Journal of the Virtual Explorer

It is difficult to define the basement in the central Apennines orogen because its depth and nature cannot be clearly recognised using existing seismic reflection and well data. In our section we did not constrain the depth of the basement strictly using the presumed depth of the aeromagnetic (e.g., AGIP, 1984 and models provided by Arisi Rota and Fichera, 1987) or magnetic basement (Speranza and Chiappini, 2002) because, as also suggested by Bally et al. (1988), magnetic depth determination beyond 9-10 km is somewhat imprecise and should be corroborated by other data. Moreover, it should be also considered that the magnetic basement is different from the crystalline one.

At the eastern side of our cross-sections, we extrapolated the depth of the basement starting from the easily recognisable Top Messinian evaporites horizon by adding the stratigraphic thicknesses derived from a review of well data in the Adriatic. Moving west, the depth of the basement has been evaluated on the basis of structural and stratigraphical constraints, such as the thickness of the different superposed tectonic units according to the modelled structural setting. We also assumed, on the basis of the known regional flexure of the Apulian lithosphere supported by the Adriatic Moho geometry (e.g., Mele et al., 2006; Di Luzio et al., 2009), a gently westward dipping basement (from about 5 km to 13-14 km). The main detachment level, at the top of the supposed basement, shows the same attitude. If we assume the regional flexure of the Apulian lithosphere to be the consequence of continental subduction of the Apulian plate underneath the belt, we should then suppose that the main active detachment level cannot have remained sub-horizontal westward towards the Tyrrhenian side but must have plunged steadily to accommodate the flexure and to form an upper boundary for subducting plate (Doglioni 1993; Carminati et al., 2004).

At the western side of the cross-section we have chosen to vanish with depth our structural reconstruction since in this area there are very few constraints. In this way, we intend to propose a conservative interpretation and to avoid the debate about the possible basement involvement in the Apennines (for a discussion about this issue see Ghisetti et al., 1993; Tozer et al., 2002; Butler et al. 2004; Scrocca et al., 2005; Billi et al., 2006; and references therein). However, it should be noted that a deep involvement of the crystalline basement in the deformation processes of the Central Apennines seems unlikely, as basement involvement has not been documented in similar orogens associated with west-directed subductions (e. g., Doglioni et al., 1999; 2007).

The major structural domains traversed by our cross-section are characterised by strong variations in stratigraphy and facies. In the Upper Triassic/Lower Liassic, extensional tectonics affected the outer margin of the Africa plate (Adria microplate), leading to the development of subsiding carbonate platforms (Apenninic and Apula) and pelagic basins (Umbria-Marche, Pescara and Molise) linked by transitional domains (Sabina and Gran Sasso). Stratigraphic successions show large variations in thickness and facies of the Mesozoic formations due to the Jurassic syn-sedimentary extension that is also recorded within the basinal sequences (as documented in the Umbria-Marche domain). In the transitional and pelagic sequences, the presence (or the absence) of resedimented carbonates (turbidites and debris flow sediments) make it very difficult to have good regional estimates for primary thicknesses of these stratigraphic units.

Our sections were originally drawn at a scale of 1:100.000; therefore, the stratigraphy of the region had to be simplified and homogenised. The stratigraphic sequences of the different domains crossed by our sections have been grouped into several domains with the same average stratigraphic thickness (Lepini, Simbruini, Magnola-Velino, Sirente carbonate platform domains, Gran Sasso and Sabina-Fogliano type transitional sequences, Umbria-Marche, Pescara, and Molise pelagic sequences). Within the pelagic sequences, we have seldom shown significant thickness changes due to the Jurassic extension.

Several wells penetrate the Upper Triassic evaporites (Burano formation), revealing variable thickness that reach a maximum of about 2000 m (around 1500 m in Trevi 1, 1850 in Antrodoco 1, and 1400 in Perugia 2). The about 1800 m penetrated in Burano 1 are due to the steep dips in excess of 70° that dominate the lower half of the penetrated section, suggesting intensive deformation in the core of the Burano anticline. It is impossible to determine the original stratigraphic thickness of the Burano formation and therefore we assumed an original thickness in the range of 1200-1500 m.

In the eastern/Adriatic area, continental red sandstone (Permian?) have been drilled below the Late Triassic Burano formation (e.g., Alessandra 1 well). Furthermore, across the Adriatic domain, seismic and well data document the widespread occurrence of locally thick Permian-Middle Triassic sedimentary units (e.g., Grandić et al., 2002; Franciosi and Vignolo, 2002). The thicknesses of these units may be highly variable due to the continental origin of the Permian deposits and to the syn-rift nature of the lower-middle Triassic formations. However, since no detailed information about the actual thickness of these units along the section trace is available, we have assumed an average thickness of about 500 m.

The stratigraphic horizons that have been chosen as markers in the cross-section delimit formations with the highest competence contrast within the pelagic sequences (top Burano, Calcare Massiccio, Maiolica, Scaglia and Upper Miocene). These sequences are covered by occasionally thick siliciclastic wedges that mark the progressive eastward migration of the subsiding foredeep. In the carbonate platform domains (including their marginal areas) we have used time lines corresponding to top Lower Lias, Lower Cretaceous and Upper Cretaceous as stratigraphic markers. These sequences are conformably overlain by Middle Miocene bryozoa limestone and Orbulina marls that have been grouped in another unit; on top of these sequences we separated the foredeep deposits.

In both pelagic and carbonate domains, we have used the same colour code to show the different ages of the foredeep deposits.

The local tectonic style is controlled by ductility contrasts that occur within the stratigraphic sequences. It is generally assumed that Triassic evaporites (Burano Formation) are the main detachment level both in platform and basin domains. Secondary compressional décollement phenomena are documented for the following formations: i) Rosso ammonitico, Marne a fucoidi, Scaglia cinerea, and Messinian evaporites for the pelagic sequence, and ii) Marne ad Orbitolina, bauxite levels, and Marne a Orbulina for carbonate platform domains.

Furthermore, stratigraphic units around these décollement levels are often intensively deformed, suggesting disharmonious folding and intensive deformation. These processes make it very difficult to define reliable stratigraphic thicknesses. At the original 1:100.000 scale of our sections, it has proved difficult to incorporate these local details into the regional profile.

The sequences in this area are buried underneath thick Quaternary alluvium and volcanics of the Latina Plain. This area has been dissected by NW-SE normal faults in Plio-Pleistocene time.

The wells drilled in the Latina Plain (Pianura Pontina), e.g. Fogliano 2 (Parotto and Praturlon, 1975), reveal a transitional sequence, which could be correlated with the similar outcrops of Circeo Mt to the south, or the Sabina to the north. Therefore, in the subsurface of the Latina Plain, southwest of the Lepini Mts, a stratigraphic transition from the carbonate platform facies (i.e. Lepini Mts) to a pelagic basin sequence has been inferred.

The pre-orogenic stratigraphy of this sector is characterized by thick and monotonous Triassic to the Upper Cretaceous-Paleocene carbonate platform sequences overlain by Middle Miocene bryozoa limestone. Thick thrust sheets piled up in Neogene time with mainly NW-SE trending structural axes. The main thrust fronts are generally located on the NE margin of the main carbonate ranges. From the inner part of the thrust (or duplex?) package towards the outer, the major thrusts are: the Lepini Mts thrust, Simbruini Mts thrust, and the Gran Sasso thrust.

According to Tallini (1994) and Cavinato et al. (1992) the southwest margin of the Simbruini - Ernici Mts is marked by an extensional fault system. The presence of hydrothermal fluids (sulfurous spring), and volcanic activity confirm that this fault system involves deep crustal levels, as discussed in section 2.3 (extensional fault).

The structural setting of the Simbruini range has been constrained taking into account the detailed geological map proposed by Devoto (1970), data from the deep Trevi 1 well (Dondi et al., 1966), and the structural interpretation proposed by Tallini (1994). According to Tallini (1994), the main superficial thrusts (Val Roveto thrust and Fosso Fioio thrust) have been correlated with the deepest tectonic discontinuities detected in the Trevi 1 well. The proposed kinematic model shows the development of some small duplexes at the level of the upper Triassic units, followed by the main thrusting phase (from Messinian to possibly Lowermost Pliocene time). During this latter deformation phase, out-of-sequence thrusting could have occurred. We consider out-of sequence thrusting to be active thrusting in an internal position with respect to the leading edge of the thrust belt (sensu Morley, 1988). In this reconstruction, the Vallepietra-Filettino fault (a structure which has caused much debate and has been interpreted alternatively as an extensional fault, a transpressional fault, and an out-of-sequence thrust) has been represented as an extensional fault (with a low angle and linked with deep ramp). An alternative reconstruction assuming the same fault geometries could include an initial compressional phase followed by an extensional reactivation. Finally, high angle normal faults dissected the previously generated structure.

The Roveto Valley is characterized on its northeastern margin by a major left-lateral strike-slip fault (Val Roveto fault) (Serafini and Vittori, 1986; Montone and Salvini, 1993). This fault, NW-SE trending, could be interpreted as the superficial expression of a deep, crustal, tectonic discontinuity as suggested by soil-gas investigations (Ciotoli et al., 1993).

Moving eastward, we represented the fault on the NE margin of Mt Sirente and the one which affected the SW border of the F. Aterno valley as thrust faults (Cavinato et al., 1994).

Further east, the Gran Sasso chain is one of the main structural culminations of the Apennines, where Meso-Cenozoic carbonate units were emplaced over the Upper Miocene - Lower Pliocene foredeep deposits (Laga Flysch). Some outcropping faults have been alternatively interpreted as out-of-sequence thrusts (Ghisetti and Vezzani, 1991) or as low angle "younger-on-older" tectonic contacts (D’Agostino et al., 1998).

However, we represent the overall structural setting of the Gran Sasso domain as a break-through fault-propagation-fold, later transported with significant displacement onto the footwall units.

Recent foredeep deposits from upper Messinian to Pliocene or younger sedimentary covers mainly outcrop in the onshore area. The carbonate units penetrated in subsurface by several deep wells show a pelagic facies and a structural setting characterized by stacked thrust units (among many others, Bally et al., 1988; Ghisetti et al., 1993; Calamita et al., 2002 and references therein).

In the footwall units of the main Gran Sasso overthrust we have represented two distinct foredeep basins:

- the westernmost (more internal), which is filled by the siliciclastic deposits of the Messinian Laga Flysch;

- the eastern basin, which corresponds to some turbiditic deposits of late Messinian age related to the Teramano foredeep basin (sensu Patacca et al., 1992b).

We assumed also in this section the existence of a buried peripheral bulge zone, which originally separated these two foredeep basins and then deformed with a buried ramp anticline setting.

The thickness of the upper Messinian (post 5.5-Ma volcaniclastic level, Odin et al., 1997) foredeep deposits on top of this unit is about 1000 m, because we consider this unit to be a northern analogue of the Queglia unit (sensu Patacca et al., 1992b). We do not include the C. le Maddalena outcrop, attributed to the Gran Sasso unit, in the Queglia unit. This unit shows thick upper Messinian foredeep deposits (up to 3000 m). One possibility is that the foredeep stage in this unit started after the Messinian evaporite deposition, and had its maximum development in upper Messinian time (as suggested by Canzano 1 well some km to the north). We consider this unit to be a northern prolongation of the Morrone unit with Meso-Cenozoic pelagic facies, according to the regional transition from carbonate platform sequences to pelagic environment (also documented in the Maiella unit).

The offshore north-eastern side of this section is partly based on a geological profile published by Bally et al. (1988).