Rotational differencies between the northern and southern Tyrrhenian domains: paleomagnetic constraints from the Amantea basin (Calabria, Italy)

We report on a palaeomagnetic study of upper Miocene sediments from the Amantea basin, located on the Tyrrhenian coast of Calabria. The magnetic mineralogy is dominated by greigite and subordinate magnetite in the Tortonian-Messinian clays (ten sites), and by hemoilmenite and magnetite in the underlaying sands and volcanic ashes (three sites), which have not been dated. Data from the Tortonian, Messinian clays pass both a reversal and a fold test, and define a 19° ± 11° clockwise rotation (with respect to the geocentric axial dipole field direction) for the whole basin. The variable amounts of westward declinations observed in the underlaying sands and volcanic ashes can be due to (1) a large counterclockwise rotation episode occurring before the clockwise rotation, (2) the effects of a transitional geomagnetic field in these rapidly deposited sediments, or (3) the observed complex magnetic mineralogy. The new results, when compared with previous palaeomagnetic studies from other Calabrian basins, show that the Neogene drifting of the Calabro-Peloritan block from the eastern margin of Sardinia to the present-day position was accompained by a (probably Pleistocene) 15-20° rigid clockwise rotation recorded in both the Tyrrhenian and Ionian margins. This tectonic regime is shown to be very different from the one observed by previous studies in the northern Tyrrhenian domain, where large rotation associated with thrust sheet activity in the external Apennines were coeval with the onset of an irrotational extentional regime in the Tuscan and Latium Tyrrhenian margins. Palaeomagnetism thus confirms the significant geodynamical differences between the southern and northern Tyrrhenian Sea spreadings