Age of the Corsica-Sardinia rotation and Liguro-Provencal Basin spreading: new paleornagnetic and Ar/Ar evidence

摘要

The age of spreading of the Liguro-Provencal Basin is still poorly constrained due to the lack of boreholes penetrating the whole sedimentary sequence above the oceanic crust and the lack of a clear magnetic anomaly pattern. In the past, a consensus developed over a fast (20.5-19 Ma) spreading event, relying on old paleomagnetic data from Oligo-Miocene Sardinian volcanics showing a drift-related 30degrees counterclockwise (CCW) rotation. Here we report new paleomagnetic data from a 10-m-thick lower-middle Miocene marine sedimentary sequence from southwestern Sardinia. Ar/Ar dating of two volcanoclastic levels in the lower part of the sequence yields ages of 18.94 +/- 0.13 and 19.20 +/- 0.12 Ma (lower-mid Burdigalian). Sedimentary strata below the upper volcanic level document a 23.3 +/- 4.6degrees CCW rotation with respect to Europe, while younger strata rapidly evolve to null rotation values. A recent magnetic overprint can be excluded by several lines of evidence, particularly by the significant difference between the in situ paleomagnetic and geocentric axial dipole (GAD) field directions. In both the rotated and unrotated part of the section, only normal polarity directions were obtained. As the global magnetic polarity time scale (MPTS) documents several geomagnetic reversals in the Burdigalian, a continuous sedimentary record would imply that (unrealistically) the whole documented rotation occurred in few thousands years only. We conclude that the section contains one (or more) hiatus(es), and that the minimum age of the unrotated sediments above the volcanic levels is unconstrained. Typical back-arc basin spreading rates translate to a duration greater than or equal to 3 Ma for the opening of the Liguro-Provencal Basin. Thus, spreading and rotation of Corsica-Sardinia ended no earlier than 16 Ma (early Langhian). A 16-19 Ma, spreading is corroborated by other evidences, such as the age of the breakup unconformity in Sardinia, the age of igneous rocks dredged west of Corsica, the heat flow in the Liguro-Provencal Basin, and recent paleomagnetic data from Sardinian sediments and volcanics. Since Corsica was still rotating/drifting eastward at 16 Ma, it presumably induced significant shortening to the east, in the Apennine belt. Therefore, the lower Miocene extensional basins in the northern Tyrrhenian Sea and margins can be interpreted as synorogenic "intra-wedge" basins due to the thickening and collapse of the northern Apennine wedge. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 78]