku and Cande, 1999, 2000; Whittaker et al., 2007, 2008;Müller et al., 2008; Tikku and Direen, 2008; Williams et al., 2011;Gibson et al., 2013). Much of the disagreement centers on: (1) whichreconstruction provides the best paleogeographic fit between thetwo continents (c.f. Powell et al., 1988; Williams et al., 2011; Gibsonet al., 2013), (2) establishingwhen sea-floor spreading began betweenthe two plates (c.f. Tikku and Direen, 2008; Direen, 2011; Direen et al.,2012) and (3) which fracture zones are considered to be conjugates ofone another during the computation of Euler poles (c.f. Whittaker etal., 2007, 2008; Tikku and Direen, 2008; Williams et al., 2011). Weexamine each of these issues individually, first by discussing thebackground to each problem, and by showing graphical examplesand potential solutions to each issue.
Please note that all Era, Period, Epoch, Stage and magneticisochron ages reported in this paper refer to those in the most recentinternationally recognized geological time scale (Gradstein et al.,2012), unless otherwise specified. This was done by adjustingthe age from the various time scales used in different papers toGradstein et al. (2012) geological and magnetic time scales. However,some workers did not report which time scale was adopted in theirstudy, so we were unable to update a particular age to Gradstein etal. (2012). In these cases we report the ages as they were originallypublished and highlight them with a superscript asterisk (⁎).
2. Geological Criteria used to evaluate reconstructions ofeastern Gondwana
Australia was part of eastern Gondwana during the Jurassic toupper Cretaceous (Powell et al., 1980). The development of extensivehalf graben systems in the Bight and Duntroon Basins at ~165–144 Ma indicates that rifting had begun between what is now rec-ognized as the Australian and Antarctic plates (Totterdell et al.,2000; Totterdell and Bradshaw, 2004). Rifting was driven by thefragmentation of Gondwana, which resulted in a triple junctionwith arms extending between present-day India–Antarctica, India–western Australia and southern Australia–Antarctica (McGowran, 1973;Deighton et al., 1976; Hegarty et al., 1988; Stagg et al., 1990; Willcoxand Stagg, 1990; Norvick and Smith, 2001; Totterdell and Bradshaw,
2004), eventually resulting in sea-floor spreading between India,Australia and Antarctica (Norvick and Smith, 2001).
It therefore follows that many of the rocks will have similar char-acteristics across each margin. This is demonstrated in the geologicalobservations (e.g. mapping, petrology, geochemistry and geochronol-ogy) that have been conducted over several decades along each mar-gin (c.f. Fitzsimons, 2003; Boger, 2011; Gibson et al., 2013; Veevers,2012 and references therein) (Fig. 1). So any given plate reconstruc-tion should position the plates in a manner that is consistent withthese observations. We therefore used the Pplates reconstructionsoftware developed at the Australian National University to testthe validity of published reconstructions by examining how theyposition conjugate geological terrane boundaries and key piercingpoints that occur along the margins.
2.1. Piercing points between the Australian and Antarctic plates
The best piercing points for reconstructions of Gondwana arenear-vertical, planar structures of the same age such as dykes or faultsthat formed after Gondwana coalesced and before it dispersed(i.e. between ~750 Ma and ~165 Ma) (Reeves and de Wit, 2000).For the Australian–Antarctic margin these include the correlation of:(1) the Neoproterozoic proto–Darling Fault (Australia) with its pro-posed extension to an unnamed fault beneath the Scott and DenmanGlaciers (Antarctica) (Harris, 1995; Fitzsimons, 2003; Boger, 2011),(2) the Paleozoic Avoca–Sorell fault system (Australia) with theLeap Year or the Lanterman faults (Antarctica) (Gibson et al., 2011,2013), and (3) the Coorong Shear Zone (Australia) with the MertzShear Zone (Antarctica) (Gibson et al., 2013). The Mertz Shear Zonehad previously been correlated with the Kalinjala Mylonite Zone inSouth Australia but this interpretation is now considered less likelyfollowing the discovery of meso-Archean crust east of the KalinjalaMylonite Zone (Fraser et al., 2010), indicating that the edge of theDelamerian orogeny occurred much further east than earlier sup-posed (e.g. Di Vincenzo et al. 2007; Goodge and Fanning, 2010). Fur-ther supporting this revised interpretation is the observation that theCoorong and Mertz shear zones both lie along strike of the George VFracture Zone whose location is thought to have been predeterminedby these two opposing crustal-scale structures (Gibson et al., 2013)(Fig. 1).
Several workers have proposed that particular outcrops orsamples identified on each margin can be directly correlated, andtherefore represent good piercing points for plate reconstructions.This includes the correlation of the Cape Hunter Phyllite (Antarctica)with the Price-Wangary Paragneiss (Australia), which are essentiallythe same age and have similar compositions (Oliver and Fanning,1997) (Fig. 1). Glacial erratics from the Terre Adelie Craton are alsostrikingly similar in age and composition to the Gawler RangeVolcanics (Peucat et al., 2002) (Fig. 1). These similarities provide fur-ther support for the idea that the terranes that are exposed on eithermargin were once connected, but they should not be used as tiepoints for plate reconstructions as they do not match the criteria forpiercing points specified in Reeves and de Wit (2000). This is becauseit is difficult, if not impossible to establish the precision of such corre-lations because the surface expression of a geological unit can bequite extensive (particularly for a unit with a shallow dip), or becausea unit might only be exposed in certain locations on either margin.The precision is even lower in the case of sampling sites of glacialerratics, which have clearly been transported after being exposed on
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Continent-Ocean Transition Zone
Transitional crust
Undifferentiated Continental Crust
New England Fold Belt
Terrains and/or Gondwana derived turbiditic sediments accreted 510-300 Ma
Reworked passive margin sediments (Ediacaran to Cambrian)
Pinjarra Belt (1330-1140 Ma)
Albany-Fraser Belt (1330-1140 Ma)
Coompana Block (undifferentiated Palaeo and Mesoproterozoic)
Gawler Range Volcanics (Aus.) / Moraine equivalents (Ant.)
Tie Point (Fault)Rayner Belt (990-900 Ma)
ANT: Crohn CratonAUS: North Australian Craton
Cambrian collisional belts (530-490 Ma)
Tasman Sea
Southern Ocean B
alleny FZ
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an FZ
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Leeuwin FZNaturaliste FZ
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Tasmanides
PSFZ
CoS
Z
LYF
Fig. 1. Tectonic element map of Australia, Antarctica and the Southern Ocean showing the terrane and structural boundaries used here. The geological boundaries and structureswere adapted from Harris (1995), Oliver and Fanning (1997), Yuasa et al. (1997), Tikku and Cande (1999), Peucat et al. (2002), Whittaker et al. (2007), Boger (2011) and Gibsonet al. (2011). Abbreviations: FZ = fracture zone, DF = Darling Fault, CoSZ = Coorong Shear Zone, AF = Avoca Fault, MSZ = Mertz Shear Zone, LFZ = Lanterman Fault Zone,LYF = Leap Year Fault, UF = unnamed fault (underlying the Scott/Denman Glacier), COB (continent–ocean boundary), WSTR (West South Tasman Rise) and ESTR (East SouthTasman Rise). The COB polygons were provided by colleagues at Geoscience Australia and are essentially the same as those shown in Direen et al. (2012).
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the surface. Given these limitations, none of these features were usedas piercing points in the reconstructions presented in this paper. Weinstead used the position of terrane boundaries as per the maps ofBoger (2011) and Gibson et al. (2011; 2013) (Fig. 1) as the locationand age of these terranes is relatively well known along the Australianand Antarctic coastlines, defined from studies by workers such asOliver and Fanning (1997) and Peucat et al. (2002).
2.2. Piercing points between the Indian and Antarctic plates
We also investigated how the position of the Indian plate inGondwana can be influenced by reconstructions of the Australianplate with respect to Antarctica. To evaluate existing reconstructionsof the fit of India with Australia and Antarctica we used a similarrationale as proposed above. That is, we used the position of conjugateterrane boundaries on the Indian and Antarctic plates as per Boger(2011) to test which of the existing reconstructions produced thebest fit. In this example we also used position of the Lambert Rift(Antarctica) with the Mahanadi Rift (India) and the Robert GlacierRift (Antarctica) with the Pranhita–Godavari Rift (India) after thegeological maps (not the reconstructions) of Harrowfield et al. (2005)and Veevers (2012). These represent Permian rift basins that weredissected when India and Antarctica broke apart. They are consideredexcellent piercing points for full-fit reconstructions of India and Antarc-tica as the edges of the rifts are delineated by relatively steep faults andbecause the rocks from each basin are the same age on each margin(Federov et al., 1982; Harris, 1994; Mishra et al., 1999; Boger and
Wilson, 2003; Lisker, 2004; Golynsky et al., 2005; Veevers and Saeed,2008, 2009; Ferraccioli et al., 2011; Veevers, 2012). We have alsoused the map/extent of Greater India as was proposed by Ali andAitchison (2005) in the reconstructions of the Indian plate.
3. Reconstructing eastern Gondwana before break-up
3.1. Australia–Antarctica
A significant amount of lithospheric stretching occurred beforeAustralia and Antarctica separated (Lister et al., 1986; Powell et al.,1988; Veevers and Eittreim, 1988; Lister et al., 1991; Sayers et al.,2001; Totterdell and Bradshaw, 2004; Espurt et al., 2009, 2012).This deformation cannot be accounted for in traditional reconstruc-tions, as tectonic plates are treated as rigid blocks that are rotatedabout the surface of the Earth. Nor can this deformation be quantifiedby using sea-floor magnetic anomalies as these are not produced inthe earliest phases of continental rifting, or if they are, they aremore likely magnetic anomalies associated with exhumed peridotites(e.g. Sibuet et al., 2007). Tectonocists have therefore used the geom-etry of bathymetric contours or the edge of the continent–oceanboundary to create “best-fit” rotation poles to account for thecrustal extension related to plate break-up (e.g. Carey, 1958; Sprolland Dietz, 1969; Powell et al., 1988), while some have also made anattempt to restore the amount of stretch crust along the margin(e.g. Williams et al., 2011; Veevers, 2012).
a
b
c 165 Ma: Williams et al. (2011)
165 Ma: Royer and Sandwell (1989)
165 Ma: Powell et al. (1988)
e 120 Ma: Müller et al. (2008)
d 120 Ma: Powell et al. (1988)
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Though many of these reconstructions have not been adequatelytested with geological data and others have been produced withpaper “cut-outs” that do not account for distortion associated withprojected two-dimensional plates being rotated around a sphere.We therefore propose that the most appropriate reconstructions ofAustralia and Antarctica's pre-rift fit will be those that most faithfullyreposition the plates so that the conjugate geological terranes andstructures (see Section 2) are aligned. In order to test this propositionwe restored the position of the Australian plate with respect toAntarctica at ~165 Ma (Fig. 2a–c) and ~120 Ma (Fig. 2d–e) accordingto the Euler poles summarized in Supplementary File 1. We also com-pared several reconstructions of Australia and Antarctica at ~100 Ma(Supplementary File 2).
The best fit of geological terrane boundaries for the conjugate riftmargins in Australia and Antarctica is obtained by the reconstructionsof Powell et al. (1988) and Veevers and Eittreim (1988) (Fig. 2a, d, Sup-plementary File 2). The other reconstructions shift Australia further tothe east with respect to Antarctica and this produces sub-optimal fitsbetween the conjugate terranes (Fig. 2b, c, e and SupplementaryFile 2). These misfits are most likely produced by attempting to mini-mize the overlap of the South Tasman Rise and Victoria Land, as isshown in many of the 165 Ma–99 Ma reconstructions (Fig. 2 andSupplementary File 2). While the Powell et al. (1988) reconstructionhas the best overall match of terrane boundaries between Australiaand Antarctica, it does however have a greater overlap of the SouthTasman Rise and Victoria Land relative to the reconstructions ofVeevers and Eittreim (1988), Royer and Sandwell (1989), Müller et al.(2008), Williams et al. (2011) and Gibbons et al. (2012).
3.2. Impact of Australia–Antarctica (mis)fits on the Indian plate
The pre-rift fit of Australia with respect to Antarctica has significantimplications for where other plates are positioned in reconstructionsof Gondwana.We showhere how someworkers have rotated the Indianplate with respect to Australia in their reconstructions of Gondwana(e.g. Müller et al., 2008), whereas others have rotated India relative toAntarctica (e.g. Powell et al., 1988; Gibbons et al., 2012). If the Indianplate is rotated relative to the Australian plate, and Australia is rotatedrelative to Antarctica, then India's location will be influenced by whereAustralia is positioned with respect to Antarctica. As we have shownthat there are considerable differences in where the Australian plate ispositioned with respect to Antarctica (Fig. 2 and Supplementary File2), it follows that there will be repercussions for the positioning of theIndian plate. This is highlighted if we use the Euler poles of Müller etal. (2008) to rotate India relative to Australia, and then rotate Australiarelative to Antarctica according to the 165 Ma rotation pole proposedby: (1) Powell et al. (1988) (Fig. 3c); (2) Royer and Sandwell (1989)(Fig. 3d) and (3) Williams et al. (2011) (Fig. 3e) and compare howeach of these reconstructions positions terrain boundaries and tie pointsbetween India and Antarctica.
This comparison indicates that the best geological fits for India–Australia–Antarctica are obtained by using the Powell et al. (1988)and Royer and Sandwell (1989) Euler pole data (Fig. 3a–b). TheWilliams et al. (2011) “hybrid-pole” shifts Australia too far eastwardwith respect to Antarctica, so India is similarly shifted eastward,resulting in a large overlap between the Indian and Antarctic plates,as well as a significant misfit between the conjugate geologicalboundaries of India and Antarctica when using the Euler poles ofMüller et al. (2008) for India's position relative to Australia (Fig. 3c).
Fig. 2. Reconstructions proposed by various workers for Australia's position relative toAntarctica at 165 Ma and 120 Ma. Reconstructions use the Euler poles of: (a) Powell etal. (1988); (b) Royer and Sandwell (1989), (c) Williams et al. (2011); (d) Powell et al.(1988) and (e) Müller et al. (2008). The best geological fit for both intervals is obtainedusing the Euler poles of Powell et al. (1988).
Greater India*
Greater India*
Australia rotated relative to Antarctica (Powell et al. 1988)India rotated relative to Antarctica (Powell et al. 1988)
Australia rotated relative to Antarctica (Williams et al. 2011)India rotated relative to Australia (Müller et al. 2008)
Australia rotated relative to Antarctica (Royer and Sandwell 1989)India rotated relative to Australia (Müller et al. 2008)
Australia rotated relative to Antarctica (Powell et al. 1988)India rotated relative to Australia (Müller et al. 2008)
Australia rotated relative to Antarctica (Gibbons et al. 2012)India rotated relative to Antarctica (Gibbons et al. 2012)
dc
ba
e
Greater India*
Greater India*
Greater India*
Continent-Ocean Transition Zone
Ediacaran-Cambrian collisional belt (550-520 Ma)
Maud-Natal Belt (1130-1060 Ma)
Coats Land Block - folded basement and undeformed Mesoproterozoic volcanic rocks (1100 Ma)
Ediacaran collisional belt (580-550 Ma)
Reworked passive margin sediments (Ediacaran to Permian)
Fault / Shear Zone
Current position of the Indus-Tsangpo Suture Zone rotated with IndiaCurrent position of the Main Frontal Thrust rotated with India
Fig. 3. The effect of error propagation in a plate circuit (where one plate is rotated with respect to another). In this case, several Euler poles were used to rotate Australia with re-spect to Antarctica at 165 Ma (see Supplementary File 1). The Indian plate was rotated with respect to the Australian plate using the full-fit Euler pole of Müller et al. (2008)(Latitude: 11.8°/Longitude: −175.8°/Angle: 63.81°/Age: 136.2 Ma). The best geological fit is obtained for the Australian, Antarctic and Indian plates when using the Euler polesof (a) Powell et al. (1988). The other reconstructions (b–c) translate Australia further to the east and produce a poorer geological fit between India and Antarctica. The error prop-agation can be reduced if the Indian plate is rotated relative to Antarctica instead of being tied to the Australian plate using the poles of (d) Powell et al. (1988) and (e) Gibbons et al.(2012). In this case the best geological fit is again obtained with the Euler poles of Powell et al. (1988). Significant misfits are produced in the Gibbons et al. (2012) reconstruction.This also highlights how different Euler poles and plate circuits can influence impressions of the extent of Greater India.
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Interestingly, rotating India relative to Australia according to theMüller et al. (2008) poles, means that the Greater India polygon ofAli and Aitchison (2005) could be extended by ~50 km so that it
extends to the Wallaby–Zenith plateau (Fig. 3b–d). This hasimplications for establishing the timing of India–Asia collision or theaccretion of island arcs to the northern margin of India as it moved
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northward towards Eurasia, providing that the position and shape ofthe ancient southern margin of Eurasia and/or Tethyan islandarcs could be accurately and precisely located. While numerouspaleomagnetic studies have attempted to define the extent of the an-cient margins, they all result in a range of latitudes that effectivelydemonstrate that the uncertainty of the results derived from thistechnique can be in the order of 1000 km.
The need to extendGreater India is reduced if India is instead rotatedrelative to Antarctica according to the poles proposed by Powell et al.(1988) (Fig. 3d) and Gibbons et al. (2012) (Fig. 3e). The best geologicalfit is obtainedwith the Euler poles proposed by Powell et al. (1988)whorotate Australia with respect to Antarctica, and India with respect toAntarctica (Fig. 3d). However, this reconstruction would mean thatthe northern extent of Greater India (as per Ali and Aitchison, 2005)must be shortened slightly to remove its overlap with the Wallaby–Zenith Plateau. Trimming this northern margin of Greater India wouldresult in a younger age of India–Asia, or India-arc collision. However,this again presupposes that the estimated shape of a continent can beused as a constraint and then only if the position and extent of Eurasiaand the arcs can be established precisely.
The fit of the Ali and Aitchison (2005) Greater India is essentiallyperfect when using the Euler poles of Gibbons et al. (2012). However,this also means that there is a significant misfit between the Indian–Antarctic terrane boundaries (Fig. 3e). As the fit between GreaterIndia and the Wallaby–Zenith Plateau is essentially perfect in Gibbonset al. (2012), we wonder if the Ali and Aitchison (2005) Greater Indiapolygon was used as a constraint to fit India to Australia and Antarctica,instead of using other data such as the position of conjugate geologicalterranes? In any case, this demonstrates that there is some uncertaintyas to where the Indian plate is positioned with respect to Australia andAntarctica, and that there is some flexibility in defining the northern ex-tent of Greater India if it cannot extend further than theWallaby–ZenithPlateau (as proposed by Ali and Aitchison, 2005). Yet, the obvious geo-logical misfits highlight the importance of using geological observationsto constrain interpretations of geophysical data.
As the best geological fit for India–Antarctica is obtained with thepole of Powell et al. (1988), we used these to restore India relative toAntarctica and trimmed the northern extent of Ali and Aitchison'sGreater India so that it does not extend further than the Wallaby–Zenith Plateau. This polygon is provided as an ArcGIS shapefile forthose workers who are interested in investigating this subject further(Supplementary File 3).
4. The initiation of sea-floor spreading between Australiaand Antarctica
Rifting of Australia and Antarctica continued episodically from~165 Ma and eventually the lithosphere was stretched so much thatsubcontinental lithospheric mantle was exhumed before seafloorspreading initiated. The position of the plates during this time is poor-ly constrained. The first appearance of true oceanic crust representsan important component of reconstructions of Australia and Antarcti-ca as the first oceanic fracture zones and magnetic anomalies on thesea-floor allow us to establish the position of the plates with respectto one another. However, there is considerable debate as to whatrepresents the first oceanic crust (e.g. Sayers et al., 2001; Tikku andDireen, 2008; Whittaker et al., 2008), so we have reviewed thevarious arguments and the available data to help us produce a revisedreconstruction of the margin.
4.1. The timing of break-up according to interpretation of sea-floormagnetic anomalies
4.1.1. Australian marginThe earliest work on the magnetic isochrons between Australia
and Antarctica was taken to indicate that sea-floor spreading initiated
between 49.34 Ma and 42.30 Ma (chrons 22–20) (Weissel and Hayes,1971, 1972; Weissel et al., 1977). However, Cande and Mutter (1982)stated that chrons 22 to 19 of Weissel and Hayes (1972) weremisinterpreted, and were, in fact, anomalies 34 to 20. This meantthat sea-floor spreading must have initiated between Australia andAntarctica by 84 Ma, but could have begun earlier (110–90 Ma)(Cande and Mutter, 1982).
This was contested by Veevers (1986), who proposed that chron34 of Cande and Mutter (1982) did not represent oceanic crust,but represented a magnetic anomaly at the youngest edge of thecontinent–ocean boundary. Veevers (1986) calculated an age of99 ± 5 Ma at the oldest edge of the continent–ocean boundary by ex-trapolating the spreading rate between it, and the oldest magneticanomaly (chron 34y). Several reconstructions adopted the 99 ±5 Ma age as the time when sea-floor spreading initiated (Powell etal., 1988; Veevers and Eittreim, 1988; Veevers et al., 1991).
Other workers considered that sea-floor spreading may havebegun earlier due to the interpretation of Mesozoic (“M-Series”)anomalies south of the Eyre sub-basin and west of the Cedunasub-basin (Stagg et al., 1990; Stagg andWillcox, 1992). Interpretationof these data indicated that sea-floor spreading between Australiaand Antarctica could have occurred during the Hauterivian(134–130 Ma) west of the Ceduna sub-basin, and continued crustalextension, or a second phase of sea-floor spreading produced theyounger magnetic isochrons that are observed eastward.
However, subsequent interpretations of magnetic and gravity dataindicated that the initiation of spreading was much younger (Tikkuand Cande, 1999). Tikku and Cande (1999) proposed that the edgeof the magnetic quiet zone was not a true magnetic isochron as itresulted in an overlap between Tasmania, the South Tasman Riseand Wilkes Land. Tikku and Cande (1999) therefore tentatively pro-posed that anomaly 34y (83.64 Ma) was the oldest possible magneticisochron between Australia and Antarctica. They also stated that theanomalies 34y, 33o and 32y could have been falsely identified astrue magnetic anomalies, and thus that spreading perhaps did notinitiate until after 32y (71.45 Ma).
The assertion that spreading had begun by ~84 Ma was supportedby the work of Sayers et al. (2001), who showed that the basementridge complex in the Great Australian Bight was most likelycomposed of serpentinized peridotite ridges and mafic magmatismderived by mantle upwelling and limited partial melting. Thesemagmas were said to cool during chron 34 (125.93–83.64 Ma),producing a distinctive magnetic anomaly that was unrelated tosea-floor spreading (Sayers et al., 2001). Later reconstructions ofAustralia–Antarctica break-up therefore used the 83.64 Ma age asthe timing of the initiation of sea-floor spreading (e.g. Norvick andSmith, 2001) and interestingly this break-up age was the preferredage for break-up in the earlier reconstruction of Tikku and Cande(2000).
Other workers reverted to an older age for Australia–Antarcticabreak-up in their reconstructions (Whittaker et al., 2007; Boger,2011). Whittaker et al. (2007) proposed that break-up occurred at96 Ma* based on an extrapolation of the spreading rate between theoldest magnetic anomaly (chron 34: ~83.64 Ma) and the edge ofthe magnetic quiet zone, the same rationale adopted by Veevers(1986). Tikku and Direen (2008) queried why Whittaker et al.(2007) adopted this age, as previous work clearly stated that themagnetic quiet zone was an inappropriate constraint on the timingof break-up. This was because: (1) on the Australian margin someof what had previously been interpreted as magnetic anomaliesolder than chron 20n(o) (43.43 Ma) were actually serpentinisedperidotites and that chron 33n(o) (79.90 Ma) was the oldest truemagnetic isochron (e.g. Tikku and Cande (1999); Sayers et al.(2001)) and; (2) investigations of the Antarctic margin showed thattrue oceanic crust was erupted on the Antarctic Wilkes marginbetween chron 32n(y) (71.45 Ma) and 33n(y) (74.31 Ma) (Colwell
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et al., 2006). This means that the older Euler poles proposed byWhittaker et al. (2007) could not have been picked from true mag-netic isochrons. Whittaker et al. (2008) responded by stating thattheir work presented an alternate hypothesis, where long symmetricmagnetic anomalies were produced due to slow, relatively amagmaticspreading. However, this reasoning was not adopted in subsequentreconstructions produced by the same authors who reverted to a~84 Ma break-up age (Müller et al., 2008; Williams et al., 2011).
Boger (2011), in a more recent reconstruction of Australia andAntarctica, adopted the 96 Ma* break-up age proposed by Veevers(1986). The adoption of a single age for break-up was laterquestioned by Direen (2011) who argued instead that the initiationof sea-floor spreading between Australia and Antarctica wasdiachronous as evidenced by information gleaned from deep-seadredging and the interpretation of seismic reflection data. Indiscussing the idea of diachronous sea-floor spreading initiation fur-ther, Direen et al. (2012) reported that spreading was thought tohave first occurred in the west, off the Naturaliste–Bruce Rise andBremer Basin (93–87 Ma: Beslier et al. (2004); Halpin et al. (2008)),before progressing into the central Great Australian Bight(85–83 Ma), and then followed by separation in the western Bight(~65 Ma), and the Terre Adelie–Otway region (~50 Ma) (Fig. 1). Inview of this progressive development, Direen et al. (2012) statedthat it is highly unlikely that the age of break-up can be definedusing one isochron. Rather, in their view the age of break-up shouldbe determined using all available (e.g. magnetic isochron picks, stra-tigraphy and the interpretation of reflection seismic data combinedwith petrographic information obtained from dredge samples anddrill holes).
We investigated this issue further by plotting the magnetic anom-aly identifications of Whittaker et al. (2007) with GeoscienceAustralia's current estimate of the continent–ocean boundaries for
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79.90 Ma71.45 Ma
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E
Fig. 4. The magnetic isochron identifications of Whittaker et al. (2007) for the Australian and(a) the Australian margin, and (b) the Antarctic margin.
the Australian and Antarctic margins (Fig. 4). This shows that theoldest end of chron 33n (79.90 Ma) is the oldest anomaly located inoceanic crust along Australia's southern margin, and chron 34(83.64 Ma), which is often taken as the timing of the initiation ofsea-floor spreading, is entirely within the Australian continent–ocean transition zone. Further to the east, the magnetic anomaliesare younger according to Royer and Rollet (1997) (Fig. 5). Off thewest coast of Tasmania, the oldest magnetic anomalies are chron18n.2no (40.15 Ma) and chron 17n.3no (38.33 Ma). However, Royerand Rollet (1997) also identified several other anomalies to thesouth of Tasmania and the South Tasman Rise that were at least asold as 53.98 Ma (chron 24n.3no), and potentially as old as 69.27 Ma(chron 31no) (Fig. 5).
4.1.2. Antarctic marginThe oldest continuous magnetic anomaly on the Antarctic margin
is the youngest end of chron 21n (45.72 Ma) (Fig. 4). The majority ofolder magnetic anomalies on the Antarctica margin are at least par-tially, if not entirely within the continent–ocean transition zone(Fig. 4). This means that those anomalies that have been identifiedwithin the transition zone cannot represent true magnetic isochronsproduced during symmetric seafloor spreading, and thus cannot beused reliably if at all to calculate Euler poles for a plate reconstruction.Alternatively, the Geoscience Australia continent–ocean boundarymay not be everywhere correct and need further refinement. Howev-er, as this continent–ocean boundary is based on the interpretation ofseismic reflection data from both the Australian and Antarcticmargins (it is similar to that shown in Direen et al., 2012 which wasderived from most of the same sources) we are confident that thisis the best estimation of the location of the continent–ocean bound-ary and continental–oceanic transition zone.
31.03 Ma
45.72 Ma
QZB
a14
0°E
35°S
40°S
Linear magnetic
features that may
have previously been
mistaken for isochrons
25.99
28.28
31.03
33.71
43.4343.43
45.7245.72
53.9853.98
62.2262.22
69.2769.27
71.4571.45
79.9079.90
83.6483.64
QZBQZB
Magnetic Anomaly Age (Ma)
53.98 Ma
45.72 Ma83.64 Ma
79.90 Ma
71.45 Ma
43.43 Ma
b
60°S
65°S
150°
E
Seamount B
Antarctic margins plotted against the geological terrane boundaries shown in Fig. 1 for:
150°E140°E
40°S
50°S
60°S
Magnetic Anomaly Age (Ma)
2.58
6.03
11.65
15.16
16.72
17.53
18.52
19.72
20.71
21.16
22.27
23.30
24.47
25.99
27.44
28.28
29.97
31.03
33.71
35.29
36.70
38.33
40.15
43.43
45.72
53.98
59.28
68.27
250 km
19.72 Ma20.71 Ma22.27 Ma23.30 Ma24.47 Ma25.99 Ma
27.44 Ma
31.03 Ma
28.28 Ma29.97 Ma
36.70 Ma38.33 Ma40.15 Ma43.43 Ma53.98 Ma59.28 Ma?
33.71 Ma35.29 Ma
68.27 Ma?
40.15 Ma
38.33 Ma
53.98 Ma
ES
TR
WS
TR
Fig. 5. The age of the magnetic anomalies west and south of the South Tasman Rise aswere interpreted by Royer and Rollet (1997). Their interpretation indicates that theisochrons are N10 Ma younger than those that have been identified west of the GeorgeV Fracture Zone, but also N10 Ma older than those identified west of the TasmanFracture Zone. WSTR = West South Tasman Rise, ESTR = East South Tasman Rise.
991L.T. White et al. / Gondwana Research 24 (2013) 984–998
The fact that linear magnetic features occur within the continental–oceanic transition zone of both the Australian and Antarcticmargins hasbeen noted before (e.g. Tikku and Direen, 2008) and is usually attribut-ed to the presence of exhumedmantle rocks (Sayers et al., 2001) as hasbeen observed elsewhere wherever stretching occurs between twoplates (e.g. Lister et al., 1986, 1991; Sutra and Manatschal, 2012). Inthe present instance, themantle rockswere exhumed as elongate ridgesalong different segments of the Australian and Antarctica plates(e.g. Niida and Yuasa, 1995; Yuasa et al., 1997; Sayers et al., 2001;Direen et al., 2012). These belts of exhumed mantle material are mag-netic, so it is possible that earlier workers mistook the magnetic signalsas symmetrical stripes that were produced due to sea-floor spreading.As these mantle rocks were exhumed due to symmetric (e.g. Direen etal., 2012) or asymmetric stretching (e.g. Espurt et al., 2012) ratherthan symmetric sea-floor spreading, they cannot be used to constrainthe position of the Australian plate with respect to the Antarctic plate.This might also apply to other plates where there is contention as towhat the oldest evidence of magnetic anomalies (e.g. the discrepancyin ages of interpreted magnetic anomalies between the Indian and
Antarctic plates— cf. Gaina et al., 2007; Jokat et al., 2010). This scenarioalso depends on competing hypotheses as to whether Australia–Antarctica rifting occurred symmetrically (e.g. Colwell et al., 2006;Direen et al., 2012) or if it involved an early phase of symmetric rifting,followed by asymmetric rifting prior to eventual sea-floor spreading(Espurt et al., 2012). However, further investigations as to the natureof stretching along the margin are required as the results of the Espurtet al. (2012) model are dependent on the 84 Ma rotational pole pro-posed by Whittaker et al. (2007) that has since been disproved (Tikkuand Direen, 2008; Direen et al., 2012; This Paper). A resolution to thisproblem is important, as it contradicts another interpretation that indi-cates that the evolution of the Australian–Antarctic margin duringbreak-up was dominantly symmetrical (Direen et al., 2011, 2012). Thissaid, it is possible that some sections of each margin developed due tosymmetric rifting, and others due to asymmetric rifting where exhumedmantle is juxtaposed against sections of the upper plate along transferfaults on both the Australian and Antarctic margins (c.f. Fig. 3 of Listeret al., 1986). So a resolution to this issue will not be reached withoutstudies of numerous sections along theAustralian andAntarcticmargins.
4.2. Stratigraphic record
The stratigraphic record indicates that there were successivephases of rifting and subsidence between Australia and Antarcticaprior to continental break-up. Much of this work has focused on thegeology of Australia's southernmargin sedimentary basins, conductedby Geoscience Australia (and its predecessor organizations) overmany years for the purpose of understanding the evolution of hydro-carbon systems. The stratigraphic record is generally divided intoseveral tectonic/temporal phases, yet it is also recognized that thelithospheric thinning has been a progressive process from west toeast (Deighton et al., 1976; Hegarty et al., 1988; Willcox and Stagg,1990; Totterdell et al., 2000; Norvick and Smith, 2001; Direen et al.,2012).
The extension between Australia and Antarctica led to a broadzone of faults forming between the Australian and Antarctic plates.This is commonly referred to as the Southern Rift System (SRS)after Stagg et al. (1990, 2005). The SRS extends from Broken Ridge–Naturaliste Plateau and Kerguelen Plateau in the Indian Ocean tothe South Tasman Rise, south of Tasmania (Stagg et al., 1990, 2005;Direen et al., 2011). The earliest phase of rifting of this system is doc-umented in the western Bight Basin in the Callovian (166–163 Ma)and consisted of ~300 km of NW–SE directed extension (Willcoxand Stagg, 1990; Totterdell et al., 2000; Norvick and Smith, 2001;Blevin and Cathro, 2008). Rifting progressively moved eastwards,producing the Duntroon sub-basin and Otway and Gippsland basinsduring the Tithonian (152–145 Ma). As rifting was progressivealong the margin, the structural style of the western Bight Basin hadchanged to thermal subsidence by the Valanginian (139–134 Ma)while fluvio-lacustrine rift sedimentation continued until theBarremian (131–126 Ma) in the Duntroon sub-basin and the Otwayand Gippsland basins (Totterdell et al., 2000; Norvick and Smith,2001).
A second phase of rifting (oriented NNE–SSW) began during theBerriasian (145–139 Ma) and is observed in the western OtwayBasin (Blevin and Cathro, 2008). This was associated with a periodof slow movement between what were to eventually become theAustralian and Antarctic plates (Norvick and Smith, 2001). Thisphase resulted in stress reorganization, the uplift in eastern Australiaand different evolutionary paths of the southern margin basins(Norvick and Smith, 2001).
A third phase of rifting (oriented NNE–SSW) began in theTuronian (93.9 Ma) to Late Maastrichtian (66.0 Ma) and is observedin the Otway, Sorell and Bass basins (Blevin and Cathro, 2008). Thisphase of extension coincides with the cessation of spreading in theTasman Sea, the onset of rapid northward motion of the Australian
a
b
c
e
d
84 Ma: Powell et al. (1988) & Veevers et al. (1991)
84 Ma: Royer and Sandwell (1989) & Müller et al. (1997; 2008)
84 Ma: Tikku and Cande (1999; 2000)
84 Ma: Whittaker et al. (2007)
992 L.T. White et al. / Gondwana Research 24 (2013) 984–998
plate (~45 Ma) and culminated in the final separation of Australia/South Tasman Rise and Antarctica at ~34 Ma (Totterdell et al., 2000;Norvick and Smith, 2001). It coincides with a major change in the pat-tern of sedimentation where a thin layer of marine/shelf carbonateswas deposited between the Paleocene–Early Eocene and the MiddleEocene (McGowran, 1973) and it also corresponds with a period ofmantle exhumation within the continent–ocean transition zone ofthe Australian plate from ~84 Ma to possibly 45 Ma, thus explainingthe elongate magnetic anomalies that are exposed along the southernmargin (Sayers et al., 2001) (e.g. Figs. 4, 5). This said, we must alsoconsider that rifting, sedimentation style and sea-floor spreadingwere diachronous along the margin (Norvick and Smith, 2001;Direen, 2011), and that changes in sedimentation style are not neces-sarily caused by tectonic forces and could instead reflect processessuch as subsidence and/or sea-level rise.
5. Comparison of Australia–Antarctica reconstructions at the timeof break-up
As we showed that there were considerable differences in recon-structions of Australia and Antarctica at ~165 Ma, ~120 Ma and~100 Ma (Fig. 2 and Supplementary Data 2), we consider it importantto make another comparison at the time of break-up. We thereforecompare several reconstructions of Australia relative to Antarctica at84 Ma (Fig. 6), as this is the age that most workers consider whensea-floor spreading began between the two continents.
All of the 84 Ma reconstructions that are shown in Fig. 6 producean overlap of the Australian and Antarctic continent–ocean bound-aries along most of the margin. However, this makes no account forthe amount of stretching anticipated to have occurred during riftingin order that subcontinental lithospheric mantle be exhumed alongpart of the Australian margin. This point aside, the models of Powellet al. (1988), Royer and Sandwell (1989) and Müller et al. (1997,2008) all produce an overlap of the continent–ocean boundaries ofAustralian and Antarctic plates in the west, and would also give riseto an underlap of the plates in the east were it not for the SouthTasman Rise. The importance of the South Tasman Rise in reconstruc-tions was first raised by Tikku and Cande (1999, 2000) (Fig. 6a–b)who tried to resolve the underlap issue, although in doing so, theyproduced a significant overlap of the South Tasman Rise with Antarctica,as was discussed by Gaina et al. (1998) and Whittaker et al. (2007)(Fig. 6c).
The Tikku and Cande (1999, 2000) reconstructions also imply thatthe initial phase of spreading had a N–S orientation although this hasbeen questioned by several workers because the orientation of thefirst formed extensional faults are interpreted to indicate that the ini-tial phase of spreading was oriented NNW–SSE (Willcox and Stagg,1990). However, McClay et al. (2002) have shown that similar differ-ences between fault and stress orientation are generated in analogmodels, suggesting that the initial spreading direction proposed inthese reconstructions could be valid.
Whittaker et al. (2007) tried to address this issue by rotatingAustralia with respect to Antarctica by matching the Leeuwin(Perth) Fracture Zone (115°E, 37°S) on the Australian plate (Fig. 1)to what they referred to as the Perth South Fracture Zone, offshoreWilkes Land, Antarctica (110°E, 65°S) (Fig. 1). This resulted in a betterfit between the west coast of Tasmania and north coast of WilkesLand, and it ensured that the spreading direction was orientedNW–SE between ~95 Ma* and 53 Ma, thus matching what Willcoxand Stagg (1990) had proposed from structural observations.
84 Ma: Williams et al. (2011)Fig. 6. Reconstructions comparing Australia's position relative to Antarctica at 84 Ma.Constructed using the Euler poles of: (a) Powell et al. (1988) and Veevers et al. (1991);(b) Royer and Sandwell (1989) (which was adopted by Müller et al., 1997, 2008);(c) Tikku and Cande (1999; 2000); (d) Whittaker et al. (2007), and; (e) Williams et al.(2011).
f
ba
c
e
d
Australian
Plate
ESTR
WSTR
ETP
ESTRWSTR
ETP
ESTR
WSTR
ETP
ESTRWSTR
ETP
ESTRWSTR
ETP
ESTRWSTR
ETP
Antarctic Plate
Antarctic Plate
Antarctic Plate
Antarctic Plate
Antarctic Plate
Antarctic Plate
Australian
Plate
Australian
Plate
Australian
Plate
Australian
Plate
Australian
Plate
Significant overlap of the STR at
84 - 96 Ma
Significant overlap of the STR at
84 - 96 Ma
Significant overlap of the STR at
84 - 96 Ma
Restoring the displacement of the Colac-Rosedale Fault (CRF)
creates space for the STR
Restoring the displacement of the Colac-Rosedale Fault (CRF)
creates space for the STR
There is no overlap ofthe STR with Australia
and Antarctica at 45 Ma and no need to
invoke CRF
Fig. 7. Reconstructions of southeast Australia, East Antarctica and the South TasmanRise (STR). Images (a–b) and (d–e) show Australia's position relative to Antarctica at84 Ma according to Tikku and Cande (1999, 2000), while (c) shows how the Eulerpoles proposed by Whittaker et al. (2007) position the two major plates. Note thesignificant overlap of the East South Tasman Rise (ESTR) and West South TasmanRise (WSTR) when using the Euler poles proposed for these microplates by (a) Royerand Rollet (1997) and (b–c) Gaina et al. (1998). (d–e) The overlap at 84 Ma is reducedif Tasmania is translated along the Colac–Rosedale Fault (CRF), however, significantoverlaps of the continental fragments (as well as gaps) are still produced if we usethe poles for the WSTR and ESTR proposed by (d) Royer and Rollet (1997) or(e) Gaina et al. (1998). There is limited evidence available to support the existenceof the Colac–Rosedale Fault. (f) So if it does not exist the STR can only be positionedbetween Tasmania and northern Victoria Land if Australia is only rotated with respectto Antarctica to its position at 45 Ma (according to Tikku and Cande, 1999, 2000).
993L.T. White et al. / Gondwana Research 24 (2013) 984–998
However, the Whittaker et al. (2007) reconstruction also shifted theAustralian plate several hundred kilometers eastward with respectto the reconstruction of Tikku and Cande (1999, 2000) (Fig. 6c–d).This is because Tikku and Cande (1999, 2000) chose different fracturezones as conjugates. In their model the Leeuwin Fracture Zone was aconjugate of the Vincennes Fracture Zone (102°E, 63°S) instead of thePerth South Fracture Zone (Fig. 1). Whittaker et al. (2008) argued thattheir reconstruction (Whittaker et al., 2007) produced a better fit,with no overlap between Tasmania/Tasman Rise and Cape Adare.Slight modifications to this reconstruction have also been proposedin Williams et al. (2011) and Gibbons et al. (2012). These later recon-structions do not rotate Australia as far south as in earlier reconstruc-tions (Tikku and Cande, 1999, 2000; Whittaker et al., 2007), and sothere is less overlap between the South Tasman Rise and WilkesLand when it is held fixed to the Australian plate. However these re-constructions still produce an overlap of the Australian and Antarcticcontinent–ocean boundaries west of Tasmania (Fig. 6e–f).
We compared two of these reconstructions ((1. Tikku and Cande,1999, 2000) and 2. (Whittaker et al., 2007)) to one another in anearlier paper (Gibson et al., 2013) to determine which one produceda better fit between conjugate onshore continental–scale faults.These continental–scale faults controlled the location and initiationof oceanic fracture zone development and transfer fault propagationduring break-up (Gibson et al., 2013) as shown here in Fig. 6 withthe tie points and terrane boundaries discussed in Section 2. Thistest indicated that the Tikku and Cande (1999, 2000) reconstructionproduced a better geological fit. Though, other good geological fitsat 84 Ma are obtained in the reconstructions of Powell et al. (1988);Veevers et al. (1991); Royer and Sandwell (1989) and Müller et al.(1997, 2008) and Tikku and Cande (1999, 2000) (Fig. 6a–c). In com-parison, the reconstructions of Whittaker et al. (2007), Williams etal. (2011) and Gibbons et al. (2012) all shift the Australian plate toofar to the east, resulting in a poorer fit between the conjugate terraneboundaries and transform faults (Fig. 6d–f). These misfits are alsoevident in the reconstructions presented in Whittaker et al. (in press).It is therefore apparent that some of the choices that were adopted inthe reconstruction of Whittaker et al. (2007) were also carried overinto these other reconstructions (Williams et al., 2011; Gibbons et al.,2012; Whittaker et al., in press) which focused on the earlier aspectsof Gondwana break-up (as was discussed earlier in Section 3 andshown in Figs. 2 and 3).
6. Reconstructing the South Tasman Rise
The South Tasman Rise and the East Tasman Plateau both formlarge submarine plateaux made up of continental crust (Exon et al.,1997; Royer and Rollet, 1997) (Fig. 1). Many workers recognizethe issue of overlap between the South Tasman Rise and thereconstructed position of Australia and Antarctica and have proposedmodels to account for this by positioning the South Tasman Risebetween western Tasmania and North Victoria Land (Stump et al.,1986; Willcox and Stagg, 1990), or (2) south of Tasmania and eastof North Victoria Land/Ross Sea Shelf (Grindley and Davey, 1982;Gray and Norton, 1988; Veevers and Eittreim, 1988; Lawver andGahagan, 1994; Bernecker and Moore, 2003; Gibson et al., 2011).
Other workers sub-divided the South Tasman Rise into westernand eastern segments (Exon et al., 1997; Royer and Rollet, 1997;Gaina et al., 1998). Royer and Rollet (1997) considered that the west-ern segment was initially attached to Antarctica and was rifted fromthis continent in the Late Paleocene/Early Eocene, while the easternsegment rifted off Tasmania and the East Tasman Plateau during theseparation of Australia from Antarctica between chrons 33y and 30y(74.31 Ma and 66.40 Ma). However, Gaina et al. (1998) showedthat this reconstruction produced a gap between the eastern SouthTasman Rise, the East Tasman Plateau, the Gilbert Seamount Complexand the Lord Howe Rise when they were restored to their position
prior to break-up. Gaina et al. (1998) also showed there was lessspace for the two South Tasman Rise blocks in the Tikku and Cande(1999) reconstruction (e.g. Fig. 7a–c) and therefore stated that partial
994 L.T. White et al. / Gondwana Research 24 (2013) 984–998
closing of embayments and the reduction of the size of the plateau toaccount for crustal extension associated with rifting would produce abetter fit. In addition, Gaina et al. (1998) proposed that the SouthTasman Rise started moving slowly southward relative to Australiaduring the opening of the Tasman Sea thereby producing a rift betweenTasmania and the South Tasman Rise, and E–W sea-floor spreadingbetween the South Tasman Rise and the East Tasman Plateau. In theirreconstruction, the eastern South Tasman Rise became attached toAustralia shortly before chron 31y (68.37 Ma) and the west SouthTasman Rise became fixed to the east South Tasman Rise at 40 Ma,after about 70 km of left-lateral strike-slip motion between the twoblocks (Fig. 7d–e). Yet, this still results in a significant overlap betweenAustralia, Antarctica and the South Tasman Risewhen using the terraneboundaries that we have adopted for this study (Fig. 1).
The over/underlaps of the continental fragments are perhaps whythere has been a recent tendency to translate Australia further to theeast with respect to Antarctica (e.g. Whittaker et al., 2007; Müller etal., 2008; Williams et al., 2011; Gibbons et al., 2012) (Figs. 2 and 6and Supplementary File 2). However, as we stated earlier, this east-ward shift is an untenable solution as it is done at the expense of asignificant misfit of the terranes on either side of the margin. One pos-sible solution to this problem is to use the Euler poles that producethe best geological fit between Australia and Antarctica, and then totranslate Tasmania and the South Tasman Rise N200 km to the eastwith respect to Australia, thus ensuring that there is room for theSouth Tasman Rise between Tasmania and Victoria Land (Fig. 7d–e).This idea was proposed by Stump et al. (1986) and Elliot and Gray(1992) who stated that during/after break-up, Tasmania was movedwestward along the Colac-Rosedale Fault, and northern VictoriaLand moved southward along a postulated left-lateral strike-slipfault (Fig. 7d–e). A similar idea was proposed in Betts et al. (2002)who proposed that a left lateral strike-slip fault developed due to con-tinued propagation of faults along Australia's southern margin associ-ated with the rotation of Australia with respect to Antarctica duringthe early phases of break-up (see Fig. 20 of Betts et al., 2002). Howev-er, there is a potential problem with these interpretations, as no~E–W trending structures are observed in the high-resolution aero-magnetic data now available for Bass Strait (Direen and Crawford,2003a, 2003b; Gibson et al., 2011). Casting further doubt on the exis-tence of this structure are belts of greenstone and other correlativesequences that extend continuously across Bass Strait from northernTasmania into Victoria without any obvious offset or disruptionthat might point to the presence of a strike-slip fault (Gibson et al.,2011). It follows that moving Tasmania along a strike-slip faultthrough the Bass Strait is not a justifiable option according to thedata currently available. This is not to say that: (1) Tasmania's posi-tion did not change at all during the evolution of the southern margin,especially considering that there was some movement of Tasmaniaassociated with the opening of the Bass Basin (e.g. Norvick andSmith, 2001; Veevers, 2012), or (2) that higher-resolution geophysi-cal datasets that are collected in the future (e.g. seismic tomograms)will not delineate vertical ~E–W trending faults in the Bass Basin, inwhich case the reconstructions will need to be revised.
Earlier we discussed the problem of identifying magnetic iso-chrons in the continent–ocean transition zone (e.g. Figs. 4, 5). Consid-ering the arguments presented above, it follows that Euler polesderived from the older isochrons that were assumed to extendalong the length of the margin are not appropriate constraints forthe position of the Australian plate with respect to Antarctica. So wehave ignored the previously published Euler poles derived from iso-chrons older than 46 Ma and have only rotated the Australian plate(with respect to Antarctica) back as chron 21n (45.72 Ma) forwhich there is some certainty regarding its origin as a true magneticisochron produced during symmetric sea-floor spreading. If this isdone, then there is sufficient room to position the South TasmanRise between Tasmania and northern Victoria Land without the
need of dividing it into smaller fragments with different movementhistories (e.g. Fig. 7a–d). This also ensures that the distinctive curvedshape of the southern edge of the South Tasman Rise continentalcrust fits to the corresponding curved shape of the continental crustof northern Victoria Land, as was proposed as the best fit accordingto the available gravity data (Gibson et al., 2011) (Fig. 7f). However,this does not preclude the possibility that the South Tasman Rise con-sists of two parts that had different “plate” motions.
7. Reconstructing Australia–Antarctica break-up: A clean slate
We have shown that reconstructions of the Australian plate rela-tive to Antarctica are fraught with uncertainties, not least of whichis the exact time sea-floor spreading began between the two conti-nents. The problem lies in establishing which isochrons are real, andwhich are exhumed magnetized mantle rocks that are emplacedwithin the transitional zone between stretched continental andoceanic crust. This problem is common to most other, if not all,“rift–drift”margins around the world (e.g. between India and Antarc-tica c.f. Jokat et al. (2010), and the Atlantic opening: c.f. Sibuet et al.,2007; Sibuet and Tucholke, 2012; Sutra and Manatschal, 2012) andwe are only beginning to understand the impact of these observationson existing plate reconstructions.
Disparities between where the Australian plate has been posi-tioned with respect to the Antarctic plate (e.g. Figs. 2 and 6) haveramifications for where other plates (e.g. India) and “microplates”(e.g. the South Tasman Rise) were positioned in the past (Figs. 3and 7). From these evaluations we have drawn attention to thosereconstructions that are most faithful to geological observations sothat these can be used as a framework to build more accuratereconstructions of Gondwana, which is what we attempt here.
We produced a simplified reconstruction of Australia–Antarctica–India break-up using the PaleoArc ArcGIS application developed byCambridge Paleomap Services Ltd. and FrogTech Pty. Ltd. In this re-construction we rotated Australia with respect to a fixed Antarcticato the oldest identifiable isochron that extends along the margin(chron 21n — 45.72 Ma) according to the poles of Tikku and Cande(1999, 2000). From this position, we rotated Australia about a pole(Latitude:−25.153°/Longitude:−157.776°/Angle: 30.198°) between45 Ma and 165 Ma, before rifting began (Norvick and Smith, 2001;Totterdell and Bradshaw, 2004). This ensures that (1) there was min-imal eastward or westward translation of Australia with respect toAntarctica; (2) the best geological fit was obtained between the twoplates at all times and; and (3) that there is sufficient room for partif not all of the South Tasman Rise between southern Tasmania andnorthern Victoria Land. While this is perhaps an oversimplified histo-ry it at least remains faithful to the geological boundaries on eithermargin and means that we have not used any magnetic anomaliesthat are located within the continent–ocean transition zone.
This reconstruction is also faithful to the idea that Coorong andMertz Shear Zones, and the Avoca and Lanterman and Leap Yearfault zones are conjugate structures on the Australian and Antarcticmargins (Gibson et al., 2011, 2013). These relationships are shownat 165 Ma in Fig. 8a. The reconstruction also highlights how thesetransfer faults are potentially the pre-cursors to the George V andTasman fracture zones that developed as oceanic transfer faultsafter the plates had stretched further and finally began to separate(Fig. 8a–f). Whether the (Mount) Darling Fault in Western Australiaand its unnamed conjugate on the Antarctic plate (Fig. 1) similarlyacted as transform faults during the evolution of the margin is lesscertain. This uncertainty stems from the fact that we do not observeany oceanic fracture zones developing along strike from these on-shore features in the reconstruction (Fig. 8a–f). However, the conju-gate terrane boundaries do match in this reconstruction, so it mightsimply mean that these faults did not influence the future location
165 Ma
100 Ma
45 Ma
80 Ma
125 Ma
0 Ma
a b
c d
e f
Fig. 8. Revised reconstruction of Australia–Antarctica–India break-up from 165 Ma to the present. (a) Rifting between the three major plates begins at 165 Ma. Continued stretchingled to the eventual break-up of India from Australia and Antarctica between (b) 125 Ma and (c) 100 Ma. Rifting continued episodically from west to east along the Australian mar-gin from (a) 165 Ma until approximately (d) 80 Ma, when the first magnetic isochrons were produced between the two plates. Seafloor spreading was very slow between(d) 80 Ma and (e) 45 Ma, but (f) Australia moved rapidly northwards after 45 Ma. A movie of this reconstruction can be viewed at 5 Ma increments (http://vimeo.com/user18925575/gondwana).
995L.T. White et al. / Gondwana Research 24 (2013) 984–998
of the oceanic transfer faults as has been proposed further east(e.g. Gibson et al., 2011, 2013).
We also propose another Euler pole (Latitude: 6.750°/Longitude:−135.300°/Angle: 22.800°) to restore the pre-break-up position ofthe South Tasman Rise between 45 Ma and 140 Ma that is faithful tomatching the terrane boundaries identified for Victoria, the SouthTasman Rise and northern Victoria Land (Gibson et al., 2011). ThisEuler pole remains faithful to the idea that there was sinistraldisplacement between the South Tasman Rise and Tasmania. However,this reconstruction also means that a small gap remains between theSouth Tasman Rise and northern Victoria Land. This could beremoved if the Australian plate was rotated further southward. For
example, if sea-floor spreading initiated at 50 Ma instead of 45 Ma. Al-ternatively, this gap could be a function of the simple rigid plate recon-struction that we have shown that does not consider deformation ofthe South Tasman Rise, or the continental–oceanic crust surrounding it.
India's position in this reconstruction (Fig. 8) is reliant on theEuler poles for India's motion relative to Antarctica as per Patriatand Ségoufin (1988) and Patriat (1987) (i.e. the same poles thatwere adopted by White and Lister (2012)) between 0 Ma and84 Ma and as per Powell et al. (1988) between 84 Ma and 165 Ma.Further details are shown in Supplementary File 1. We also used themodified version of Ali and Aitchison's (2005) Greater India polygonas was discussed in Section 3.2. These data ensure that there is a
996 L.T. White et al. / Gondwana Research 24 (2013) 984–998
good geological fit between the Australian, Antarctic and Indianplates (Fig. 8f), and this will mean that equally good fits should beobtained with the other major plates and microplates of Gondwana.
In order to achieve an optimal continentalfitwe refrained fromusingany of the existing Euler poles previously proposed for Australia–Antarctica break-up between 45 Ma and 165 Ma. We adopted thisapproach because we cannot be certain that any of the previously iden-tified 45–84 Mamagnetic features are true isochrons as all occur withincontinental or transitional continental–oceanic crust (e.g. Figs. 2, 3).Future workers may show that the continent–ocean boundaries thatwe have adopted in this study are inappropriate, and therefore justifythe use of N46 Ma magnetic sea-floor anomalies.
While we did not want to advocate a discrete age for the initialphase of seafloor spreading, we considered it important to try toshow how the seafloor magnetic anomalies might have been generat-ed after break-up between Australia and Antarctica. Synthetic iso-chrons were generated at 4 Ma increments with PaleoArc accordingto the rotation poles for Australia and Antarctica between 84 Maand 0 (Fig. 8d–f) and the modern day geometry of the spreadingcenter (Fig. 1). This shows that magnetic isochrons may havedeveloped at various places discontinuously along the marginbetween 84 and 80 Ma (Fig. 8d). Spreading was very slow between80 Ma and 45 Ma (Fig. 8e), but the Australian plate moved rapidlynorthward after this time (Fig. 8f). While the geometries of thesynthetic isochrons are not perfect, they do match very closely tothe modern day fracture zone geometry and this gives us someconfidence about what is shown in the reconstruction, which canbe observed as a movie at 5 Ma increments (http://vimeo.com/user18925575/gondwana).
8. Implications for basin evolution along Australia'ssouthern margin
Our new reconstruction shows that Australia–Antarctica riftingbegan in the west, and crustal extension would have propagated east-ward as the Australian plate rotated clockwise relative to the Antarcticplate (Fig. 8a–c). This scenario is consistent with the pattern and timingof rifting in the sedimentary basins along Australia's southern margin,where rifting and spreading began in the west, and propagatedeastward (Halpin et al., 2008; Direen et al., 2012).
The reconstruction is also consistent with the structural evolutionof the sedimentary basins and the Southern Rift System, where thedominant stretching direction in the early stages of the system wasproposed to be NW–SE, followed by NNW–SSE before changing toN–S as rifting progressed (Stagg et al., 1990; Norvick and Smith,2001; Blevin and Cathro, 2008). While this reconstruction showsthe evolution of this system in a simplified manner, we are contentthat it is consistent with the geological and geophysical data collectedalong the margin. So we hope that this might form the basis for moredetailed reconstructions to be built in the future, where perhaps therotation of Australia with respect to Antarctica between 165 Ma to45 Ma can be subdivided into different periods of rifting that betterreflect changes in the orientation of stress/strain during its evolution,relative to what we have shown as a steady-state rotation over a longinterval.
9. Conclusions
We have shown that there are considerable differences in wherethe Australian plate is positioned in various tectonic reconstructions.These differences relate to the different data and timescales that wereadopted, but also, the choices as to which fracture zones are consid-ered as conjugates for the calculation of Euler poles. Such choiceshave led to several reconstructions being proposed in recent yearsthat incorrectly position the Australian plate so that the terranesthat have been mapped by geologists over many years do not match
when the plates are restored to their pre-break-up position. This isa major problem, and can have a flow-on effect to misfits with otherplates if they are rotated relative to Australia (e.g. the Indian plate).We therefore went back to the basics, and presented a very simplereconstruction of the break-up of the Australian, Antarctic and Indianplates with the hope that this forms a framework for geologists andgeophysicists to build more detailed, but geologically feasiblereconstructions of Gondwana. While geophysical data are oftenconsidered more quantitative when compared to geological data,our review of existing reconstructions highlights that geologicaldata cannot be removed from the equation.
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.gr.2013.06.009.
Acknowledgments
Research support was provided by the Australian–Indian StrategicResearch Fund “Towards a unified East Gondwanaland reconstructionand its implications for Himalayan Orogeny” and by a consortium ofoil companies that sponsor the South East Asia Research Group.G. M. Gibson publishes with permission from the CEO, GeoscienceAustralia. Sam Hart is thanked for his efforts in developing the Pplatescode. Alan Smith and Lawrence Rush of Cambridge Paleomap ServicesLtd. are thanked for providing a copy of PaleoArc and instructions onits use. We are grateful for discussions about the evolution of thesouthern margin with J. Totterdell, C. Mitchell and A. Stacey and forthe insight and recommendations of Nick Direen and Steve Boger intheir reviews of the paper.
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Lloyd White graduated with a BSc (Hons) from theUniversity of NSW (2006), before briefly working forGeoscience Australia (2006–2008) on various geological-based projects. He later completed a PhD at the AustralianNational University (ANU) (2008–2011), where he investi-gated the tectonic history of the Himalayan orogen. Thiswas followed by an Australian–Indian Strategic ResearchFund post-doctoral fellowship at ANU (2011–2012), andanother post-doctoral fellowship with the Southeast AsiaResearch Group at Royal Holloway University of London(2012–). His research interests revolve around structural ge-ology, geochronology andplate reconstructions andhe is cur-rently focusing on unraveling the tectonic history of SE Asia.
George M. Gibson is a graduate of Edinburgh and Otagouniversities with extensive research and leadership experi-ence in the structure and tectonic evolution of Proterozoicorogenic belts, including Broken Hill and Mount Isa. His re-search interests lie in structural geology, geochronologyand tectonic analysis which have taken him to projects inEurope, Australia, New Zealand and Antarctica. He is cur-rently a theme leader (Geodynamics) in the InternationalGeological Correlation Program and for the previous fouryears served on various committees for the highly successful34th International Geological Congress (Brisbane, 2012). Be-fore joining Geoscience Australia in 1995, he was employed
in the private and university sectors.
Gordon Lister is a Professor at ANU: PhD ANU (1975), lec-tured at Leiden University (1974–1979), Utrecht University(1979–1984), Bureau of Mineral Resources (1984–1987),Columbia University, NY (1986), Professor of Earth Sciences,Monash University (1987–2003), and Research Schoolof Earth Sciences, ANU (2003–present). His research hasfocused on structural geology and tectonics, computers inthe geosciences, and economic geology. His current researchinterests include seismotectonics and cellular automata,the role of extreme extension during mountain building,argon geochronology and the 4D evolution of the planetarylithosphere.
