1 Introduction
The southern Apennines fold and thrust belt results from the convergence between the Africa-Apulian and European plates since Late Cretaceous time (Mazzoli and Helman, 1994, and references therein). The Apennine orogen comprises several superposed tectonic units characterized by Mesozoic-Tertiary shallow water to slope-facies carbonates (Apennine and Apulian carbonate platforms) and pelagic basin (Lagonegro) successions, resulting from the deformation of the Apulian continental paleomargin, as well as unconformably overlying Miocene-Pliocene wedge-top and foredeep basin deposits (Bonardi et al., 2009; Mazzoli et al., 2008). The orogenic pile is tectonically covered by remnants of an accretionary wedge (including the Nord-Calabrese and Parasicilide Units in Fig. 1) tectonically superposed onto the Apennine Platform domain in Early Miocene time (Bonardi et al., 2009; Ciarcia et al., 2009). The ‘Internal’ Units (Bonardi et al., 1988; Ciarcia et al., 2009) which include, besides the Nord-Calabrese and Parasicilide Units, the Frido and Sicilide Units (not exposed in the area of the present study), consist of sedimentary successions derived from oceanic (Neothethys) to thinned continental (Apulian) crust floored basins (Ciarcia et al., 2009). The Nord-Calabrese Unit and the metamorphic (HP-LT) Frido Unit also include remnants of the magmatic oceanic basement (Bonardi et al., 1988). All of these units, that were accreted as a result of NW-dipping subduction (Knott, 1987), are unconformably overlain by Miocene wedge-top basin deposits (Cilento Group, Monte Pruno, Albidona and Monte Sacro Fms) (Amore et al., 1988; Ciarcia et al., 2009; Selli, 1962). The Parasicilide Unit (or Terreni ad Affinità Sicilide Bonardi et al., 1988) crops out from the Sele River Valley (Ciarcia et al., 2009) to the Torrente Pietra Valley (Fig. 1), extending southeast of our study area, as also does the Nord-Calabrese Unit outside the Cilento area shown in Fig. 1 (Bonardi et al., 1988). The aim of this paper is to provide, for the first time, a structural analysis of both the Parasicilide and Nord-Calabrese Units cropping out in the Cilento area. The study area comprises the Castelnuovo Cilento village area, where the Parasicilide Unit crops out in a tectonic window exposed in the footwall to the Nord-Calabrese Unit (APAT, 2005; Cammarosano et al., 2000), and the areas around Pioppi and Punta Telegrafo (Figs. 1 and 2). The results of this study also provide new insights into the Miocene geodynamic evolution of the ‘Internal’ Units in the southern Apennine sector of the Neotethys realm.
Geological sketch map of the Cilento area (after Bonardi et al., 2009, modified).
Carte géologique du Cilento, d’après Bonardi et al. (2009), modifiée.
Geological sketch map and cross section of the Castelnuovo Cilento area (after APAT, 2005, modified).
Carte et coupe géologique de Castelnuovo Cilento, d’après APAT (2005), modifiée.
2 Geologic setting
The Parasicilide Unit, otherwise known as Castelnuovo Cilento Unit in the Cilento area (Cammarosano et al., 2000), includes four formations (Ciarcia et al., 2009). These are (from bottom to top; Fig. 3):
- (i) micaceous sandstones, clays, shales and marls of the Postiglione Fm;
- (ii) marly limestones, marls and calcarenites of the Monte Sant’Arcangelo Fm;
- (iii) marls and whitish limestones of the Contursi Fm;
- (iv) sandstones of the Arenarie di Albanella Fm.
Stratigraphic sketch of the basin successions cropping out in the Cilento area.
Schéma stratigraphique des successions de bassin affleurent dans la région de Cilento.
The ascertained age of these deposits ranges between the Middle Eocene and the Burdigalian (Ciarcia et al., 2009); however, the lower part of the succession could be as old as Upper Cretaceous (Bonardi et al., 1988; Ciarcia et al., 2009) (Figs. 2 and 3). The thickness of whole succession is about 800–1000 m.
The Nord-Calabrese Unit is formed by a Lower Jurassic–Burdigalian? (Bonardi et al., 2009) ophiolite-bearing succession characterized, from bottom to top, by pillow lavas, slates, quartz-arenites, limestones and cherts of the Timpa delle Murge Fm, black shales and slates of the Crete Nere Fm, and finally limestones, marls and sandstones of the Saraceno Fm. The Crete Nere and Saraceno Fms, the only two formations of the Nord-Calabrese Unit cropping out in the Cilento area (Fig. 3), represent a dominantly siliciclastic and calciclastic succession deposited on oceanic crust (Neotethys) during both passive margin and younger foredeep basin stages (Bonardi et al., 1988; Bonardi et al., 2009).
The Cilento Group (Fig. 3) consists of an Uppermost Burdigalian to Serravallian, dominantly siliciclastic succession unconformably overlying previously deformed ‘Internal’ Units (Bonardi et al., 1988). This wedge-top basin succession includes the Pollica and San Mauro Fms both passing, laterally and southward, to the Torrente Bruca Fm (Amore et al., 1988) and, southeast of the area shown in Fig. 1, to the Albidona Fm (Selli, 1962). In the Cilento area the top of the succession (Fig. 3) consists of unconformable coarse-grained deposits of the Upper Tortonian Monte Sacro Fm (Selli, 1962).
3 Tectonic setting and structural analysis
3.1 Parasicilide Unit
In the study area, the Parasicilide Unit crops out – in a tectonic window – in the footwall to the Nord-Calabrese Unit. The tectonic contact separating the two units is not well exposed, therefore hindering a complete and detailed kinematic analysis; however, field relationships clearly show the tectonic superposition (Fig. 4a). The Parasicilide Unit is affected by an inhomogeneous deformation with strain localization occurring especially in the pelitic layers. Often the succession shows a chaotic appearance as a result of fracturing and variable degree of disruption of the competent beds. This is probably due to the fact that, during the superposed deformation events, the temperature was not high enough to allow the rocks to flow in a fully ductile fashion.
Examples of structural features in the Parasicilide (a to d) and Nord-Calabrese (e to g) Units. (a) North-directed view of the Castelnuovo Cilento tectonic window. (b) Superposition of conjugate second stage kink folds (F2PS) on first stage isoclinal fold (F1PS) (Salento). (c) F2PS kink fold (Castelnuovo Cilento). (d) Superposition of third stage fold (F3PS) on isoclinal F1PS fold (Castelnuovo Cilento). (e) Superposition of F2NC folds on F1NC isoclinal fold (Punta Telegrafo). (f) F2NC ptigmatic folds with crenulation cleavage (S2NC) (Punta Telegrafo). (g) Thrust fault, showing minor displacement, associated with third stage folds (Pioppi).
Exemples de structures de déformation dans les Unités Parasicilide (a–d) et Nord-Calabrese (e–g). (a) La fenêtre tectonique de Castelnuovo Cilento (vue vers le nord). (b) Superposition de plis « kink » conjugués F2PS sur un pli isoclinal F1PS (Salento). (c) Pli « kink » F2PS (Castelnuovo Cilento). (d) Superposition d’un pli F3PS sur un pli isoclinal F1PS (Castelnuovo Cilento). (e) Superposition des plis F2NC sur un pli isoclinal F1NC (Punta Telegrafo). (f) Plis ptigmatiques F2NC et clivage de crénulation S2NC (Punta Telegrafo). (g) Chevauchement avec modeste déplacement associé à la troisième phase de plissement (Pioppi).
The first deformation episode produced chevron to sinusoidal isoclinal folds (F1PS) (Fig. 4b). An axial plane cleavage in pelitic rocks and a spaced, convergent cleavage in more competent lithologies are weakly developed. The isoclinal folds are refolded by open to tight kink folds (F2PS), generally displaying conjugate axial planes (Fig. 4b and c). Locally the F2PS folds are related to thrust faults showing minor displacements. The interference pattern between F1PS and F2PS folds (Fig. 4b) is comprised between types 2 and 3 of Ramsay's classification (Ramsay, 1967). Both F1PS and F2PS fold sets are refolded by gentle to open, rounded folds (F3PS) developed mainly in the steep F2PS limbs and showing sub-horizontal to moderately dipping axial planes (Fig. 4d). F1PS fold hinges (A1PS) display a girdle distribution around a NE-SW striking, sub-vertical plane (Fig. 5a), whereas poles to axial planes (AP1PS) are scattered (Fig. 5b). F2PS fold hinges (A2PS) are characterized by a dominant north-south trend (Fig. 5c), the related axial planes (AP2PS) showing both west and east dip directions and variable angles of dip (Fig. 5d). F3PS fold hinges (A3PS) are generally NW-SE trending (dominantly gently to moderately plunging to the south-east; Fig. 5e), while the related axial planes (AP3PS) tend to be sub-horizontal to moderately dipping in various directions (with a dominant north-east gentle dip; Fig. 5f).
Orientation data (lower hemisphere, Schmidt net) for main deformation structures in the Parasicilide (a to f) and Nord-Calabrese (g to l) Units. A1, A2, A3: axes of first, second and third folding stage, respectively; AP1, AP2, AP3: axial planes of first, second and third folding stage, respectively. PS: Parasicilide Unit; NC: Nord-Calabrese Unit.
Projections stéréographiques des principales structures de la déformation dans l’Unité Parasicilide (a–f) et l’Unité Nord-Calabrese (g–l). A1, A2, A3: axes, respectivement, de la première, deuxième et troisième phase de plissement; AP1, AP2, AP3 planes axiales, respectivement, de la première, deuxième et troisième phase de plissement. PS : Unité Parasicilide ; NC : Unité Nord-Calabrese.
3.2 Nord-Calabrese Unit
Two outcrop areas of the Nord-Calabrese Unit have been analyzed in this study: those of Punta Telegrafo and Pioppi (Fig. 1). In these areas the lower part of the Saraceno Fm, characterized by calcareous-pelitic turbidites, crops out. As it occurs for the Parasicilide Unit, the Nord-Calabrese Unit is also characterized by the superposition of three fold sets (F1NC, F2NC and F3NC) and associated planar and linear structures. In the pelitic layers, the main foliation consists of a slaty cleavage (S1NC) sub-parallel to F1NC fold axial planes (AP1NC). In the competent arenitic beds a spaced, disjunctive cleavage is present. F1NC folds display geometries ranging from tight to isoclinal (Fig. 4e and f). F2NC folds are characterized by larger interlimb angles with respect to preexisting F1NC folds (Fig. 4e and f). Fold interference patterns range from perfectly coaxial (type 3; Ramsay, 1967) for the Crete Nere Fm to moderately non-coaxial (intermediate 2–3 type; Fig 4e and f) for the Saraceno Fm. A crenulation cleavage (S2NC) occurs in the pelitic layers (Fig. 4f). F3NC open to tight folds are often associated with SW verging thrust faults showing moderate displacements (Fig. 4g). F1PS fold hinges (A1PS) show a girdle distribution characterized by dominant steep plunges (Fig. 5g), whereas poles to axial planes form two clusters of dominantly south-east and north-west steeply dipping planes (Fig. 5h). F2NC fold hinges are sub-horizontal and NE-SW trending (Fig. 5i), while F2NC fold axial planes (AP3NC) mainly dip to the north-west (Fig. 5j). F3NC fold hinges dominantly plunge moderately to the NW (Fig. 5k), whereas F3NC axial plane poles (AP3NC) show a girdle distribution around a NE-SW oriented great circle (Fig. 5l).
4 Discussion
The reconstructed tectonic setting of the study area is characterized by the tectonic superposition of the Nord-Calabrese Unit onto the Parasicilide Unit (Fig. 1). The tectonic contact clearly truncates folds, both in footwall and hanging-wall successions, thus suggesting that it postdates at least the early folding events in both units (as it may be seen in Fig. 2). This contact could either represent a late, out of sequence thrust fault, or a late low-angle normal fault related to horizontal extension affecting the previously overthickened and uplifted accretionary wedge. Early exhumation of the Parasicilide Unit in various sectors of the Cilento area (e.g., Monte Sacro and Monte Centaurino, Fig. 1) where this unit is unconformably overlain by the Cilento Group strata (Bonardi et al., 1988) is consistent with tectonic omission produced by extension. Accretionary wedge uplift was probably related to footwall imbrication involving the inner (i.e. southwestern) portion of the Apennine Platform carbonates. This process could also have been responsible for the development of regional, broad NE-SW trending folds in the Alento River area (Fig. 1) that refolded the whole tectonic pile overlying the Apennine Platform carbonates and also appears to control the outcrop pattern of the Parasicilide Unit in the tectonic window of Castelnuovo Cilento.
The structural analysis allowed us to unravel three main folding stages in both analyzed tectonic units. The first two folding events are weakly non-coaxial for the Parasicilide Unit, indicating NW-SE and west-east shortening, respectively, whereas they are broadly coaxial and resulting from NW-SE shortening for the Nord-Calabrese Unit. The girdle distributions of F1PS and F1NC axes (Fig. 5a and g) can be interpreted as two branches of small circles (flexural slip fold; Ramsay and Huber, 1987; p. 483) indicating a rotation of F1PS and F1NC axes around the F2PS and F2NC axes, respectively. The original angles between the two mean fold axes, of ca. 45° for the Parasicilide Unit and ca. 10° for the Nord-Calabrese Unit, have been estimated – as a first approximation and taking into account the limitations of the method – by measuring the angle between the mean directions of horizontal F1PS–F1NC axes and F2PS–F2NC axes, respectively. These two folding events can be related to horizontal shortening of the Nord-Calabrese and Parasicilide successions as they were accreted into the subduction complex. NW-SE shortening of the ‘Internal’ Units successions is also recorded in the Sele River Valley (Ciarcia et al., 2009). Furthermore, in northern Calabria (south-east of the area shown in Fig. 1) the innermost portions of the Apennine Platform domain record top-to-the-east-northeast thrusting (Iannace et al., 2007) in Burdigalian time (Vitale and Mazzoli, 2009).
The third folding event unravelled in both the Nord-Calabrese and Parasicilide Units can be correlated, in terms of fold style and orientation, with that characterizing the stratigraphically overlying Cilento Group Fm (Zuppetta and Mazzoli, 1997). This folding stage can be interpreted as a result of late shortening related to the overthrusting of the accretionary wedge, together with the unconformable wedge-top basin deposits, onto the Apennine Platform domain. Based on the youngest deposits of the Cilento Group involved in folding, this deformation stage can be dated as post-Serravallian.
The first folding stage has generated, at a regional scale, a train of south-east-vergent overturned folds with parasitic folds in the Parasicilide Unit, as shown in the cross section of Fig. 2. These structures have been refolded by F2PS folds and subsequently crosscut by low-angle tectonic contacts (thrusts or low-angle normal faults) presently separating the Nord-Calabrese Unit in the hanging-wall from the generally overturned succession of the Parasicilide Unit in the footwall (e.g., Castelnuovo Cilento tectonic window). Dramatic fold truncation suggests significant tectonic excision of the Parasicilide footwall succession. Deformation stages for the various analyzed successions, their interpreted correlation and chronology are summarized in Table 1.
Corrélation et âge des phases de la déformation dans les unités tectoniques étudiées.
| Burdigalian | Post-Serravallian | |||
| Parasicilide Unit | DPS1 | DPS2 | DPS3 | |
| Nord-Calabrese Unit | DNC1 | DNC2 | DNC3 |
In order to insert the envisaged structural evolution in a more general framework, a tentative reconstruction of the geodynamic evolution of the Apennine accretionary wedge between the Late Aquitanian and the Early Langhian is provided (Fig. 6). In a first stage (Fig. 6a), the Nord-Calabrese succession is covered by the foredeep deposits of the Saraceno Fm (sandstones). Subsequently the Nord-Calabrese Unit is accreted into the accretionary wedge and deformed by overall NW-SE shortening (DNC1) developing isoclinal FNC1 folds (Fig. 6b). During this stage foredeep sedimentation occurs on top of the Parasicilide domain with the deposition of the Arenarie di Albanella Fm. In Burdigalian time (Fig. 6c) the Nord-Calabrese Unit experiences continued NW-SE shortening (DNC2) and the Parasicilide Unit is accreted into the wedge (DPS1), leading to the development of FPS1. During the Burdigalian (Fig. 6d) the inner sector of the Apennine Platform carbonate domain is overthrusted by the accretionary wedge, while sedimentation of the Bifurto Fm (Selli, 1957) occurs in the newly developed foredeep. The Parasicilide Unit is deformed by roughly east-west shortening (DPS2) and FPS2 folds are developed. Later (Fig. 6e) the accretionary wedge undergoes horizontal stretching and vertical shortening, probably due to previous overthickening and footwall imbrication at the expenses of the tectonically underlying Apennine Platform carbonate succession, producing uplift of the ‘Internal’ Units. Low-angle extensional detachments associated with synorogenic extension favor the development of accommodation space in wedge-top basin depocentres (Fig. 6f) that are filled by the Cilento Group deposits.
Cartoon showing reconstructed geodynamic evolution of the ‘Internal’ Units in the studied segment of the southern Apennines.
Schéma de la reconstruction de l’évolution géodynamique des Unités « Internes », dans le segment de l’Apennin méridional étudié.
5 Conclusions
This work, representing the first comprehensive structural analysis of the Nord-Calabrese and Parasicilide Units in the Cilento area, unravelled a complex deformation history for these units, within the general framework of the geodynamic evolution of the Miocene Apennine accretionary wedge. The Parasicilide succession has been overthrusted by the Nord-Calabrese Unit in the Burdigalian and then accreted into the wedge. Shortening within the Nord-Calabrese and Parasicilide Units is recorded by three-fold sets in both units, recording NW-SE (DNC1 and DNC2) and then NE-SW (DNC3) for the Nord-Calabrese Unit and NW-SE (DPS1), west-east (DPS2) and then NE-SW (DNC3) shortening for the Parasicilide Unit. Accretionary wedge overthickening and uplift, probably associated with thrusting involving carbonate footwall units, was followed by wedge thinning, which produced the accommodation space for the sediments of the Cilento Group in a series of wedge-top depocentres.
Acknowledgements
The paper benefited from detailed and constructive reviews by A. Michard and two anonymous referees. The authors wish to thank Fabio Laiena, Francesco Mittiga, Angelo Noviello and Francesco Tramparulo for the help in the fieldwork.