PALEOMAGNETISM OF JURASSIC CARBONATE ROCKS FROM SARDINIA - NO INDICATION OF POST JURASSIC INTERNAL BLOCK ROTATIONS

Several Paleomagnetic studies on Carboniferous and Permian sedimentary and volcanic rocks from Sardinia and Corsica have recently demonstrated (a) the tectonic coherence between southern Corsica and northern Sardinia as well as (b) significant rotations between individual crustal blocks within Sardinia itself. The geodynamic significance of these rotations, however, is not clearly understood mainly because of uncertainties in defining their timing and causes. In order to contribute to these issues, a pioneering study on Jurassic carbonates from the Baronie- Supramonte (Monte Albo) regions (eastern-central Sardinia) has been extended regionally, and detailed paleo- and rock-magnetic investigations across the whole Jurassic sequence of Sardinia have been carried out. A total of 280 oriented drill cores were taken from 44 sites of Mid and Late Jurassic age in the Nurra, Baronie- Supramonte, Barbagia- Sarcidano and Sulcis regions. Despite generally weak remanent magnetization intensities, on the order of less than 1mA/m, thermal and alternating field demagnetization was successfully applied to define a characteristic remanent magnetization component in about 60% of the samples. Site mean directions show rather good agreement after correction for bedding tilt, and yield Middle and Late Jurassic overall mean directions of D = 270.3° and I = 45.0° (alpha_{_95} = 8.1°, k = 14, n = 22 sites) and D = 275.5° and I = 50.7° (alpha_₉₅ = 7.2°, k = 45.3, n = 10 sites), respectively. Positive regional and local fold and reversal tests demonstrate the primary character of the natural magnetic remanence, which is carried by magnetite. These results indicate no post-Jurassic rotations within the island of Sardinia. The resulting Middle and Late Jurassic paleopoles (Lat = 16.9°, Long = 299.8°, dp = 6.5°, dm = 10,2° and Lat = 23.4°, Long = 301.2°, dp = 6.5°, dm = 9.7°), corrected for the opening of (1) the the Liguro- Provencal Basin and (2) the Bay of Biscay using well defined rotation parameters fall onto the coeval segment of the European apparent polar wander path.

These results constrain the timing of large differential block-rotations found in Permian rocks to a pre Jurassic age, and lead to exclude Alpine related tectonics for such rotations.