Ediacaran Global Reconstructions
Extended Abstract – Prato 2004

D.A.D. Evans & T.D. Raub
Dept Geology & Geophysics Yale University, New Haven CT 06520, USA
(emails:dai.evans@yale.edu, timothy.raub@yale.edu)

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Running header text: Ediacaran Global Reconstructions

The Ediacaran Period presents Earth scientists with an extraordinary plethora of unusual events and circumstances. Within the realm of continental reconstructions, the primary tools employed are palaeomagnetism and tectonic comparisons; palaeobiogeography can provide further constraints. Tectonic features of the Ediacaran cratons show that Gondwanaland was assembling between 630 and 530 Ma. At the same time, several passive continental margins, notably those encircling Laurentia, developed from earlier rifts. The Avalonian-Cadomian arc was a major locus of subduction. By Cambrian time, there were four major continental blocks: Gondwanaland, Laurentia, Baltica, and Siberia.
Despite the apparent simplicity of solving a four-piece puzzle on the globe, reconstructions of these continents have proved difficult. Each of them is embroiled in a controversy among palaeomagnetists regarding which data are reliable and which can be neglected. The best data sets are those from Australia and Laurentia. Australian palaeomagnetic data derive from sedimentary successions of the Flinders Ranges and Amadeus Basin, whose relative stratigraphic ages are unambiguous, despite their impressive lack of numerical age calibration. These results indicate a near-equatorial position for the entire Ediacaran Period, but with substantial, apparently oscillatory, map-view rotations. If Gondwanaland can be considered nearly assembled during that interval, then Australia's rotations imply rapid cross-latitudinal translations for the Amazonian and West African sectors of the large supercontinent. Laurentian palaeomagnetic data derive primarily from precisely dated igneous intrusions but more complicated magnetic signatures, commonly involving multiple components of uncertain relative ages within single rock units. The best data suggest a series of rapid, possibly oscillatory, cross-latitudinal translations similar in character to those described above for Amazon and West Africa. The absence of numerical ages within the Australian successions prohibits testing whether the oscillations recorded by the two continents are synchronous, but some constraints are provided by the need to avoid continental overlap and maintain a consistency with the geological records of rifting, subduction, and collision.
There are several ways to interpret these data. The first is to discard the most anomalous data implying the most rapid rotations and translations, and generate a palaeogeography that is reasonably supported by geological data. This approach, endorsed by Joe Meert, Trond Torsvik, and perhaps the majority of palaeomagnetists, satisfies the principle of least astonishment but must unfortunately neglect what seem to be some of the most reliable data. A second approach is to accept all reliable data and postulate an alternative mechanism such as true polar wander (TPW), which is a plausible geodynamic process for generating the implied rapid, oscillatory rotations and translations. This approach, sponsored mainly by ourselves, Joe Kirschvink, and a few others, is attractive in explaining the data but demands a somewhat nonuniformitarian view of the Ediacaran world. A third category of interpretation is to question some of the fundamental assumptions underlying palaeomagnetic methods in general, namely that the Earth's geomagnetic field was a quasi-stable axial dipole during Ediacaran time. This last approach has not been developed in publication, as our understanding of even the present geodynamo is rudimentary and substantial departures from the axial dipole hypothesis are a Pandora's Box that most palaeomagnetists dare not open without trepidation. Any solution of this type must satisfy strict requirements of the data, which, although widely scattered within each continent's database, nevertheless are patterned enough to guide hypothesis formulation. In addition, any unusual Ediacaran geodynamo must show some behavior that is remarkably reminiscent of that which we know from the Cenozoic and Mesozoic Eras, with polarity reversals scattered aperiodically through continuous stratigraphic sections.
At present we favor the true polar wander model for the strange Ediacaran palaeomagnetic database. This would imply large relative sea level variations on those continental margins experiencing cross- latitudinal motion, such as Laurentia, Amazon, and West Africa, due to delayed mantle viscous relaxation to changes in position relative to the equatorial bulge (fixed to the geomagnetic reference frame by assumption). The TPW model also predicts large-magnitude methane clathrate mobilization, which can account at least partly for dramatic Ediacaran carbon-isotopic variations of seawater. Rigorously detailed palaeomagnetic studies of well preserved stratigraphic successions can generate the data that might help resolve these remarkable problems in a most exciting period of Earth history.