Palaeozoic geography of the Alpine realm, new palaeomagnetic data from the Northern Greywacke Zone, Eastern Alps

Abstract

The Palaeozoic units of Alpine Europe are generally accepted as having been part of the northern margin of Gondwana throughout the Cambro-Ordovician. By Permo-Carboniferous times, northern Gondwana had collided with Laurasia, forming Pangaea, but the palaeogeographic affinities of the Alpine units in intervening times, and whether they remained an integral part of Gondwana or formed part of the Armorican terrane assemblage (ATA), remains unclear. In order to resolve this problem, a palaeomagnetic study of Late Silurian and Mid Devonian rocks from the Kitzbuhel Alps of the Northern Greywacke Zone, Eastern Alps (part of the Norian-Bosnian terrane, or Proto-Alpine terrane), has been carried out. Three ancient directions of magnetisation have been identified in this study. The youngest (component B) is identified in the most deformed rocks, and represents a remagnetisation event of Alpine age and indicates rotation of the region with respect to stable Europe since the remagnetisation event. The other two directions are interpreted to be primary in origin, and Late Silurian (component C) and Mid Devonian (component D) in age. Both components are identified as high unblocking temperature directions of magnetisation, and stereographic projection of site mean directions from each age shows a clear, small circle distribution after bedding correction, which is related to local and small-scale vertical axis block rotations. Both data sets pass the inclination-only fold test, demonstrating the primary nature of these magnetisations. The resulting mean inclination values obtained for the Silurian and Devonian rocks are 65° (α95=1.7°) and 42° (α95=4.6°). Using the negative polarity option, these yield palaeolatitudes for the southern Proto-Alpine terrane of 47°S (+2.3° and –2.1°, upper and lower 95% confidence level) during the Silurian, and 24°S (+3.7° and –3.4°) during Mid Devonian times. In terms of palaeogeography, therefore, these new results indicate that in Mid Palaeozoic times, the Proto-Alps were located significantly further south than, and were independent of, the ATA. The relationship with northern Gondwana is more complex, given the uncertainty of the APW path for Gondwana. Palaeobiogeographic indicators, however, support the interpretation that the Proto-Alps formed an independent microplate, rather than having remained part of the north Gondwana margin.