Integrated seismic and magnetic study of the Torre del Greco submerged volcanic structure (Naples Bay, Italy) and relationships with the Vesuvius volcanology

A three dimensional magneto-seismic reconstruction of a large volcanic structure located offshore the town of Torre del Greco (Naples Bay, Southern Tyrrhenian sea) and representing the seaward prolongation of the Vesuvius volcano has been carried out, on the basis of the integrated geological interpretation of densely-spaced high resolution seismic and magnetic profiles. This reconstruction has provided new insights into the knowledge of marine geophysics and volcanology of the Vesuvius volcano. Several geological and geophysical surveys focusing on the Vesuvius volcano have been undertaken. These have produced significant results concerning the shallow crustal structure of the volcano and the relationships between seismic velocities and rock lithologies in volcanic environments based on a passive seismic tomography of the volcano.Onshore seismic reflection data on the volcano indicated a SW lateral collapse, which probably occurred between 35 and 11 ky ago.Other volcanic structures, such as buried adventive craters, dykes and pyroclastic deposits were detected on the eastern side of the volcanic complex. Seismic stratigraphy of the volcanic structures shows acoustically-transparent seismic facies and high contrasts of acoustic impedance with respect to the overlying sediments. The structures have mound-shaped external geometry and average dimensions measurable in terms of kilometres. The base of the Torre del Greco volcanic structures is not acoustically evident, because it overlies the volcanic seismic unit correlated to the "Campanian Ignimbrite" (CI) pyroclastic flow deposits. Consequently, no contrast of acoustic impedance between the two volcanic units is clearly evident on the seismic profiles. The top of the structure is irregular and eroded and shows several culminations. A map showing the main magnetic structures of the area highlighted by our analysis, together with lineaments located by reflection seismics has been constructed. The map also shows the lateral boundaries of the main volcanic structures and calderas of the area. It is likely that the area's different magnetic signatures are due to the different characteristics of the pyroclastic deposits forming these bodies. The location of main volcanic structures has been represented. The unit BV represents submerged and/or buried parasitic vents genetically related to the Somma-Vesuvius volcanic complex,corresponding to a NNW-SSE trending magnetic anomaly field. The unit B is related to dome-shaped, isolated and buried volcanic structures,not correlated with magnetic anomalies. The extension of the seismic unit CI in the subsurface has also been mapped. The unit CL represents the Vesuvian lava flows, historical in age, corresponding to slight magnetic anomalies offshore the town of Torre Annunziata. The three-dimensional reconstruction of the Torre del Greco structure has been carried out using seismic constraints and correlations of volcanic structure to bathymetry. The sea floor topography is compared to the top of the Torre del Greco volcanic structure represented by contour depth of corresponding seismic horizons. A striking coincidence between the long-shaped topographic high on the seafloor (towards the east)and the high in the top of the volcanic structure underneath can be explained as a consequence of sea floor deformation due to the underlying volcanic culminations. Smaller volcanic mounds, buried and fossilised by marine sediments, are not related to any magnetic anomaly, probably due to their composition of volcanic tuffs.