When did Sardinia stop rotating? New palaeomagnetic results

Abstract

Previous palaeomagnetic results from Tertiary volcanic rocks of Sardinia suggest that the island underwent a quick counterclockwise rotation between 20.5 and 19 Ma. In order to test this hypothesis, a new palaeomagnetic study was carried out on sediments and volcanic rocks with well-controlled biostratigraphic or radiometric ages younger than the suggested age for the end of the rotation. Unrotated palaeomagnetic directions obtained from the Upper Ignimbrites (radiometric age: 18.5–19.7 Myr) and the Aquitanian tuffs collected in the Anglona region (N Sardinia) would suggest that the rotation ended earlier than 20.5 Myr. The directions found in the volcanics are perhaps the result of secular variation, rather than differential rotation of the Anglona region with respect to surrounding regions. Two sites belonging to the upper ignimbritic unit (SI 2 ) in the Monte Traessu-Monte Rispisu area (Logudoro region) exhibited a large amount of rotation ( D =320.4°, I =60°) which may be age related (radiometric age: 19.9 Myr). Four dikes (radiometric age: 18.3–16.7 Myr) in the Arcuentu volcanic complex (Arburese region) yield results ( D =348.4°, I =33.8°) that indicate a further amount of rotation after their emplacement. Stable end-point directions were successfully isolated from three Upper Burdigalian/Langhian sedimentary sites (biostratigraphic age: 18–15Ma) from the Marmilla and Logudoro regions. Consistent paleomagnetic directions obtained from these sites ( D =352°, I =49°) indicate that a small amount of rotation was still to be completed in the Langhian time. This result is consistent with the history of Sardinian volcanism, which did not end until 13 Ma.