Journal of the Virtual Explorer

Taking into account the stratigraphic constraints previously described, we have attempted to reconstruct a kinematic-structural scheme for the central Apennines (Plate. 4). In this scheme, the time interval in which a particular sector has been involved in the chain is indicated in full color. This kind of data is provided by the age of the piggyback deposits recognized in the study area, which are represented in the kinematic-structural scheme with the half tone of the color used for that tectono-sedimentary event. Superimposed dots of different colors represent the relationship of the foredeep outcrops to different tectono-sedimentary events. The colored superimposed dots, which show the foredeep deposits of a particular tectono-sedimentary event, generally are drawn above a full colored area which indicates the time-interval in which that foredeep domain has been involved in the chain. In those sectors in which the stratigraphical-structural analysis has shown out-of-sequence reactivation (Olevano-Antrodoco-Sibillini Mts areas and Gran Sasso front), colored bands show the out-of-sequence reactivation age, superimposed on the full color relative to the first phase of the mountain building.

The proposed kinematic-structural map (Plate 4) prominantly displays a first order structural discontinuity with NNE-SSW trend that cuts the central Apennine chain. This discontinuity allowed the independent evolution of the northern Apennine and the central Apennine orogenic systems (Fig. 11), releasing the two sectors in which the compressive deformation migrated towards the Adriatic foreland with different rates (Vai, 1987; Cipollari and Cosentino, 1996). In this paper we wish to underline the correct displacement of the Apennine orogenic system in correspondence with this discontinuity. In the proposed scheme, this displacement is evidenced by the non-alignment of coeval sectors of the Apennine accretionary wedge.

Figure 11. Neogene kinematic evolution of the central-northern Apennines

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Kinematic scheme showing the different migration between 18 and 3.5 Ma for the central and northern Apennine orogenic systems. Different velocities of propagation for the northern and central Apennine orogenic systems are guaranteed to happen for the occurrence of a lithospheric discontinuity separating northern and central Apennines.

An additional point that emerges from the analysis of this kinematic model is the prominent Adriatic foreland propagation of the Apennine compressive deformation, already evidenced by several authors (Bally et al., 1988; Endignoux et al., 1989; Bigi et al., 1991; Sage et al., 1991; Cipollari and Cosentino 1992, 1995; Patacca et al. 1992b; Cavinato et al., 1994; Patacca et al., 2008). In this paper, the timing of the propagation of the Apennine compressive front and the structures involved in the different deformational phases is more accurately shown.

Unfortunately, for the northern sector of the examined orogen, the stratigraphical constraints are insufficient to reconstruct the timing of the foreland propagation, because of the limited extent or the absence of syn-tectonic deposits linked to the evolution of the orogenic system. Deposits that do provide information are limited to the inner Marnoso-Arenacea outcrops (upper Burdigalian-lower Serravallian) (Valle dell’Aia, Martani Mts, Mt Subasio, etc.), the Vallocchia-Castelmonte marl outcrops, the Belvedere Fm. (middle-upper Serravallian) (Cipollari and Cosentino, 1997) and the terrigenous deposits of the Camerino basin (Calamita et al., 1979) (Middle Tortonian-Messinian) and of the Laga Fm (Messinian). Owing to this, the reconstruction of the evolution of the orogenic system for this portion of the chain is not complete, as we currently lack information about the Serravallian foredeep and have scarce data concerning the Tortonian foredeep.

In contrast, in the southern sector of the study area, extensions of the syn-tectonic deposits both of the foredeep and of the piggyback basins enable us to create a more detailed reconstruction of the evolution of the Apennine orogenic system from the Late Tortonian tectono-sedimentary event.

In its outer portion, the Tortonian Apennine chain included the Volsci chain (Lepini Mts, Ausoni Mts and Aurunci Mts), at the front of which the Latina Valley foredeep basin was developed (Cipollari and Cosentino, 1995). During the building of the chain, instead of coarse clastic material being routed exclusively to the foredeep, several satellite basins received the coarse sediment (Gavignano and Gorga Units, Alberti et al., 1975) (Cipollari, 1995; Cosentino et al., 2003).

During the lower Messinian tectonic phase, with the migration of the orogenic system towards the Adriatic foreland, the whole Ernici-Simbruini sector was involved in the chain.

The Monti della Laga area, as far as the Val Roveto-Valle del Salto-Tagliacozzo and at least the western portion of the Marsica domain, were part of the Messinian foredeep. In the literature there are different opinions about the terrigenous deposits cropping out on the Montagna Grande area: were they deposited in the Messinian foredeep or in a younger foredeep basin? Following Patacca et al. (1992b), the flysch cropping out in this area is related to a younger foredeep, developed in a time interval corresponding to the Messinian Lago-Mare-Lower Pliocene p.p., while according to Corrado et al. (1995), on the basis of the optical indicators of maturity in the organic matter dispersed in the terrigenous sediments cropping out in the same area, the Montagna Grande foredeep basin should have evolved during the Messinian. This latter opinion is reported also by Ghisetti et al. (1993). The existence of a siliciclastic post-gypsum deposit in the Anversa degli Abruzzi sector would support the Patacca et al. (1992b) hypothesis, but at present the stratigraphical relation of this deposit and the Montagna Grande carbonatic succession is still under discussion.

On Plate 4 the siliciclastic deposits cropping out in the Anversa degli Abruzzi and Valle del Tasso-Sagittario sectors have been considered as belonging to a Messinian Lago-Mare/Early Pliocene foredeep. Then on a regional scale, along a SW-NE transect, the kinematic evolution of the Volsci chain, the Valle Latina, the Simbruini-Ernici Mts, the Val Roveto and the Marsica region, up to the Maiella more external domain, followed a piggyback sequence, with a NE-ward propagation of the Apennine frontal thrust. Except for some out-of-sequence reactivations that affected the central Apennine chain, it is possible to recognize a migration of the orogenic system towards the Adriatic foreland following the activation of thrusts in more external position. In the proposed model, the new element of the chain, which accretes the Apennine orogenic wedge during each tectono-sedimentary event, is built up simultaneously (Cipollari and Cosentino, 1992), developing a system of synchronous thrusts within the accreted wedge. Therefore, the accreted chain element is limited externally by its frontal thrust, activated for the first time during that tectono-sedimentary event, while on the inner side it is bounded by the frontal thrust of the previous tectono-sedimentary event. This inner bounding thrust generally could remain active during the younger tectonic event.

In the central Apennines, the out-of-sequence activity of regional thrust systems, such as the frontal thrust of the Olevano-Antrodoco-Sibillini Mts (Cipollari and Cosentino, 1992) and that of the Gran Sasso chain (Ghisetti and Vezzani, 1991), is superimposed on the structures of an already deformed chain. In those areas, the Olevano-Antrodoco-Sibillini Mts thrust system reactivated a chain sector already built during the previous Early Messinian tectonic phase. The thrust system that characterizes the Gran Sasso chain transversely cuts the N-S structures linked to the Acquasanta and Montagna dei Fiori-Montagnone ridge, built up during the Messinian Lago-Mare-Early Pliocene event. The age of the activation of the first thrust system is constrained by the age of the top of the Laga Fm. (Upper Messinian, post salinity crisis; Cantalamessa et al., 1982; Centamore et al., 1990; 1991; 1992; 1993).

These out-of-sequence re-activations were accompanied by rotation of the more superficial structural units involved in the deformation, as shown by palaeomagnetic analyses carried out in the two sectors (Ghisetti et al., 1992; Mattei et al., 1992; 1994; 1995; 1999; Speranza et al., 1997b; Satolli and Calamita, 2008).

In the northern area of the central Apennines, this deformational event reactivated the area between the Olevano-Antrodoco-Sibillini Mts thrust system, to the east, and the more internal tectonic units of Sabini Mts and Spoleto Mts to the west. In particular, on the Spoleto Mts, signals of out-of-sequence reactivation have been observed in the Belvedere and Vallocchia areas, where low-angle thrust surfaces cause the superposition of the Calcare massiccio Fm onto a tectonic unit consisting of either the Belvedere Fm (Decandia and Giannini, 1977) or the Vallocchia marls (Cipollari, 1995), both belonging to the stratigraphical succession of a piggyback basin (sensu Ori and Friend, 1984) (Cipollari and Cosentino, 1997). Moreover, in the whole Sabine portion of the Apennine orogen, evidence of multiple phases of deformation (re-folded meso-folds, macro- and meso-folds truncated by thrust surfaces, and multiple domains of striae on the same fault plane) has been observed (Cosentino, 1988; Mattei et al., 1986; Calamita et al., 1987; Cosentino and Parotto, 1988; 1989; 1992; Cosentino and Montone, 1991).

In the southern area of the central Apennines, evidence for a late Messinian reactivation of some inner structures of the Apennine chain have been recently found in the southern Latina Valley (Pasquali et al., 2007). In that area, thin thrust-sheets consisting of Early-Middle Miocene shallow-water carbonates are thrust onto siliciclastic deposits sedimented in a wedge-top basin. The timing of deformation can be discerned in the Monte San Giovanni Campano area, where the thrust tectonics affect clays with Messinian gypsum (Pasquali et al., 2007), suggesting a tectonic event younger than the Messinian salinity crisis.

Considering a single synchronous event for the reactivation of those internal areas of the Apennine chain in the northern sector, the younger stratigraphical constraints are provided by the more ancient age of the continental deposits of the Tiberino Basin, developed in a syn-extensional post-orogenic realm. Recently, the older stratigraphic succession of the Tiberino Basin has been recognized in the Fosso Bianco Unit (Basilici, 1992; Ambrosetti and Basilici, 1994) cropping out in the south-western branch of the basin. The age of the Fosso Bianco Unit, which according to Basilici (1992) and Ambrosetti and Basilici (1994) is between the Middle Pliocene and the upper part of the Early Pliocene, seems instead to have been deposited only during the Middle Pliocene (R. Pontini pers. comm.).

In the southern sector, this out-of-sequence thrusting is constrained by the occurrence in the subsurface of the southernmost Latina Valley of an Early Pliocene (top Zanclean) syn-rift basin (Pasquali et al., 2007).

Several geometrical and chronological constraints help pinpoint the age of the out-of-sequence thrusting in the Gran Sasso chain. The thrust tectonics are limited by the age of the siliciclastic deposits that conformably rest above the Cenozoic platform-to-basin carbonate succession in a foreland setting. These deposits show that the area was involved in a foredeep domain during the early Messinian (Ghisetti and Vezzani, 1990); Patacca et al., 1992b). As a consequence, the first orogenic deformation is related to a tectonic event younger than the early Messinian tectonic phase. A further constraint on the timing of the Gran Sasso out-of-sequence thrusting is given by the age of the Conglomerati di M. Coppe, which lay unconformably above already deformed units. The Conglomerati di M. Coppe have been dubitatively referred to the Messinian (Ghisetti and Vezzani, 1990). The authors do not provide any chronological reference for these conglomerates, but instead present an age for the arenaceous-pelitic succession resting conformably above those conglomerates. In the Mt Paradiso area, this arenaceous-pelitic succession is referenced, still with uncertainty, to a generic Messinian age, while a definite infra Pliocene age (Sphaeroidinellopsis zone) is provided for the Mt. Coppe sector, and a top Early Pliocene age (G. margaritae and G. puncticulata zones) is provided for the Colle dei Cavatori area (Ghisetti et al., 1993).

According to us, the sedimentary basin of the Conglomerati di M. Coppe and the overlying arenaceaous-pelitic succession was deposited in a thrust-top basin of the Apennine chain, therefore linked to the first phase in which this area was involved in the chain.

The kinematic model proposed in this paper for the Gran Sasso chain is slightly different from that suggested by Ghisetti and Vezzani (1990) and Ghisetti et al. (1993). The deformation of the EW Gran Sasso front should be related to a compressive event that occurred late in the Early Pliocene. We derive this conclusion after considering the Latium-Abruzzi area, containing the Gran Sasso chain, was a foredeep domain not only during early Messinian, but also up to the Late Messinian, having recorded the Messinian salinity crisis (Castorina et al., 1994). It is also noteworthy that conglomerate deposits in the Latium-Abruzzi area associated with the piggyback basins active during the Messinian Lago-Mare-Early Pliocene event (Le Vicenne, Colacicchi et al., 1967; Cipollari et al., 1999a; Mt. Mezzana, Praturlon, 1980) are very similar to the Conglomerati di M. Coppe, unconformably overlying a deformed substratum.

To summarize, the first mountain building event of the Gran Sasso chain must be related to a tectono-sedimentary phase younger than the event which formed the foredeep basin of Laga-Valle del Salto-Tagliacozzo-Marsica (early Messinian). The out-of-sequence activation of the Gran Sasso thrust front must be related to a younger tectono-sedimentary event. Therefore, the first compressional deformation that affected the area should be referred to the Messinian Lago-Mare-Early Pliocene event. During this time interval, all the Latium-Abruzzi domain was involved in the chain, and the deposition of the Conglomerati di M. Coppe occurred in a piggyback basin coeval with those of Le Vicenne and Mt Mezzana. On the basis of these time constraints, the out-of-sequence thrusting must be related to the tectonic event that occurred in central Apennines at the top of the Early Pliocene. This event caused the accretion of the chain with the more external units.