Structural architecture

To illustrate the main structural features of the Southern Apennines, a regional cross-section that cuts across the entire thrust belt-foredeep-foreland system has been modeled. This cross-section is based on the interpretation of the CROP-04 deep seismic reflection profile (Fig. 4), which was acquired, between 1989 and 1990, within the framework of the Italian deep crust exploration project (CROP Project; Scrocca et al., 2003). This profile provides valuable new information on the structure and tectonic evolution of the Southern Apennines. A special issue of the Italian Journal of Geosciences dedicated to the CROP-04 profile collects scientific contributions concerning the geology of the Southern Apennines and the interpretation of this specific profile (Mazzotti et al., 2007 and references therein).

Figure 4. Regional geological cross-section

Regional geological cross-section

Regional geological cross-section built along the CROP-04 seismic reflection profile, location in figure 1 (modified after Scrocca et al., 2005 and 2007).

The interpretation of the CROP-04 seismic reflection profile proposed in this paper has been developed combining the results of field surveys, carried along the section trace, and the interpretation of industrial seismic lines and well logs made accessible by the oil industry [see also Scrocca et al., 2005; 2007]. The resulting cross-section can be considered a conservative interpretation of the Southern Apennines structure down to depths of about 10 km (Fig. 4).

Unfortunately, a rigorous structural balancing cannot be properly carried out along the whole cross-section, since the plane strain deformation requirement is not fulfilled in some areas (e.g. strike-slip tectonics or motions out of the plane of section). However, balancing techniques (e.g., key-bed lengths balancing) have been applied where possible in some portions of the cross-section. In this way, the pre-deformational extent of the sedimentary cover (e.g., Apennine platform and Lagonegro basin) or the shortening at top Apulian carbonates horizon have been estimated.

The cross-section will be described starting from its north-eastern edge and then moving towards the Tyrrhenian side. In this description two main structural and geological elements will be considered: the Apulian carbonate platform and the allochthonous units.

Apulian Carbonate Platform

Moving from the foreland toward the foredeep, the top Apulian carbonates horizon (Sella et al., 1988; Nicolai and Gambini, 2007), easily identifiable from seismic reflection and well data (Fig. 5), is characterised by an increasing dip of the regional monocline (Mariotti and Doglioni, 2000) that highlights the flexural geometry of the Apulian Platform (Royden et al., 1987).

Both below the foredeep and in some sectors of the Southern Apennines thrust belt, another deeper, strong reflector can be recognised on seismic reflection data (Roure et al., 1991; Mazzoli et al., 2000). This horizon, generally subparallel to the top of the Apulian carbonate, is interpreted as the bottom of the Apulian carbonates and attributed to the acoustic impedance contrast between the upper Triassic dolomites and the underlain Permo-Triassic clastic deposits (Ricchetti et al., 1988; Mazzoli et al., 2000; Patacca and Scandone 2007a).

At a regional scale the Apulian Platform shows an almost constant time-interval thickness of about 2.4 s TWT below the foredeep domain. Assuming an average velocity for the Apulian carbonates of about 6000 m/s, this time thickness corresponds in depths to about 7400 m.

In the proposed cross-section the Late Triassic-Miocene Apulian Carbonates have been represented with a simplified stratigraphy, characterised by an average thickness of about 7400 m. A constant thickness of 1500 m has been assumed for the underlying middle Triassic-upper Permian deposits, notwithstanding their recognised syn-rift nature (Merlini et al., 2000; Patacca and Scandone 2007a), due to the lack of information about their lateral regional thickness variations.

Moving westward, along the axial zone of the Southern Apennine thrust belt and below the Lagonegro and Molise allochthonous units (described in the next section), Mesozoic to Tertiary shallow water carbonates were penetrated by several wells. These carbonate deposits, according to their facies and to the age of the stratigraphically overlying foredeep deposits, can be interpreted as portions of the western side of the Apulian carbonate Platform. Available seismic reflection data indicate that these Apulian carbonates are part of imbricated units, forming a buried antiformal stack (Mostardini and Merlini, 1986; Casero et al., 1988; Mazzoli et al., 2000; Menardi Noguera and Rea, 2000; Patacca and Scandone, 2001). A sole thrust separates the antiformal stack from the relatively undeformed part of the Apulian domain, while another major thrust represent the boundary with the overlying allochthonous units (Figs. 4 and 5).

Unfortunately, as often happens in complex thrust belts, seismic reflection data provide a relatively good definition only of the hanging wall of the thrust units while both the thrust faults and their footwalls, and the deeper horizons (e.g., the bottom Apulian carbonates), remain generally poorly imaged (e.g., Fig. 5). Moreover, the westward extension at depth below the Apennine carbonate platform of the Apulian imbricate units is a matter of debate (e.g., compare interpretations of Menardi Noguera and Rea, 2000 vs. Mazzotti et al., 2000).

Figure 5. Central segment of the CROP-04 seismic profile

Central segment of the CROP-04 seismic profile

Central segment of the CROP-04 seismic profile - Detail of the central part of the CROP-04 seismic profile (location in figure 1) showing the main structural features of the allochthonous units. Horizons and faults are based on the integrated interpretation of well data and industrial seismic reflection profiles available in the area (modified after Scrocca et al., 2007).

In the cross-section presented in figure 4, the structural setting of the Apulian unit has been reconstructed starting from the relatively well constrained hanging wall geometry. Then, thrust faults and related footwalls have been modelled assuming a minimum displacement criterion (see also Scrocca et al., 2005 for details). The main thrusts likely offset both the bottom of the Apulian carbonates and the underlying lower Triassic-upper Permian deposits (e.g., Mazzoli et al., 2000; Shiner et al., 2004).

Below the Apennine platform and Lagonegro thrust sheets, the possible presence of a thrust unit with Apulian affinity has been interpreted following the interpretation of seismic facies on the CROP-04 seismic reflection profile (see also Mazzotti et al., 2000; Scrocca et al., 2007; Patacca and Scandone, 2007a).

Figure 6. Western segment of the CROP-04 seismic profile

Western segment of the CROP-04 seismic profile

Figure 6. Western segment of the CROP-04 seismic profile - Detail of the western part of the CROP-04 seismic profile (location in figure 1). In this interpretation the Lagonegro units and the Apulian platform extend westward below the Apennine carbonate units (modified after Scrocca et al., 2007).

In this reconstruction, a conservative estimate of about 20 km of shortening (mainly accommodated by thrust faults affecting the top Apulian horizon) has been evaluated, without taking into account pressure solution phenomena possibly affecting the Apulian carbonate. It should be noted that, since the position of the footwall cut-offs for both the top and the bottom Apulian horizon for the two westernmost ramp anticlines is largely unconstrained, larger shortening may not be ruled out.

The onset of contractional deformation in the inner portion of the Apulian domain likely started at the end of the Early Pliocene (e.g., Cello and Mazzoli, 1999) while the main tectonic phases affected this domain during the Late Pliocene-Early Pleistocene (Menardi Noguera and Rea, 2000; Sciamanna et al., 2004).

Allochthonous units

The Tertiary basinal deposits of the Molise Unit (Tufillo-Serrapalazzo and Daunia units sensu Patacca et al., 1992b) are the easternmost and more advanced thrust sheets outcropping in the Southern Apennines thrust belt. The same units represent also the lowest thrust sheets resting above the Early Pliocene deposits overlying the Apulian carbonates (Figs. 4 and 5).

Available subsurface information allows an estimation of the cumulative forward displacement of the allochthonous nappes occurred in the Late Pliocene-Early Pleistocene. A first estimate has been provided by Patacca and Scandone (2001). According to these authors, the nappe advance occurred in two phases (from 3.70 to 3.30 Ma and from1.83 to 1.50 Ma) with at least 30 km of displacement. A second assessment has been proposed by Sciamanna et al. (2004) who have calculated almost 40 km between 3.57 and 0.66 Ma.

Allochthonous units, made up by lithologically monotonous basinal sequences such as the so called “Argille Varicolori” (Varicoloured Shale) outcrop further west. These units, often of unknown age due to the lack of diagnostic fossils, have been attributed to different paleogeographic domains. In the interpretation proposed in figure 4, the large majority of these outcrops have been interpreted as the Late Cretaceous-Early Miocene detached upper portion of the Lagonegro basin, represented by the Sannio Units (Patacca and Scandone, 2007b and references therein).

The Lagonegro units crop out along the axial part of the belt. Available well and seismic reflection data (e.g., Patacca, 2007; Scrocca et al., 2007) document a very complex structural setting, which will be analysed in detail in a following section.

In the south-western part of the cross-section (Fig. 6), the Sicilide units thrust over the Apennine shallow water carbonates, which in turn tectonically overlay the Lagonegro units.

The Apennine carbonate sequence is characterised by a transparent seismic facies, about 1.8-2 s TWT thick that corresponds to about 5000 m in depth, which rests above a very reflective and well stratified seismic facies, less than 1 s TWT thick (Fig. 6). This facies, well known from industrial seismic lines and clearly recognisable also on the western side of the CROP-04 profile, has been interpreted by Menardi Noguera and Rea (2000) as the seismic evidence of a huge slice of Paleozoic basement. However, if well data and the whole seismic image provided by the western side CROP-04 profile are considered, a different interpretation can be proposed (e.g., Mazzotti et al., 2000; Scrocca et al., 2005; Patacca and Scandone, 2007a).

The reflective and well stratified seismic facies has been penetrated by some deep exploration wells (e.g. Contursi 1 well and S. G. Magno 1 wells; Patacca, 2007) where it resulted to be associated to Lagonegro units. Apulian carbonates have been also documented below the Lagonegro units.

Both the stratified seismic facies and the horizon associated to the top Apulian carbonates deepen westward below the Apennine platform thrust sheets as clearly recognisable on the CROP-04 seismic data (Fig. 6). Moreover the CROP-04 profile, due to the higher penetration with respect to industrial seismic reflection data, provides a further support to this interpretation. Across the SW end of the CROP-04 line, at about 8 s TWT, a strong seismic event can be observed that could be interpreted as a near bottom Apulian carbonates reflector.

Several cases of high-angle normal faults, related to both extensional and strike-slip tectonics widely documented by both seismological data and surface geology, can be also observed across the western side of the CROP-04 profile (Figs. 4 and 6). Normal faults NW-SE trending affects Monte Marzano where they reflect the present day extensional tectonic field responsible for the 1980 Irpinian earthquake (Pingue et al., 1988). The Alburno-Cervati massif is delimited on each edge by WNW-ESE sub-vertical, strike-slip, fault systems active in the Late Pliocene (Ascione et al., 1992; Berardi et al., 1996). Also the western side of the M. Soprano ridge is displaced by a major fault system.