Journal of the Virtual Explorer

The study area is located on a peri-cratonic region, which experienced several deformation events in response to the Neogene tectonic interaction between the European and African plates, leading to the peri-Mediterranean orogeny. In particular, during the post middle-Tortonian orogenic phases of the Apennines, the Adria microplate (D’Argenio and Horvath, 1984; Anderson, 1987) played a significant role. The boundary interaction between Adria and the surrounding continental plates controlled the evolution of the Adria-verging orogenic system. These plate interactions caused the building of both the Apennine and of the Dinaric segments of the peri-Mediterranean chain.

The post-middle Tortonian Apennine chain consists of tectonic units derived from the deformation of both Meso-Cenozoic shallow water limestones (carbonate platform domains), and Meso-Cenozoic deeper-water carbonates (slope and pelagic basin domains) (Fig. 9 and Plate 3). The structural setting of these tectonic units is similar to other post-collisional thrust-belts, and consists of basement and cover thrust-sheets developed in an ensialic context. The geometry of the chain, the diachronism of the eastward migrating foredeep basins in this area, and the different ages of the forethrusts (Fig. 10) are consistent with a regional foreland propagating model for the central Apennines (Bally et al., 1988; Endignoux et al., 1989; Sage et al., 1991; Cipollari and Cosentino, 1992, 1995; Patacca et al., 1992a; 1992b; Cavinato et al., 1994; Patacca et al., 2008).

Such a geodynamical setting controlled the origin and the evolution of several syntectonic sedimentary basins (foreland basin systems, sensu De Celles and Giles, 1996) which developed during the evolution of the Apennine chain (Ori and Friend, 1984; Patacca and Scandone, 1989; Boccaletti et al., 1990; Cosentino et al., 2003; Patacca et al., 1992a; 1992b; 2008).

Figure 9. Structural sketch map of the Apenninic chain

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Structural sketch map of the central-southern Italy. 1) Plio-Quaternary marine and continental deposits; 2) Quaternary volcanics; 3) Post-Burdigalian fordeep siliciclastic deposits (undifferentiated ages); 4) “Flysch Rosso” belonging to the Sicilide, Lagonegro and Molise units (Oligocene-Upper Cretaceous); 5) Meso-Cenozoic carbonate platform sequences and carbonatic ramp deposits (Middle Miocene-Upper Triassic); 6) Meso-Cenozoic pelagic sequences and transitional shelf-to-basin deposits (Middle Miocene-Upper Triassic); 7) Liguride and Sicilide units, deriving from the deformation of internal domains, with Lower Miocene thrust-top basin deposits (S. Mauro, Pollica and Albidona Fms.); 8) thrust front of the Apenninic chain; 9) thrust; 10) normal fault; 11) strike-slip fault.

The present structural setting of central Apennines is mainly a result of the superimposition of two tectonic processes, which affected central Italy in slightly different times. The general northeastward migration of the central Apennine orogenic system, which follows a piggyback sequence of the main thrusts, entails an out-of-sequence re-activation (out-of-sequence thrusts) of some chain sectors previously involved in the thrust belt. Generally, these compressional tectonic phases were followed by post-orogenic extensional and strike-slip tectonics.

Within this general framework, the tectonic units considered in Plate 3 are bounded by both the main thrusts activated during the different piggyback migration phases of the orogenic system, or the main out-of-sequence thrust fronts that characterize this part of the Apennines. In this context, the tectonic units distinguished in Plate 3 show a regional significance and can be considered a series of structural sub-units.

Figure 10. Chain-foredeep migration

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Bio-chronostratigraphical scheme adopted for the micropaleontological analysis of the syn-tectonic terrigenous deposits of the central Apennines (zonal schemes are after Martini 1971 (modified) and Okada and Bukry, 1980). In the right half, the recognized central Apennine tectonic events are shown. For the Serravallian foredeep stage (in white) no siliciclastic deposits are recognizable in the study area.

In this paper, the description of the tectonic units that make up the central Apennines will follow the geometrical sequence, beginning with the geometrically higher units in the pile of the Apennine thrust sheets, down to the lower ones.

In a piggyback sequence of the main thrusts responsible for the growth of the Apennine chain, the geometrical criteria matches with the chronological one. In some places, where out-of-sequence thrusting plays an important role in the definition of the structural setting of the central Apennines, the geometrical criteria does not always match the timing of a forelandward piggyback propagation of the chain.

The central Apennine fold-and-thrust belt is characterized by the presence of two allochthonous units derived from the deformation of internal domains such as the external Ligurian, the Sicilide, and the Sannio domains. Generally, these allochthonous tectonic units are characterized by a chaotic complex consisting of varicoloured shales, calcareous and arenaceous turbidites. These allochthonous Apennine units were piled up during the earliest Apennine orogenic events and, subsequently, have been transported onto more external domains until Pliocene times.

• If we exclude the allochthonous portion of the external Ligurian and Sicilide units, cropping out in the northwestern and southeastern sectors of Plate 3, and those of the Sannio unit, cropping out in the southeastern sector of the area, the higher carbonate tectonic unit of the central Apennine thrust belt crops out in the northwestern sector of Plate 3. In particular, the highest tectonic unit of this thrust sheet is the Mt Soratte Tectonic Unit. This tectonic unit shows stratigraphical characteristics similar to those of the Tuscan succession and is subdivided into several tectonic sub-units. Its role as a tectonic unit should date back to the compressive Late Burdigalian event, while the foredeep of the inner Marnoso arenacea developed in the more outer sectors (Narni Mts and Martani Mts).

• Going towards the base of the tectonic wedge, the next tectonic unit is the Inner Umbria Tectonic Unit. This unit is bounded to the east by a right-lateral N-S strike-slip fault (southern portion of the tectonic unit) and by a thrust (Spoleto thrust), which shows multiple kinematics. This tectonic unit includes Mt Subasio, the Martani Mts, the Narni Mts, and the western Sabini Mts. Its internal structural setting is characterized by syncline and anticline macro-folds, which deform a basinal carbonate succession Lower Triassic to Lower Miocene in age. Its development should be attributed to the Serravallian compressive event, recognized in the Spoleto Mts (Cipollari and Cosentino, 1997) due to the presence of a thrust-top basin of that age (Belvedere-Vallocchia basin). Besides the presence of syn-orogenic deposits of the Belvedere-Vallocchia thrust-top basin, siliciclastic deposits are also common within this tectonic unit . These siliclastic deposits are related to sedimentation within a foredeep basin (Upper Burdigalian-Lower Serravallian), which further constraints the kinematic evolution of the area.

• The following tectonic unit is much wider than the previous one, encompassing the Sibillini Mts, the Sabini Mts, the Tiburtini Mts, and the Prenestini Mts. It derives from the deformation of shelf-to-basin and deeper-water Meso-Cenozoic limestones of the Umbria-Marche and Sabine domains. This tectonic unit (Umbria-Marche-Sabine Tectonic Unit) shows a general N-S trend with some minor thrust sheets, associated with syncline and anticline macro-folds. Generally, the orogenic transport of these structures is towards the eastern sectors. This tectonic unit is limited to east by the well-known central Apennine thrust surface: the Olevano-Antrodoco-Sibillini Mts thrust. This thrust corresponds to a very complex faulted area, characterized by multiple thrust surfaces, among which the Olevano-Antrodoco-Sibillini Mts thrust represents the more external enveloping surface (Parotto and Praturlon 1975; Salvini and Vittori, 1982; Cavinato et al., 1986; Cipollari and Cosentino, 1992; Cipollari et al., 1993). Because of both its timing and its oblique trend, if compared with the piggyback foreland migration of the central Apennine orogenic system, an out-of-sequence kinematic activity has been suggested for this structural element. Moreover, this tectonic unit, more than others, shows kinematic evidence that suggests multiple phases of deformation (re-folded folds, multiple slip direction on the same fault plane, folded cleavage, etc.).

In conclusion, this tectonic unit corresponds to a sector of the chain that was deformed during the Late Tortonian and Messinian Apennine compressional phases (in piggyback sequence) and subsequently was re-deformed due to the out-of-sequence thrusting of the Olevano-Antrodoco-Sibillini Mts thrust. The timing of this out-of-sequence activation corresponds to one of the main tectono-sedimentary events recorded in central Apennines: the Messinian Lago-Mare-Early Pliocene tectonic event.

• The Lepini-Ausoni-Aurunci Tectonic Unit derives from the deformation of the inner portion of the Apennine carbonate platform domain. It consists of Meso-Cenozoic shallow-water carbonates thrust onto Upper Tortonian terrigenous deposits related to the evolution of the Tortonian Apennine foredeep. The tectonic unit is characterized by wide homocline structures and by secondary widely open syncline structures, with a NW-SE prevailing trend.

Some small slices of the Sicilide complex are overthrust onto the carbonate of the Lepini-Ausoni-Aurunci Unit (e.g. Mt Caccume Klippe and Carpineto Romano area). At Mt Caccume, these small outcrops of Sicilide unit are tectonically overlain (sandwich-like structure) in part by a shallow-water platform succession similar to that of the Lepini-Ausoni-Aurunci Tectonic Unit.

The frontal thrust that bounded the tectonic unit on its eastern margin shows a subhorizontal geometry, as evidenced by several Klippen at its front. This thrust surface is one of the main overthrusts that lead to the build up of the central Apennine chain. In essence, the structure of the Lepini-Ausoni-Aurunci Mts represented, during Late Tortonian, the more external portion of the Apennine tectonic wedge, which was migrating with a piggyback sequence towards the Adriatic area. This conclusion is stratigraphically constrained in the Lepini Mts (Carpineto Romano area), where a sedimentary cycle unconformably overlies high deformed Meso-Cenozoic shallow-water carbonates. This unconformable sedimentary cycle (the Gorga-Gavignano unit, Cosentino et al., 2003), which consists mainly of coarse-grained deposits, was sedimented in a thrust-top setting, as testified by the compressional tectonics affecting the Gorga-Gavignano unit (Cosentino et al., 2003).

During the subsequent Messinian tectonic phase, the whole structure was further transported eastward, and the thrust front reached its present position.

• Moving towards the base of the central Apennine tectonic wedge, the Simbruini-Ernici-Matese Tectonic Unit rests beneath the previous tectonic unit. It is one of the wider tectonic units of the central Apennines and derives from the deformation of both shallow-water platform and platform-to-basin domains. As above mentioned, the trend of the orogenic system developed obliquely to the Meso-Cenozoic isopic facies. At its eastern end, this tectonic unit is bounded by a regional thrust surface, which allows the thrust of the Simbruini Mts, Ernici Mts, Mt Cairo, Venafro Mts, and Matese Mts carbonate successions onto the Messinian terrigenous syn-orogenic sequence. This latter sequence filled the Apennine foredeep basin that developed in this area during the Messinian.

As in other regional tectonic units recognized in the central Apennines, several minor tectonic structures are recognizable within the Simbruini-Ernici-Matese Tectonic Unit. These are linked to secondary thrusts that are probably synchronous with the main thrust at the base of this tectonic unit.

• The next tectonic unit, the Gran Sasso-western Marsica Tectonic Unit, is about 60 km wide and derives from the deformation both of Meso-Cenozoic shallow-water platform and platform-to-basin domains.

This tectonic unit has a very complex structural setting. Thrust tectonics linked to the piggyback evolution of the Apennine accretionary wedge was followed by out-of-sequence thrusting of the Gran Sasso chain. The first compressional phase that involved the area in the Apennine chain must have occurred during the Messinian Lago-Mare/Early Pliocene, as testified by the ages of several thrust-top basins (e.g. 1-Monte Coppe: Patacca et al., 1992b; Ghisetti et al., 1993; 2-Le Vicenne: Colacicchi et al., 1967; Cipollari et al., 1999a, 1999b; Gliozzi, 1999; 3-Monte Mezzana: Praturlon, 1968; 4-Palena: Patacca et al., 1992b). This phase was characterized by piggyback migration of the Apennine compressive front, with deformation accommodated along newly-generated N-S oriented structures. Subsequently, at the end of the Early Pliocene, an out-of-sequence thrust with a W-E trend was activated (Gran Sasso Chain: Ghisetti and Vezzani, 1986; 1990; Cipollari et al., 1997; Vezzani and Ghisetti, 1998), perpendicular to the previous N-S structures.

The tectonic unit of Gran Sasso-western Marsica is, thus, bounded to the north by the out-of-sequence thrust of the Gran Sasso chain, while to the east the boundary is uncertain, mainly in the zone between Mt Picca and Anversa degli Abruzzi.

In the area between Mt Cappucciata and Mt Picca, a thrust surface with a N-S trend is well developed. This structural feature places the Gran Sasso-western Marsica Tectonic Unit above the syn-orogenic terrigenous deposits of the La Queglia Flysch, (Patacca et al., 1992b; 2008) dated Messinian Lago-Mare/Early Pliocene. A similar structural relation is visible to the east of the Montagna Grande, where platform edge carbonates are thrust onto the terrigenous siliciclastic deposits of the foredeep. For these latter deposits, the only stratigraphical constraint is represented by the outcrop of the clays with gypsum of Anversa degli Abruzzi, which are thrust onto very fine–grained siliciclastic deposits of this foredeep. This tectonic relation implies a foredeep siliciclastic deposition post-dating the Messinian salinity crisis, thus correlative with the La Queglia foredeep (Messinian Lago-Mare/Early Pliocene).

As already stated, the external boundary of this tectonic unit in the Mt Picca and Anversa degli Abruzzi area is uncertain. A better definition of this boundary could result from detailed analysis of the terrigenous deposits cropping out in this sector and from a more accurate meso-structural analysis.

• The Mt Morrone-eastern Marsica Tectonic Unit is bounded to the east by several thrusts, such as the Mt Porrara thrust and the Montagna del Morrone thrust (Cosentino et al., 2003). The frontal thrust is located in correspondence with the western margin of the Maiella structure and leads to the tectonic superimposition of the La Queglia Flysch onto the Maiella Flysch (Lower Pliocene) and onto the Maiella Mts pre-orogenic sustratum (Patacca et al., 1992b; Patacca et al., 2008).

This tectonic unit shows strong N-wards axial plunging, which allows for exposure of only the younger syn-orogenic deposits of the local stratigraphic succession (La Queglia Flysch) (Messinian Lago-Mare-Lower Pliocene). This unit, as far it has been described, should have the same geodynamic significance of the Queglia Unit of the authors (Patacca et al., 1992b).

The correlation between the Mt Morrone-eastern Marsica sector and the Queglia Unit comes from a reconsideration of the stratigraphical data in the literature concerning the syn-orogenic deposits cropping out in the area (clays with gypsum of Anversa degli Abruzzi, Rocca Pia Flysch, gypsum-arenites of Mt Porrara). A detailed stratigraphical and structural analysis of the syn-orogenic terrigenous successions cropping out in the area is needed to confirm this hypothesis. The study of this outer sector of the Apennines is of great importance because it allows for examination of surface stratigraphical-structural features which, to the north, are buried under thousand of metres of Upper Messinian-Middle Pleistocene deposits.

• The Maiella Unit has many features in common with the previous tectonic unit. A strong axial plunging towards north causes carbonate rocks that correlate with those cropping out along the Maiella Mts ridge to be buried some thousand metres deep. The Maiella structure is characterized by a macro-anticline, overthrust towards east and cut along the backlimb by an extensional fault system (Caramanico fault Auct.). Another similarity with the nearby Mt Morrone-eastern Marsica Unit is the presence of a clear lateral variation of facies, from a shallow-water platform (in the south) to a platform-to-basin facies (in the north). The foredeep deposit related to the Maiella structure (Maiella flysch) was deposited during the tectono-sedimentary event of the Lower Pliocene post Sphaeroidinellopsis seminulina zone. In particular, recent biostratigraphical analyses (Cipollari et al., 2003) carried out on the Maiella flysch of Fonte dei Pulcini (southern Maiella) indicated the upper part of the MPl 2 or MNN 12 zones for the onset of the siliciclastic deposition in the outermost outcropping foredeep basin (Maiella flysch).

The Maiella structure has been involved in the Apennine chain during the Middle Pliocene tectonic phase.

Finally, we describe briefly a tectonic unit that is isolated if compared with the structural framework reconstructed for this sector of the Apennines. The Molise units (Agnone, Tufillo and Daunia, Patacca et al., 1992b) are characterized by thin skinned tectonics, which led to the complete detachment of the Oligocene-Middle Miocene succession from the supposed Mesozoic substratum of the Molise Basin (Calabrò et al., 2003; Corrado et al., 1997, 1998a, 1998b; Di Bucci et al., 1999; Speranza et al., 1997a). Generally, the units consist of deeper-water shales (varicoloured shales) and limestones with shallow-water carbonate resediments.

Normally, the detachment of the Oligocene-Middle Miocene succession from its substratum happens in correspondence with the varicoloured shales. At present, the authors suggest for the Molisano Basin a compressional activation similar to the Gran Sasso-western Marsica Tectonic Unit (Patacca et al., 1992b; Cipollari et al., 1999b). This interpretation is based on the age of the Agnone flysch (Messinian) and from some evidence of thrust-top basins related to the Messinian Lago-Mare/Lower Pliocene event (Conglomerato di Palena) (Patacca et al., 1992b).

In some sectors of the central Apennines, the effects of a post-orogenic strike-slip tectonics, superimposed on a complex compressional deformational history, are particularly evident (Salvini, 1991; Mattei et al., 1995). One of the most prominent tectonic elements with strike-slip character is a fault system in the western sector of the study area, in the Umbro-Sabine domain. This element (Sabine lineament and Val Serra fault) shows a vertical geometry along a N-S trend and a right-lateral stike-slip kinematics (Calamita, 1990; Alfonsi et al., 1991a, 1991b; Calamita and Pierantoni, 1994; Alfonsi, 1995). According to our kinematic reconstruction, this element has been active since the Tortonian.

An additionalstrike-slip element, with left-lateral strike slip motion, is the fault system that forms the eastern border of the Val Roveto. It is a tectonic feature with a regional significance [from the Venafro Mts, to the south (Cavinato and Sirna, 1988; Serafini and Vittori, 1986, 1988), to the Carseolani Mts (Serafini and Vittori, 1986; Montone and Salvini, 1990, 1993), to the north], along a NW-SE tectonic trend, which has also been activated as a normal fault. Tectonic structures related to strike-slip faults have also been recorded in the western Marsica (Corrado et al., 1992) and in more outer sectors (Mattei and Miccadei, 1991; Ghisetti et al., 1993; Vezzani and Ghisetti, 1995).

The present-day morpho-structural setting of central Italy is strongly influenced by the extensional tectonics that affected the study area from the Late Messinian until the Late Pleistocene and, sometimes, also until the Holocene (Barberi and Cavinato, 1993; Blumetti and Dramis, 1993; Calamita and Pizzi, 1992; Cavinato, 1993; Dramis, 1993; Cavinato et al., 1994; D’Agostino et al., 1994, 1998; Doglioni et al., 1999; Corrado et al., 1997; Calamita et al., 1999; Ciccacci et al., 1999; Cipollari et al., 1999b; Cavinato et al., 2002; Miccadei et al., 2002).

A NW-SE orientation is the prevailing tectonic trend for extensional features in the central Apennines. However, normal faults with anti-Apennine trend (NE-SW) are also present in the area. Across the suite of both extensional and compressive tectonic features, a space-time migration of the deformative front from the innermost to the outermost region of the Apennines is recognizable (Cavinato and De Celles, 1999; Cipollari et al., 1999b; Patacca et al., 1992a). This is generally inferred from the age of the different intra-mountain sedimentary basins, initiated by the extensional tectonic activity that was affecting the emerged Apennine area since the late Messinian. Along a transect transverse to the Apennine chain, the onset of these extension-related basins becomes younger going from the Tyrrhenian to the Adriatic side of the chain (Cavinato et al., 1994; Cavinato and De Celles, 1999).

A full revision of the geophysical and geological data available for the Central Italy in the area along the CROP 11 Profile (Crustal Seismic Profiling) can be found in Cavinato et al. (1994), Billi et al. (2006), Patacca et al. (2008), and Di Luzio et al. (2009), whereas a geodynamic scenario and a model of the Miocene tectonic evolution of the Northern Apennines have been described by Pialli et al. (1995; 1998), Barchi et al. (1998; 2001). The main information available on the deep structure of the central-northern Apennines is summarized below.

Seismic tomography (Ciaccio et al., 1996; Cimini, 2004) highlights a body of high velocity material dipping steeply towards the west underneath the Marche-Umbria-Tuscan area of the northern Apennines; this body has been interpreted as a retreating subducted slab of Apulian lithosphere, which is almost completely assimilated at a depth of about 250 km. This hypothesis is supported by subcrustal seismicity in the northern Apennines: the distribution of deep-focus earthquakes delimits a 40-45 degree dipping plane reaching depths of up to 90 km from the Adriatic to the Tyrrhenian sea (Amato and Selvaggi, 1991). On the contrary, the central Apennines show neither subcrustal seismicity nor high velocity anomalies in the tomographic images (Cimini, 2004).

Digital reappraisal of seismic refraction profiles data suggest a crustal doubling, affecting lower crust and mantle, under the area of the Tiber Valley in the northern Apennines (Ponziani, 1995). These features have been recently confirmed by the results of CROP 03 deep seismic line (NVR: Near vertical reflection) (Barchi et al., 1996; 1998).

In addition, Bouguer anomaly analysis (Bigi et al., 1992; Tiberti et al., 2005) points out positive anomalies along the Tyrrhenian Sea and in the Tuscan and the Latium sectors, whereas in the Umbria-Marche areas, negative values are recognizable. The anomaly values increase toward the east and reach positive values in the Adriatic Sea. According to Tiberti et al. (2005), most of the regional gravity anomalies in the central Apennines should originate within the lower crust. The transition from the western positive values and the central negatives happen across a narrow belt.

This narrow belt has been interpreted as the gravimetric expression of crustal doubling at a regional scale in the northern Apennines (Cassinis et al., 1991). Taking into account the Bouguer anomalies and the results of gravimetric modelling, Bernabini et al. (1997) hypothesized the same crustal doubling also under the Fucino Plain. More recently, the crustal data beneath the central Apennines, coming mainly from the CROP 11 seismic profile, have been synthetized by Di Luzio et al. (2009). In that paper, the authors confirm a crustal doubling just beneath the Fucino Plain, where the Adriatic Moho reaches 47 km depth, whereas the Tyrrhenian Moho rests at shallower levels at about 30 km depth.

According to Billi et al. (2006) a mid-crustal folding affects the central Apennines just beneath the Olevano-Antrodoco out-of-sequence thrust system. A fault-bend fold-like structure has been imaged in the CROP 11 seismic profile from 5 down to 8 seconds TWTT. From surface geological data, the authors suggest that the mid-crustal antiform grew as an out-of-sequence structure since late Messinian time.

Taking into account all the reported evidence, the following geodynamic setting can be assumed for the deep crustal structures of the central Apennines:

- a western thinned crust, including the Thyrrhenian Sea, the Tuscan sectors, and the area north of Rome; it is characterized by very high heath flow (Della Vedova et al., 1991; Mongelli et al., 2004) due to astenospheric uplift;

- the doubling of the Moho both beneath the Tiber Valley-Gubbio area (northern Apennines) and the Fucino Plain (central Apennines) where a "Tyrrhenian Moho" has been inferred to be superposed on an "Adriatic Moho" (Morelli, 1998; Di Luzio et al., 2009);

-an Adriatic lithosphere flexing down westward under the Apennine chain (Cimini, 2004).

The post Tortonian geodynamic evolution of the Apennines can be described in terms of post-collisional evolution of the chain driven by the passive sinking of the Apulian lithosphere and the progressive roll-back of the flexural hinge (Faccenna et al., 2004), with Tyrrhenian mantle compensation (eastward mantle flow) (Doglioni et al., 2004).