Paleomagnetism of Permian sediments and volcanic rocks from Sardinia

During the Late Carboniferous, large granitic bodies were emplaced on Sardinia related to a change of the tectonic setting from compressional to extensional. Simultaneously the newly formed fault-bounded basins were filled with continental clastic sediments, paleontologoically and radiometrically dated to be Late Carboniferous to Early Permian in age. These basins provide an ideal testing ground for tectonic models such as the controversial Pangea B to A transition, which should have occurred prior to the opening of the Atlantic Ocean in the Jurassic. The mega-shear zone required for this transition has been postulated to be situated in today's Mediterranean (Arthaud and Matte, 1977). If this is valid, Sardinia should have been caught within this shear zone and suffered massive internal deformation. Here we report paleomagnetic data from six Permian basins from different Sardinian regions: in the NW (Nurra), N (Gallura), SW (Iglesiente-Sulcis) and SE (Gerrei-Sarcidano). About 200 samples from 23 sites covering mostly continental redbeds and subordinate acidic volcanics were thermally demagnetised. The overwhelming majority of the specimens studied displayed rather simple directional behaviour. A positive intraformational conglomerate test (Gerrei-Sarcidano) as well as the general improvement of the betweensite scatter after correcting for minor tilt supports the primary character of the magnetisation. The resulting mean direction for five sites from Nurra (Dec: 126°, Inc: -24°, α95: 8°, k: 118) is in perfect agreement with data published earlier by Vigliotti et al. (1990). Three sites of the Gallura area farther to the east yield a mean site direction (Dec: 158°, Inc: -34°, α95: 23°, k: 28) which is rotated clockwise with respect to the data of Vigliotti et al. (1990). Seven sites from southern Sardinia (SW and SE) yield a mean direction with a declination of 82° and inclination of 12° (α95: 12°, k: 25). These results imply crustal coherence of Nurra and southern Corsica. The mean direction for Gallura, however, indicates significant clockwise rotation with respect to Nurra. The new data from southern Sardinia indicate approximately 40° or more degrees of counterclockwise rotation with respect to Nurra. These results are essentially coherent with the paleomagnetic data for the Late Paleozoic dyke swarms (Emmer et al., 2005) from the island of Sardinia and support the interpretation that Sardinia cannot longer be considered as a coherent block, but suffered massive internal deformation of pre-Triassic age.