Vs models in the Gulf of Napoli

Abstract

Lithospheric 1-D shear wave velocity (Vs) models (up to 73 km of depth) are defined in the Gulf of Napoli through the non-linear inversion (Hedgehog method) of group velocity dispersion curves of fundamental-mode Rayleigh surface wave, extracted by frequency-time analysis (FTAN method) from seismic noise cross-correlation functions between two receivers. The eastern margin of the Tyrrhenian Sea is characterized by a number of basins evolved during the latest Neogene-Quaternary across the structural boundary between the Apennine fold and thrust belt and the Tyrrhenian back-arc extensional area. These basins, which include the present-day Gulf of Napoli, formed in response to large-scale orogen-parallel extension and associated transtensional tectonics that accompanied the anti-clockwise rotation of the Apennine belt and lithospheric stretching in the Tyrrhenian basin. Seismic noise data were recorded by four stations belonging to the Osservatorio Geofisico - sez. Sismologia of the Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse (University of Napoli Federico II). The local group velocity dispersion data (from 1.7 s to 6.5 s) were inverted together with average phase and group velocity ones obtained from regional tomography (from 10 s to 150 s) (Panza et al., 2007). The structural model below the westernmost part of the Gulf of Napoli is characterized by a coverage of marine sediments and volcanic products, about 1 km thick, on the carbonate basement with Vs ranging from 2.2-2.6 km/s to 3.0-3.6 km/s. However, the presence of lava bodies within the carbonate rocks cannot be excluded in the light of the same density and seismic velocity values. A main feature is represented by a sharp increment of velocity around 10 km of depth (Vs of 3.7-4.0 km/s), which can be attributed to the presence of metamorphic rocks, overlying a low velocity layer at about 15-16 km of depth. The depth of the low velocity layer has a regional extension and corresponds to the brittle-ductile transition identified in the peri-Tyrrhenian area at 10-15 km of depth with temperatures of 400°±170°C. Towards the northeastern part of the Gulf of Napoli, a low velocity layer is detected at about 6-7 km of depth, which continues towards Mt. Somma-Vesuvius. It may be explained by partially (about 5-10%) melted rocks. At about 12-13 km of depth, a layer is found, whose velocity (Vs of 3.4-3.8 km/s) is consistent with that (velocity reversal) detected in the westernmost sector of the Gulf of Napoli at about 15-16 km of depth. The Moho discontinuity (Vs of 4.1-4.3 km/s) is at about 25-27 km of depth, according to previous deep seismic surveys beneath the Campi Flegrei and the regional Vs model. The results of this study are coherent with the geodynamic context of the southern Tyrrhenian Sea: stretching of the continental lithosphere, that produced thinning and passive upwelling of hot asthenosphere, accompanied by magmatism and high surface heat flow. They also support the model of the Campanian Volcanic Zone (CVZ), hypothesized on the basis of the recent geochemical, volcanological and geophysical data, extending from Roccamonfina in the north to the Capri Island in the south, and bounded by the Tyrrhenian margin in the west and by the southern Apennine in the east.