The Torre del Greco seismic and magnetic volcanic structure: a hypothesis for a submarine prolongation of the Somma-Vesuvius volcanic complex (Naples Bay)

The Torre del Greco seismic and magnetic volcanic structure has been investigated in detail through seismic and magnetic data. A hypothesis that it represents a submarine prolongation of the Somma-Vesuvius volcanic complex is here presented. An integrated interpretation of densely-spaced seismic and magnetic profiles recorded by the CNR-IAMC Institute of Naples onboard of the R/V Urania (CNR) has been carried out in order to reconstruct the physiography of a large volcanic structure located offshore the Torre del Greco town (Naples). This structure is framed in the regional geological setting of the Gulf of Naples, owing its complexity to the interaction between volcanic and sedimentary processes during the Late Quaternary. Acoustically-transparent seismic units occurring in the subsurface of the gulf are here interpreted as volcanic deposits erupted during poliphasic activity of Somma-Vesuvius and Phlegrean volcanic districts. Offshore the volcano the magnetic properties have allowed to discriminate the volcanic nature of geological units identified through reflection seismics. A semi-quantitative integrated interpretation of bathymetric and seismic data has been carried out, resulting in a topographic and seismic reconstruction of the Torre del Greco volcanic structure. Significant results have been obtained on the shallow structure of the Somma-Vesuvius volcano and the relationships among seismic velocities and rock lithologies through tomographic techniques. Onshore seismic reflection data on the volcano have indicated a lateral collapse towards SW occurred between 35 and 11 ky ago. Buried adventive craters, dykes and pyroclastic deposits have been found on the eastern side of the volcano. The warping of lowstand marine deposits by subsurface cryptodomes located about 10 km from the top of the volcano coupled with the occurrence of normal faults has indicated a seaward collapse of the volcanic edifice. The integrated interpretation of magnetic, bathymetric and seismic data suggests a close correlation between positive magnetic anomalies offshore the volcano and three main elevated peaks of the volcanic structure located offshore the Torre del Greco town, NNW-SSE trending and located at water depths ranging between 80 and 110 m. Seismo-stratigraphic evidence is provided by acoustically-transparent seismic facies, high contrasts of acoustic impedance with respect to the overlying marine and coastal sediments, mounded-shaped external geometry and average dimensions measured in terms of kilometers. The base of the volcanic body is not acoustically evident since it overlies the seismic unit correlated with the Campanian Ignimbrite pyroclastic flow deposits. The top of the structure is deeply eroded and shows three main culminations. The thickness of the overlying Holocene sediments is significantly reduced, otherwise the quoted sediments are lacking and the structure directly crops out at the sea bottom. Some circular positive magnetic anomalies NNW-SSE trending are located in the Torre del Greco marine coastal belt, while other small and localized anomalies, weaker than the previous ones are located in the Torre Annunziata offshore. The Torre del Greco volcanic structure is located in a complex magnetic anomaly area, made up of several anomalies reaching a maximum intensity of 400 nT. Main trending of magnetic and seismic structure suggests the occurrence of NNW-SSE normal faulting.