Paleomagnetism of Late Paleozoic Dyke Swarms from Sardinia

One of the key elements of late Paleozoic tectonics in Europe is the continental dextral mega shear system postulated as early as 1977 by Arthaud and Matte, which is thought to control the structural evolution along the southern and eastern margins of Europe. It is rather obvious that this shear zone, which in the western Mediterranean realm is bordered by the Biscaye-North-Pyrenean Fault to the North and the Anti-Atlas-Fault to the South, is one of the most likely candidates along which the controversial Pangea B to Pangea A transition during Permian times might have taken place. Yet, no comprehensive paleomagnetic study has been carried out so far aiming at testing the Arthaud and Matte model and to quantify the amounts of crustal rotations within the shear zone. Here we report paleomagnetic data from Sardinia, which is positioned to the North of the Biscaye-North-Pyrenean Fault and should be directly affected by deformation within the mega shear zone. One of the most striking features of Sardinian geology is the occurrence of three major dyke swarms, which can be easily differentiated by their mean strike directions. The dykes cover a wide range of geochemical compositions but are clearly associated with different tectono-magmatic phases of late to post-Variscan age. Published age data for the three provinces range from 298 to 289 Ma in the south and central part of Sardinia and 282 to 268 Ma in the northern part. Thirty-one sites (182 samples) covering all three provinces were analysed. No tectonic correction was applied on the assumption that the dykes have not been disturbed from their original position after emplacement. Detailed paleomagnetic demagnetisation experiments yielded very well defined mean directions for almost all sites sampled. The resulting in situ mean directions (in Sardinian coordinates) of Permian age for the northern part of the island (Dec: 138°, Inc: -1°, α95: 6°, k: 20, N: 12 sites) is in very good agreement with data published by Vigliotti et al. (1990) supporting the structural coherence of northern Sardinia and Corsica. However, the mean directions for the central and southern part of the island (Dec: 089°, Inc: -3°, α95: 4°, k: 18, N: 14 sites, and Dec: 081°, Inc: -21°, α95: 4°, k: 17, N: 6 sites) determined in dykes of latest Carboniferous and earliest Permian age differ significantly from the mean direction for the northern dykes and suggest that the coherence of Sardinia in the late Paleozoic cannot be maintained. Although the age of these rotational motions is not perfectly bracketed it can be argued that rotation happened before the early Jurassic, probably during Permian times. Surprisingly, however, the mean directions for the southern and central part of the island indicate counterclockwise rotation with respect to the mean for the northern swarm by up to 60° which is `counterintuitive' to the rotations expected in a simple dextral shear system but are possibly related to local or conjugated sinistral shears within a wide dextral shear zone.