GNGTS – Atti del 22° Convegno Nazionale / 02.01

A. Argnani, C. Bonazzi and M. Rovere

Geologia Marina (ISMAR-CNR), Bologna

TECTONICS OF THE SOUTH GARGANO DEFORMATION ZONE

The Gargano promontory appears as a broad E-W elongated anticline affected by faults trending NW-SE, E-W and, to a minor extent, NE-SW. The nature of these fault systems and their chronology, however, are not well assessed, as dating of tectonic events is hampered by lack of syn-tectonic sediments. Some Authors put more emphasis on strike slip motion along the E-W trending Mattinata Fault system (e.g. Funiciello et al., 1988), whereas other Authors (e.g. Ortolani and Pagliuca, 1988) favoured N-S and NE-SW compression, considering the deformation in the Gargano promontory as related to remote stresses from the Dinarides. The Mattinata Fault system is a well expressed morphological feature that is about 50 km long, although segmented, and cuts across the southern part of the Gargano Promontory. Most Authors agree that this fault system is characterised by strike slip motion; however, both sense of motion (see e.g. Funiciello et al., 1988, vs Di Bucci and Mazzoli, 2003) and timing of activity are not well assessed. In addition, the Mattinata Fault system might have experienced multiple reactivations during its long-lasting history: i) transfer fault within the Mesozoic extensional fault system; ii) sinistral strike-slip fault in late Miocene – early Pliocene; and, iii) dextral strike-slip in late Pliocene – Quaternary (Chilovi et al., 2000). A relationship between the E-W-trending Mattinata Fault and the October- November 2002 Molise earthquakes has been suggested on the ground that the most reliable focal plane trends c.a E-W, as indicated by the swarm of aftershocks, and presents a dextral strike-slip component (INGV web site). In addition, the hypocentral depths (15-20 km) of the Molise earthquakes suggest that they are located below the Apennine fold-and-thrust belt, i.e. within the Adriatic plate, like the Mattinata fault. The nature and the geometry of the fault system buried in the subsurface of the Molise region are, however, poorly known. This contribution aims at describing a composite E-W-trending structure that is located offshore Puglia, just south of the Gargano Promontory, and that has been previously named South Gargano Deformation Belt (Argnani et al., 1993). This tectonic feature has been studied by several Authors (de Dominiciis and Mazzoldi, 1987; Colantoni et al., 1990; de Alteriis and Aiello, 1993) using commercial seismic data that, however, lack of appropriate resolution, and has been interpreted as the eastern prolongation of the Mattinata Fault, often without much evidence in support (see Argnani et al., 1993 for a critical review). The Plio-Quaternary sedimentary strata present in the offshore and the close grid of seismic profiles which has been acquired by the Institute of Marine Geology (presently ISMAR) allow to better characterise and date, within a reasonable approximation, the neotectonic activity of such a structure. The understanding of the deformational character of this structure could, in turn, help interpreting the nature and geometry of similar structures that are currently buried below the frontal part of the Apennine edifice. The observed deformation occurs in an E-W-trending belt that is about 60 km long and 20 km wide. Although the deformation appears distributed in few sets of structures, it is mainly focussed along a narrow E-W belt composed by individual segments with slightly different trends. Folding is the most commonly observed type of deformation throughout the belt. Growth strata allow to infer that most of the GNGTS – Atti del 22° Convegno Nazionale / 02.01 deformation occurred before the Pliocene, involving all of the structures. Subsequently, throughout the Plio-Quaternary, the deformation focussed along fewer structures and appears of reduced intensity.

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