Geological framework

The Southern Apennines fold-and-thrust belt (Fig. 1) developed during Neogene and Quaternary times along an eastward-retreating west-directed subduction zone. Starting from Early Miocene the subduction retreat caused the progressive eastward migration of the foreland flexure, thrust fronts, and of the extensional backarc tectonics, which migrated from the Tyrrhenian Sea to onland intra-mountainous basins (e.g., Malinverno and Ryan, 1986; Royden et al., 1987; Patacca et al., 1990; Doglioni, 1991; Doglioni et al., 1999). The progressive propagation of the contractional deformation towards the foreland is clearly documented by the development and evolution of a series of eastward-younging foredeep basins and by the occurrence of several piggy-back basins that developed on top of the advancing allochthonous units (Patacca and Scandone, 1990, 2001). Starting from the middle Miocene, the tectonic accretion within the thrust belt has been contemporaneous with extensional tectonics along the Tyrrhenian margin which produced thinning of the internal sectors of the belt (Casero et al., 1988; Patacca et al., 1990; Cello and Mazzoli, 1999). During the Late Pleistocene, the subduction retreat appears to have slowed in response to the interference of the thick continental Apulian lithosphere with the front of accretionary prism (Doglioni et al., 1994).

During the subduction hinge rollback, the Meso-Cenozoic passive margin sedimentary cover of the subducting Apulo-Adriatic plate was offscraped and piled up to form the Apennines accretionary prism.

Due to the very complex geological setting of the Southern Apennines several, often conflicting, paleogeographical models have been proposed for the passive margin of the Adriatic plate (D’Argenio et al., 1975; Mostardini and Merlini, 1986; Casero et al., 1988; Sgrosso, 1988; Patacca et al., 1992a; Marsella et al., 1995; Menardi Noguera and Rea, 2000). In this study, a paleogeographic model that honours the available stratigraphic and structural data, at least in the sector crossed by the modelled cross-section, has been adopted.

The main units cropping out in the Southern Apennines (Figs. 1 and 3), from bottom to top in the thrust pile which corresponds to a east to west transect in the original paleogeography, are the following: i) the Apulian carbonate platform, ii) the Lagonegro-Molise basins, iii) the Apennine carbonate platform, and iv) the internal oceanic to transitional Liguride-Sicilide basinal domains (internal nappes).

The paleogeography of the region was controlled by Mesozoic extensional tectonics that led to the opening of the Ligurian-Piedmont (or Alpine Tethys) and East-Mediterranean oceanic domains. In the proposed model, the Liguride-Sicilide nappes represent remnants of the Ligurian-Piedmont oceanic domain whereas the Apennine and Apulian carbonate platforms and the intervening Lagonegro-Molise basin developed along the the Adria passive continental margin. The Mesozoic Lagonegro-Molise basin, likely located on thinned continental crust, may have represented the northern marginal part of the East-Mediterranean segment of the Neotethyan ocean (e.g., Ciarapica and Passeri, 2002, Stampfli et al., 2002 and references therein). However, it should be noted that the Apennine and Apulian carbonate platforms and the Lagonegro-Molise basin were originally located on contiguous segments of the same basement belonging to the Apulo-Adriatic plate (Fig. 3).

Figure 3. Southern Apennine stratigraphy

Southern Apennine stratigraphy

Adopted stratigraphic scheme. The Apennine and the Apulian shallow water carbonate platforms and the intervening Lagonegro-Sannio-Molise basin developed during the Mesozoic rifting and the subsequent passive continental margin evolution of the Apulo-Adriatic plate (modified after Casero et al., 1988).

The complete closure of the Neotethyan domain was achieved in the Southern Apennines in the Late Cretaceous to Early Miocene, following a stage of subduction of oceanic crust (Cello and Mazzoli, 1999). After the overthrusting of the Liguride-Sicilide units onto the Apulo-Adriatic plate Mesozoic passive margin, the sedimentary cover of the passive margin itself was progressively incorporated in the Southern Apennines accretionary prism through a series of thrusting events (e.g., Patacca and Scandone, 2001).

Apulian Carbonate Platform

This units is made up of shallow-water carbonates, 5000 to 7000 m thick, Upper Triassic-Miocene in age (Fig. 3). These carbonates crop out in Apulia region (Gargano, Murge, and Salento) and represent the pre-orogenic cover of the foreland area (Ricchetti et al., 1988). Upper Messinian and Pliocene deposits stratigraphically overlain the Apulian shallow water carbonates.

The Apulian carbonates rest on the top of Permian volcanoclastic deposits (e.g., Puglia 1 well), or on Ladinian-Carnian carbonate/terrigenous deposits (e.g., Gargano 1 well).

Lagonegro-Sannio and Molise Basinal Units

The characteristics of the Middle Triassic-Early Cretaceous Lagonegro portion of this basinal domain are well established, whereas the nature of the Late Cretaceous-Tertiary section is somehow still debated.

The Lagonegro stratigraphic succession (Fig. 3) is made up of the following four formations which evolve from the fluvial conglomerates and shallow water carbonates of the “Monte Facito” (Middle Triassic), to the “Calcari con Selce” (Late Triassic), “Scisti Silicei” (Jurassic), and “Galestri” (Early Cretaceous) deep water facies (among many others, Scandone, 1967, 1972; Wood, 1981; Miconnet, 1988).

The structural setting of the Lagonegro units has been defined at a regional scale in terms of two superimposed nappes (Scandone, 1972). These nappes have been named respectively Lagonegro type II the upper one (which shows more proximal depositional characteristics), and Lagonegro type I the lower one (with more distal facies). Doubts about this matching between thrust units and sedimentary facies have been raised by Carbone et al. (1991) and Mazzoli et al. (2001). Regardless, the primary geometry of the Lagonegro units was significantly modified in the Miocene-Pliocene by thrusting, breaching, and out-of-sequence processes which generated complex imbricates (described in detail in a following sections).

Concerning the palinspastic reconstruction of the Lagonegro domains, there is substantial agreement about an original position of the Lagonegro basin between the Apennine and Apulian Platforms. However, it should be noted that an internal provenance of the Lagonegro units has been also proposed by some authors (e.g., Marsella et al., 1995).

The upper portion of the Lagonegro units was detached from its Triassic-Early Cretaceous part and transported farther east. The so-called Sannio Unit likely represent the Late Cretaceous-Early Miocene section of the basin (Carbone et al., 1988; Carbone and Lentini, 1990; Patacca and Scandone, 2007b). Tertiary basinal deposits, completely detached from their original substratum and outcropping along the eastern edge of the Southern Apennines thrust belt, where they are usually named Molise units (Tufillo-Serrapalazzo and Daunia units sensu Patacca et al., 1992a and references therein), could represent the remaing easternmost portion of this basin. The Early Messinian age of the foredeep deposits belonging to the Molise units (Patacca et al., 1992b) documents the original external paleogeographic position of these units (i.e., east of the western carbonate platform and likely at the north-eastern margin of the Lagonegro–Molise basin).

Apennine Carbonate Platform

This unit (Fig. 3) is made up of a thick pile (up to 5000 m) of shallow-water carbonates Late Triassic-Early Miocene in age (Sartoni and Crescenti, 1961; Selli, 1957, 1962). In the area crossed by CROP-04 profile, this paleogeographic domain (also known as Western or Campano-Lucana Platform) includes tidal-flat and protected shelf-lagoon facies (Alburno-Cervati unit), platform-edge (M. Marzano) and slope facies (Monti della Maddalena). The carbonate section is overlain by condensed hemipelagic and siliciclastic deposits related respectively to the flexural sinking of this domain and to the onset of the following foredeep environment (Patacca et al., 1990).

All the thrust sheets derived from Apennine Carbonate Platform are generally detached along an intra-Triassic décollement from their Paleozoic substratum, which has never been reached by exploratory wells.

Internal Nappes

This group of nappes comprises sediments derived from internal domains (Fig. 3) which could be associated with the Ligurian-Piedmont branch of the Neotethyan Ocean. The following units have been recognised.

- Liguride units, Early Cretaceous to Early Miocene sequences with incorporated ophiolitic suites. It comprises both the metamorphic Frido Unit and the unmetamorphosed Cilento Unit (Ogniben, 1969; Knott, 1987; Bonardi et al., 1988; Monaco and Tortorici, 1995). The Frido Melange has been interpreted as a part of an accretionary prism built up during the Cretaceous subduction of the Tethys oceanic lithosphere (Knott, 1987, 1994).

- Sicilide units, Late Cretaceous – Early Miocene succession of basinal deposits (Ogniben, 1969). The provenance of the Sicilide units from a basinal domain located west of the Western Platform can be inferred from their geometric position, since the Sicilide units systematically overlie the Alburno-Cervati carbonates from the Cilento area to the high Agri valley. However, an external original position (i.e., east of the Apennine Carbonate Platform) was proposed in other studies (Mostardini and Merlini, 1986; Casero et al., 1988; Pescatore et al., 1988). A discussion on this topic can be found in the work of Menardi Noguera and Rea (2000).