Central Apennines
The backbone of the Central Apennines is made up of tectonic units accreted with a main E and NE sense of transport (Bigi et al., 1990) dominated by pre-orogenic Triassic-Miocene, mainly carbonate sequences derived from the deformation of the Adria passive margin and overlain by Miocene to Quaternary syn-and post-orogenic sediments (Fig. 6 cross section B). Internal units, such as the Sub-ligurian at the top of the orogenic edifice, are scarcely preserved and are testified by olistostromes in the syn-orogenic deposits of the Latina Valley and on top of the innermost thrust sheets of this chain segment (Volsci Range).
The pre-orogenic setting is the result of various extensional events during the Triassic, Jurassic and Cretaceous-Paleogene (Centamore et al., 1971; Decandia, 1982; Calabrò et al., 2003) that created an articulated passive margin of Adria microplate where sedimentary successions in mainly carbonate platforms, pelagic basins, slope and intrabasinal highs facies range in thickness from about 1,000 (pelagic basins) to 5,000 m (carbonate platforms) (Parotto and Praturlon 1975; Damiani et al., 1991 and references therein).
Different stratigraphic domains can be distinguished from the west to the east: the Umbria-Marche-Sabina slope-to-basin, the Latium-Abruzzi carbonate platform, the Molise-Sannio basin and the Apulian carbonate platform.
The Sabina unit consists of upper Triassic-Miocene sediments mainly of pelagic and slope facies and represents the southern extension of the Umbria-Marche basin. It is arranged at least in three main N-S and NW-SE striking thrust sheets verging toward the east. In the western side of this unit a N-S right-lateral strike-slip fault dissects the earlier thrust structure. The eastern tectonic boundary of this area is marked by the Olevano-Antrodoco out-of-sequence thrust (Cipollari and Cosentino, 1992) where the pelagic sediments of the Sabina region overthrust the Apenninic carbonate platform and its associated siliciclastic deposits.
The Apenninic carbonate platform comprises shallow-water Mesozoic carbonate successions strongly influenced by the pre-existing basin architecture. The region is particularly important because it contains the most dramatic lateral change in tectono-stratigraphic units in the Apennines. These units are now telescoped into several NW-SE oriented thrust sheets showing a NE transport direction (Accordi et al., 1988). Along their south-western sides, these thrust sheets are dissected by steep NW-SE trending faults. Structural and stratigraphic data suggest both normal (prevailing) and left-lateral strike-slip motion for these faults, with earlier transcurrent faults being reactivated as dip-slip features. These faults bound NW-SE trending structural depressions filled with upper Miocene turbiditic deposits that are progressively younger (from Serravallian to Messinian age) toward NE (e.g., Cipollari and Cosentino, 1996).
In the eastern Marsica region, at the eastern boundary of the Apenninic carbonate platform, the Mesozoic facies evolve from carbonate platform to Meso-Cenozoic slope facies (Colacicchi, 1967; Miccadei, 1993). This region is organised in a series of E to ENE verging thrusts, overprinted by N-S trending strike-slip faults (Mattei and Miccadei, 1991; Corrado et al., 1992).
To the southeast of the Apenninic carbonate platform, in the Matese Mts., and to the North, in the Gran Sasso range, the main contractional structures evolve into an E-W trend with a general north vergence and the pre-orogenic facies evolve to the surrounding pelagic basins (respectively Molise-Sannio and Umbria-Marche in Scrocca and Tozzi, 1999 and Ghisetti and Vezzani, 1991). They plunge to the north under Lower Messinian turbiditic deposits (Laga and Agnone flysch).
The more external Apulian carbonate platform crops out farther to the east with a predominant NNW trend of ENE vergent thrusts, offset by mainly NNW trending normal and strike-slip faults (Miccadei, 1993; Corrado et al., 1995). The carbonate platform succession consists mainly of upper Triassic-Cretaceous deposits that locally evolve upward into slope and pelagic basin facies since the Jurassic. Upward in the succession, a Langhian to Tortonian carbonate ramp unit is overlain by upper Messinian-lower Pliocene siliciclastic deposits that become progressively younger toward the east (Patacca et al.,1992).
Farther to the east, the Apulian carbonates, unconformably overlain by Pliocene hemipelagic sediments, dip under the Sannio-Molise units and under the Plio-Pleistocene foredeep deposits (Patacca et al., 1992), to crop out again in the Apulian foreland to the east.
The Sannio-Molise pelagic units detached on the top of the buried Apulian platform, include Oligocene and lower Miocene pelagic basin deposits made up of mainly smectite rich clays, known as “Argille varicolori”. The succession evolves upward into calcarenites interbedded with marly and calcareous turbidites of Miocene age. Overlying Upper Miocene syn-orogenic deposits show a more proximal facies in the Sannio units and a more distal facies in the Molise units (Patacca et al., 1992; Di Bucci et al., 1999), thus indicating that these thrust sheets originated from different portions of the same basin located between the Apenninic and the Apulian platform. These units, detached from their Meso-Cenozoic substratum along the “Argille varicolori” detachment zone, overthrust the buried Apulian carbonates forming a N to NE-vergent thrust system (Corrado et al., 1997). On the other hand, “Argille varicolori” belonging to the inner Sicilide domain are mainly located in the Latina Valley. Since Miocene, pre-orogenic successions were deformed by E and NE verging folds and related thrusts interacting with the pre-existing structures and developing a variety of geometries. Pre-thrusting normal faults have influenced compressive deformation, have acted as focussing structures for thrust ramp localization or were positively inverted (Butler et al., 2006, Scisciani, 2009). Contraction migrating towards the East was coupled with the progressive development of syn-orogenic deposits in front (foredeeps) and on top of the fold-and-thrust belt whose age is bracketed between Burdigalian and Lower Pliocene (Patacca et al., 1992; Cipollari and Cosentino, 1995).
Besides most of accretion propagated in a piggy-back mode, several out-of-sequence reactivations or synchronous activity occurred within the belt. The most important is represented by the Olevano-Antrodoco line that marks the overthrust of the Umbria-Marche-Sabina domain onto the Latium-Abruzzi carbonate Platform in Lower Pliocene times (Parotto and Praturlon, 1975; Cipollari and Cosentino 1992). Furthermore strain partitioning along transpressional discontinuities active in Upper-Pliocene-Pleistocene marks the end of collision responsible of rotations about vertical axis (Alfonsi et al., 1991; Corrado 1995; Mattei et al., 1995; Corrado et al., 1997; Butler et al., 1997; Tavarnelli et al., 2004; Sani et al., 2004).
Extensional tectonics related to the opening of the back-arc Tyrrhenian Basin have been dissecting the compressive edifice since Miocene times (Serravallian) rejuvenating from hinterland to foreland accompanied by the development of syn-rift basins, presently well preserved at the rear of the chain. Some of these normal faults are still active in the axial zone of the chain and along the Adriatic margin (D’Agostino et al., 2001).