Seismic study of the ‘41st Parallel’ Fault System offshore the Campanian–Latial continental margin, Italy

Abstract

A set of seismic reflection lines, collected in the Southern Tyrrhenian Sea offshore the Campanian–Latial continental margin has been selected and interpreted. The aim is to characterize: (1) the structural features of the 41st Parallel Line (41PL), an E–W elongated magnetic anomaly zone separating the Northern Tyrrhenian domain from the Southern Tyrrhenian domain; and (2) to study the relationships between the 41PL and the Ortona–Roccamonfina Line (ORL), a tectonic structure transversal to the Apennines along which the Northern Apennine Arc merges the Southern Arc. The interpretation of the seismic lines is discussed in light of the available geological (stratigraphic and structural) and geophysical (gravimetric and magnetic) information. Results show that the Campanian–Latial continental margin is characterized by a series of structural lows and highs that match with the main structures on the mainland (Mt. Massico horst, the Volturno and Garigliano depressions). The study area is characterized by ESE–WNW to E–W and NE–SW striking faults. The activity of these faults developed during Pliocene–Early Pleistocene times. ESE–WNW to E–W faults display structures consistent with strike–slip movements. These faults, which are located on the maximum gradient of the E–W elongated magnetic alignment of 41PL, are responsible for the SSE translation of the offshore sector of the Mt. Massico horst. The horizontal dislocation of Mt. Massico suggests left-lateral movements for the ESE–WNW to E–W faults. The NE–SW faults that affect the Mt. Massico horst, which represents the southern tip of ORL, show seismic features consistent with normal movements, as also revealed by inland data. Since: (1) background seismicity is virtually absent along the study area; and (2) the uppermost seismic reflectors seem unaffected by faults, it is very likely that both the ORL and 41PL fault zones are now inactive, at least in the Campanian–Latial area. Seismic data indicate that ORL is older than 41PL and support the interpretation that the 41PL represents a deep-seated transfer fault system formed in response to the different rates of opening of the Tyrrhenian Sea. The strike–slip movements along the 41PL faults and the normal movements along the ORL faults are consistent with a NW–SE extension, which is responsible for the longitudinal extension in the Southern Apennines belt.