Geophysical and petrological modelling of the structure and composition of the crust and upper mantle: the case of the Tyrrhenian Sea and surroundings

Information on the physical and chemical properties of the lithosphere-asthenosphere system (LAS) can be obtained by geophysical investigation and by studies of petrology-geochemistry of magmatic rocks and entrained xenoliths. Integration of petrological and geophysical studies is particularly useful in geodynamically complex areas characterised by abundant and compositionally variable young magmatism, such as in the Tyrrhenian Sea and surroundings. A thin crust, less than 10 km, overlying a soft mantle (where partial melting can reach about 10%) is observed for Magnaghi, Vavilov and Marsili, which belong to the Central Tyrrhenian Sea backarc volcanism where subalkaline rocks dominate. Similar characteristics are seen for the uppermost crust of Ischia. A crust about 20 km thick is observed for the majority of the continental volcanoes, including Amiata-Vulsini, Roccamonfina, Phlegraean Fields-Vesuvius, Vulture, Stromboli, Vulcano-Lipari, etna and Ustica. A thicker crust is present at Albani – about 25 km – and at Cimino-Vico-Sabatini – about 30 km. the structure of the upper mantle, in contrast, shows striking differences among various volcanic provinces. Volcanoes of the Roman region (Vulsini-Sabatini-Alban Hills) sit over an upper mantle characterised by Vs mostly ranging from about 4.2 to 4.4 km/s. At the Alban Hills, however, slightly lower Vs values of about 4.1 km/s are detected between 60 and 120 km of depth. This parallels the similar and rather homogeneous compositional features of the Roman volcanoes, whereas the lower Vs values detected at the Alban Hilla may reflect the occurrence of small amounts of melts within the mantle, in agreement with the younger age of this volcano. The axial zone of the Apennines, where ultrapotassic kamafugitic volcanoes are present, has a mantle structure with high-velocity lid (Vs ~ 4.5 km/s) occurring at the base of a 40-km-thick crust. Beneath the Campanian volcanoes of Vesuvio and Phlegraean Fields, the mantle structure shows a rigid body dipping westward, a feature that continues southwards, un to the eastern Aeolian arc. The western Aeolian arc and Ustica sit over an upper mantle with Vs ~ 4.2-4.4 km/s, although a rigid layer (Vs =4.55 km/s) from about 80 to 150 km occurs beneath the western Aeolian arc. In Sardinia, no significant differences in the LAS structure are detected from north to south.