Ediacaran Paleomagnetism of Well-dated Units in Laurentia and West Avalonia: Implications for Models of Oscillatory True Polar Wander, Equatorial Dipoles and Rapid Continental Drift

English Abstract:

Ediacaran paleomagnetic data from Laurentia are complex, with inclinations of presumed primary remanences that differ by up to 90° or more, often within single geological units. These unusual data have been variously interpreted as due to magnetic overprinting, very rapid continental drift, one or more episodes of oscillatory ~90° true polar wander (TPW), or unusual behaviour of the geomagnetic field such as an equatorial dipole. Here we review the Laurentia data in the 615-565 Ma period. The ages assigned to steep and normal components, if they are primary, appear to require at least two full oscillations during the period in question. There is growing evidence (especially from the Grenville and Rideau dyke swarms for which 9 precise U-Pb baddeleyite ages are now available) indicating that the magnetic directional changes are much too rapid to accommodate either rapid drift or TPW (using current theoretical models). In addition, the paleomagnetic data do not always conform to an equatorial dipole model in which paleopoles should differ by 90°. We also review the paleomagnetic data from well-dated (606-570 Ma) Ediacaran units of the West Avalonia microcontinent, which appear to be simpler than those from Laurentia. Unlike Laurentia units, individual units of West Avalonia usually carry a single presumed primary remanence direction (of dual polarity), rather than two discrete remanence directions or directions that are streaked along a great circle that might record rapid TPW. Large directional (mainly declination) changes between units are usually interpreted as due to block rotations, but alternatively could reflect TPW or unusual behaviour of the magnetic field. However, the corresponding paleopole changes are significantly less than the 90⁰ expected for an equatorial dipole model. Taken together, the Ediacaran Laurentia and Avalonia data do not appear consistent with current models of oscillatory TPW, an equatorial dipole or unusually fast drift.