Magnetic anomalies and magmatic events in and around the island of Elba (Northern Tyrrhenian Sea)

Abstract

The island of Elba is located at the eastern margin of a N-S-trending high-intensity magnetic lineation. Measurements of magnetic susceptibility of the different lithologies that outcrop in the island indicate that only the peridotites (serpentinites), which are associated with other ophiolitic rocks such as gabbro, diabase and siliceous sediments, possess high magnetic susceptibility (4000−10,000 × 10 -5 SI units). We infer that the long intense N-S magnetic alignment in Northern Tyrrhenian sea is the expression of a segment of oceanic upper mantle and crust of the Jurassic Tethys. The oceanic rocks have been emplaced in the Alpine-Apenninic orogen (ophiolitic suture zone) that, subsequently, through rifting processes, evolved into the opening Tyrrhenian with subsequent basin formation. The island of Elba marks the site of a W-E zone of magnetic lineations related to magmatism of late Neogene age (7.5-5 Ma). The intrusive melts affected a suture site that probably represents an inherited Tethyan ridge-transform structure and have granodiorite-granite compositions and a calcalkaline affinity. Magmatic events in the Northern Tyrrhenian Sea and adjacent Tuscan areas are from Late Miocene (∼7.5–7 Ma) to late Quaternary (0.21 Ma) (Monte Amiata volcano) in age and are related to asthenospheric updoming and stretching, attenuation of the lithosphere, and possibly accompanying underplating beneath the continental crust. Some other main magnetic W-E lineations in the Tyrrhenian Sea and adjoining eastern margins are considered. They are also linked to extensive magmatic manifestations that migrated from west to east along deep sinistral strikeslip fault zones. These faults separate adjacent areas of the seafloor, the extent of opening to the east being different (greater in the south).