Note to users. If you're seeing this message, it means that your browser cannot find this page's style/presentation instructions -- or possibly that you are using a browser that does not support current Web standards. Find out more about why this message is appearing, and what you can do to make your experience of our site the best it can be.
Whittaker et al. (Reports, 5 October 2007, p. 83) presentedreconstructions for Australia and Antarctica showing a changein relative plate motion 53 million years ago, coincident withan inferred major global plate reorganization. This commentaddresses problematic areas in their assumptions and the geologicalconsequences of their reconstructions.
1 ExxonMobil Upstream Research Company, Post Office Box 2189, Houston, TX 77252–2189, USA. 2 Rensselaer Polytechnic Institute, Department of Earth and Environmental Sciences, 110 8th Street, Troy, NY 12180, USA. 3 University of Tasmania, School of Earth Sciences, Private Bag 79, Hobart, TAS 7001, Australia. 4 FrOG Tech Pty. Ltd., Post Office Box 145, Blackmans Bay, TAS 7052, Australia.
* To whom correspondence should be addressed. E-mail: anahita.a.tikku{at}exxonmobil.com
Whittaker et al. (1) reported evidence of major Australia-Antarcticplate reorganization between 50 and 53 million years ago (Ma).The change in relative plate motion in their reconstructionsis based on two assumptions. The first assumption is that the>95-million-year-old Leeuwin (Perth) fracture zone (2) offshorewestern Australia (115°E, 37°S), southeast of the NaturalistePlateau, is conjugate to the newly identified Perth South fracturezone, offshore Wilkes Land, Antarctica (110°E, 65°S).The second is that the northwest-southeast direction of separationimplied by these fracture zones persisted between the magneticQuiet Zone Boundary (QZB, 95 Ma) and chron 24o (53 Ma). Theseassumptions are open to some debate. Specifically, the identificationof the seafloor spreading magnetic anomalies and the Perth Southfracture zone are both uncertain, and the new poles give riseto other geological inconsistencies.
Integrated interpretation of geophysical data from both theAustralian and Antarctic margins indicates that the onset ofbreakup was complex and diachronous with the oldest seafloorspreading anomalies identified as chron 32y (3, 4) and chron33o (3) in the central sector of the Australian-Antarctic Basin,between 127 and 133°E on the Australian margin and 125 and128°E on the Antarctic margin. The Whittaker et al. (1)identification of seafloor spreading magnetic anomalies forchrons 33o, 34y, and the QZB in this sector is at odds withthese interpretations, and any finite reconstruction poles olderthan chron 32y are speculative.
Colwell et al. (4) interpreted that magnetic anomalies landwardof the identifications for chron 20o (to the west) to chron24o (to the east) along the Sabrina Coast Sector of the Antarcticmargin (110 to 124°E) possibly lie on transitional continental-oceaniccrust with major magmatic components. This interpretation wasbased on the same profiles as the magnetic anomalies interpretedby (1), data collected by the Australian Government in 2001and 2002. These results suggest that the magnetic anomalieslandward of chron 20o in this region may not be seafloor spreadingisochrons, but could be serpentinized ridges analogous to thepassive margins of Iberia and Labrador (e.g., 5, 6), and thereforecannot be used as constraints in the derivation of finite reconstructions.
Comparing the reconstructions of Whittaker et al. (1) and Tikkuand Cande (2), we find that the poles for chrons 20o, 21y, and24o are statistically indistinguishable at the 95% confidencelevel (Table 1); only the poles for chrons 27y and 32y differ,with those two poles being geographically very close to the24o pole for (2) and further to the southeast for (1). The geologicalconsequences are very different for these two scenarios.
Table 1. Finite rotations of relative motion for Australia with respect to East Antarctica.
In the chron 32y reconstruction of (2), the Leeuwin fracturezone is roughly aligned with the Vincennes fracture zone (102°E,63°S), which bounds the eastern margin of the Bruce Riseoffshore Antarctica, whereas it is aligned with the Perth Southfracture zone in the chron 32y reconstruction of (1). Becausethe Perth South fracture zone is 400 km east of the Vincennesfracture zone, the Australian plate is shifted eastward by thisamount in the reconstruction of (1) with respect to (2). Thegeological justifications (1) cited for this shift of Australiaare threefold. First, Gaina et al. (7) recently identified Mesozoicseafloor spreading anomalies (M2 to M9) north of the Bruce Risethought to be conjugate to crust formed west of the NaturalistePlateau on the Indian plate. Second, the shift apparently alignsArchean-early Paleozoic terranes on the conjugate Australianand Antarctic plates (8). Third, the reconstructions of (2)for chrons 33o and 34y produced an overlap of 140 km betweenTasmania and Antarctica. Although these constraints could bevalid, they are not tight or definitive. Magnetic anomaly identificationsnorth of the Bruce Rise are very noisy and therefore speculative.Based on seismic reflection data, Stagg et al. (9) characterizedthis crust as being highly variable in character and not definitelyoceanic in origin. With >350 million years between the earlyPaleozoic and chron 32y (71 Ma), apparent net eastward shiftof Australian onshore basement terranes may be partly relatedto earlier events (e.g., documented Paleozoic orogenies or Pangeabreakup). In addition, the Antarctic piercing points proposedin (1) are oversimplified with another deformed 1.69-billion-year-oldbasin (10) occurring west of Archean rocks shown in (1) andice cover, making direct correlations speculative. Last, theamount of overlap in the reconstructions of (2) between Tasmaniaand Antarctic at chron 32y, which is now taken to be the oldestisochron (3, 4), is extremely minor.
A geological constraint appears to be violated by moving Australiaeastward (Fig. 1), suggesting that the chron 32y pole of (2)is more likely than that of (1). Broken Ridge and KerguelenPlateau are conjugate rifted plateaus formed 88 to 100 Ma (11),but in the chron 32y reconstruction of (1) the eastern end ofthe ridge falls on oceanic crust dated between chrons 18y and20o (40 to 43 Ma). The entire ridge shifts 215 to 260 km tothe east using the poles of (1) versus those of (2). This impliedamount of compression is greater than can be reconciled withcontemporaneous deformation in the Kerguelen Plateau and LabuanBasin (12).
Fig. 1. Satellite-derived free-air gravity maps (14) of (top) Broken Ridge (BR) and (bottom) Kerguelen Plateau (KP), illuminated from N0°E and N270°E along with regional isochrons (15). On the top, BR (as outlined on the bottom) is reconstructed using the finite rotation parameters of (1) and (2) for chron 32y.
[View Larger Version of this Image (109K GIF file)]
Finally, evidence supporting the existence of the proposed northwest-southeasttrending Perth South fracture zone on the Antarctic plate, presumingit is a conjugate to the Perth fracture zone, is not demonstrablyrobust. Whittaker et al. (1) suggested that the fracture zoneis expressed in the downward continued satellite-derived gravityfield, but the expression in that field is weak. Moreover, downwardcontinuation is not a conventional technique for identifyingfracture zones, as it is prone to amplification of noise. Furthermore,there is no observed offset in magnetic anomalies across thesuggested fracture zone that would support its presence.
In summary, we contend that the published evidence does notsupport the hypothesized major change in relative plate motionbetween Australia and Antarctica at the Hawaiian-Emperor bendtime as argued by Whittaker et al. (1). The data do supportthe hypothesis that the rate of extension increased at chrons24y (53 Ma), 21y (43 Ma), and 18o (40 Ma) (13). More data arerequired to verify the style of development and age of the conjugateCretaceous Australian-Antarctic passive margins.
2. A. A. Tikku, S. C. Cande, Earth Planet. Sci. Lett.180, 117 (2000). [CrossRef]
3. J. Sayers, P. A. Symonds, N. G. Direen, G. Bernardel, in Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea, R. C. L. Wilson, R. B. Whitmarsh, B. Taylor, N. Frotzheim, Eds. (Geological Society, London, 2001), pp. 51–76.
4. J. B. Colwell, H. M. J. Stagg, N. G. Direen, G. Bernardel, I. Borissova, in Antarctica: Contributions to Global Earth Sciences, D. K. Fütterer, D. Damaske, G. Kleinschmidt, H. Miller, F. Tessensohn, Eds. (Springer-Verlag, Berlin, 2006), pp. 327–340.
5. J.-C. Sibuet, S. Srivastava, G. Manataschal, J. Geophys. Res.112, B06105 (2007). [CrossRef]
6. N. G. Direen, I. Borissova, H. M. J. Stagg, J. B. Colwell, P. A. Symonds, in Imaging, Mapping and Modelling Continental Lithosphere Extension and Breakup, G. D. Karner, G. Manatschal, L. M. Pinheiro, Eds. (Geological Society, London, 2007), pp. 235–261.
7. C. Gaina, R. D. Müller, B. Brown, T. Ishihara, S. Ivanov, Geophys. J. Int.170, 151 (2007). [CrossRef]
14. D. T. Sandwell, W. H. F. Smith, Geophys. J. Int.163, 79 (2005). [CrossRef]
15. R. D. Müller, W. R. Roest, J.-Y. Royer, L. M. Gahagan, J. G. Sclater, J. Geophys. Res.102, 3211 (1997). [CrossRef]
Received for publication 29 February 2008. Accepted for publication 25 June 2008.
The editors suggest the following Related Resources on Science sites:
In Science Magazine
TECHNICAL COMMENTS
J. M. Whittaker, R. D. Müller, G. Leitchenkov, H. Stagg, M. Sdrolias, C. Gaina, and A. Goncharov (25 July 2008) Science321 (5888), 490d.
[DOI: 10.1126/science.1157501] |Abstract »|Full Text »|PDF »
REPORTS
J. M. Whittaker, R. D. Müller, G. Leitchenkov, H. Stagg, M. Sdrolias, C. Gaina, and A. Goncharov (5 October 2007) Science318 (5847), 83.
[DOI: 10.1126/science.1143769] |Abstract »|Full Text »|PDF »|Supporting Online Material »
53 million years ago, coincident with an inferred major global plate reorganization. This comment addresses problematic areas in their assumptions and the geological consequences of their reconstructions. 
