30 Palaeomagnetic perspectives on the Africa-Arabia-Eurasia plate interactions

Thursday, 7 October 2010
9:30 AM - 12:30 PM
Salon C

PERMO-TRIASSIC PANGEA FITS: WHEN DID A PROPOSED 3500 KM MEGASHEAR BETWEEN GONDWANA AND LAURUSSIA HAPPEN, IF It HAPPENED AT ALL

Paper number: 30-1
Program: Scientific Sessions
Time: 9:30 AM – 9:50 AM

Authors

Abstract

It has been noted repeatedly in the last 40 years that Permian reconstructions of the Atlantic-bordering continents in conventional Pangea fits (called Pangea-A type) are not well supported by the available paleomagnetic results. The scale of the misfit (i.e., undesirable overlap between Gondwana and Laurussia) is about 1100 km. While it is widely recognized that the Atlantic Ocean opened from a Pangea-A configuration, rather different Permo-triassic configurations have been proposed that accommodate the paleopoles much better. The so-called Pangea-B fit has Gondwana in a much more east-northeasterly position with respect to Laurussia, such that the Moroccan coast of Africa was adjacent to the southern margin of the East European Craton (i.e., the Ukraine’s Black Sea coast). The transformation of Pangea-B into Pangea-A would then have taken place along a 3500 km-long, roughly ENE-WSW, and dextral megashear. The timing of movements along the megashear is uncertain: it could have been in late Early Permian times (as proposed by Muttoni and colleagues in 2003) or in the Triassic (suggested by Irving in 1977and by Torcq and colleagues in 1997). Besides these tectonic solutions to the problem, questions have been raised about the geocentric axial dipole hypothesis, which is so central for paleomagnetism. And lastly, nagging doubts that the paleomagnetic results may be less reliable, in that they do not perfectly reflect the ancient magnetic field, have not gone away. One, now widely accepted reason could be that inclination shallowing occurred in sedimentary rocks, thereby yielding paleolatitudes too close to the equator.

In an attempt to improve upon the available data, especially for the time interval of 280 – 240 Ma where the discrepancy between Laurussia and Gondwana poles is the greatest, we have studied ignimbrite flows (Puesto Viejo Fm., 265 – 240 Ma) from Argentina, late-stage dikes from the Oslo Graben (270 – 245 Ma) in Norway, trachyte dikes (~280 Ma) in Ukraine and ~270 Ma volcanics from Illinois. A fifth and useful result has previously been published for the Esterel volcanics in France (270 – 258 Ma). The new results allow a Pangea-A fit to be constructed without significant (and impossible) overlap of the continental areas of Eurasia and Gondwana for the Late Permian – Early Triassic. A new Euler pole (78.6N, 161.9E, 31 degrees) for the fit of North America w.r.t. Europe brings an improvement as well. This implies that there is no longer any need to invoke a megashear of enormous magnitude for Late Permian - Triassic times, for which there is little or no geological evidence. The question, however, remains whether a Pangea-B configuration may have existed in Late Carboniferous – Early Permian times, if it existed at all. Relevant geological observations from ancient landmasses within the shear zone (e.g., the Caucasus and northern Black Sea area, perhaps Turkey’s Pontides and Sakarya, Moesia, Adria, Iberia, Florida, etc.) and improved and well-dated earliest Permian-Late Carboniferous paleomagnetic paleopoles from Gondwana are needed to answer this question.