Ediacaran palaeomagnetism and apparent polar wander path for Australia: no large true polar wander

Phillip W. Schmidt*, George E. Williams

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    41 Citations (Scopus)

    Abstract

    We report new palaeomagnetic data for red beds from the Ediacaran Brachina and Wonoka formations in the Adelaide Geosyncline, South Australia, and discuss their place with previously determined poles in the Ediacaran apparent polar wander path for Australia. Both formations behave similarly on thermal demagnetization, displaying high-temperature components that decay to the origin at 680 °C, consistent with haematite being the only magnetic mineral present. Restoring the strata to the palaeohorizontal yielded positive fold tests for both units at 99 per cent confidence, indicating that acquisition of magnetization occurred before the early Palaeozoic Delamerian Orogeny. For the Brachina Formation (N = 91 specimens) the mean direction after unfolding is declination D = 178.2°, inclination I = -22.6° (α95 = 4.4°), indicating a palaeolatitude λ = 11.8 ± 2.5° and a pole position at latitude λp = 46.0°S, longitude φp = 315.4°E, with confidence semi-axes dp = 2.4° and dm = 4.6° The mean direction for the Wonoka Formation after unfolding (N = 70) is D = 255.9°, I = -23.7° (α95 = 6.4°), indicating λ = 12.3 +3.8/-3.4° and a pole position at latitude λp = 5.2°S, longitude φp = 30.5°E (dp = 3.6° and dm = 6.8°). The mean directions for these units and other Ediacaran units in the Adelaide Geosyncline are significantly different from each other, which excludes blanket remagnetization of the units before Delamerian folding and therefore gives strong preference to their magnetization dating from close to the time of deposition. The late Cryogenian-Ediacaran-Cambrian apparent polar wander path for South Australia spans 150 Myr from ∼635 to 490 Ma and places Australia in low palaeolatitudes throughout the interval studied. The poles differ significantly from each other, suggesting Australia underwent continual drift during that time. Whereas the directional difference between the late Cryogenian Elatina Formation and early Ediacaran Nuccaleena Formation is mainly in inclination, for most other contiguous stratigraphic units the differences are mainly in declination with minor inclination differences, indicating Australia was rotating about a nearby Euler pole in low palaeolatitudes. The large and perhaps rapid polar shifts at 615-590 and 575-565 Ma in the Laurentian apparent polar wander path are not evident in the Ediacaran apparent polar wander path for Australia. Because true polar wander should be recorded globally, in the absence of evidence for any major stratigraphic break in the South Australian succession we conclude that large true polar wander did not occur during the Ediacaran.

    Original languageEnglish
    Pages (from-to)711-726
    Number of pages16
    JournalGeophysical Journal International
    Volume182
    Issue number2
    DOIs
    Publication statusPublished - Aug 2010

    Keywords

    • Magnetostratigraphy
    • Palaeomagnetism applied to geologic processes

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