The Northern Apennines
The Northern Apennines consists of different Mesozoic Tethyan paleogeographic domains, organized in folds and thrust sheets forming a broad curved structure with an eastward (Adriatic) vergence (Fig. 3). The curvature of the arc follows the Adriatic margin of the belt, with the main structures ranging from NW-SE/WNW-ESE in the north to almost N-S in the south. Several second-order arcs developed besides the principal arc, in response to multiple compressional phases that induced both out-of-sequence nappe stacking and in-sequence external frontal accretion of the belt toward the Adriatic foreland.
The Ligurian domain constitutes the uppermost nappe of the Northern Apennines. It is represented by oceanic (Ligurides) and transitional (sub-Ligurides) domain units, characterized by the occurence of Jurassic ophiolites and their Jurassic-early Cretaceous sedimentary cover, overlain by Cretaceous-Oligocene flysch sequences. The Ligurian domain units overthrusted eastward the Tuscan domain units starting from the late Oligocene (Principi and Treves 1984). Upper Triassic to Miocene marine carbonates and sandstones constitute the Tuscan unmetamorphosed sequence, deformed in an array of thrust sheets (Tuscan nappe). The Umbria-Marche domain consists of sedimentary sequences deposited on a continental margin with basal late Triassic evaporites, platform carbonates (Lias) and pelagic sequences (Jurassic-Eocene) enriched upward in terrigenous deposits (Paleogene) and flysch sequences (Miocene).
The tectonic evolution of the Northern Apennines is characterized by the progressive migration of the orogenic front toward the Adriatic foreland, which is marked by the onset of siliciclastic deposits which get progressively younger toward the Adriatic-Ionian foreland. The onset of siliciclastic deposition occurred in Northern Apennines during late Cretaceous in the oceanic Ligurian domain. The Ligurian oceanic domain was deformed during late Cretaceous to early Eocene time, and formed a double vergent accretionary wedge, now outcropping from Corsica to Italian peninsula (Treves 1984; Carmignani et al. 1994). Starting from the Oligocene onwards, foredeep basins migrated eastward and formed on the top of continental sequences, belonging to the passive margin of Apulia. Their incorporation into the Apennines orogenic wedge marked the subduction of Adriatic continental lithosphere underneath Europe. Afterwards, during the Neogene, foredeep basins further migrated toward the Apulia foreland in front of the migrating thrust nappes. In Northern Apennines such process is well documented by stratigraphic and seismic studies, which precisely constrain the Neogene evolution of the foredeep basins in the front of the Apenninic chain (e.g., Patacca et al. 1992; Calamita et al. 1994; Pieri et al. 1994). Foredeep basins formed on the top of progressively easternmost (external) units, up to the Adriatic foreland, where the foredeep Quaternary deposition is no more active. The formation and evolution of foredeep basins were driven by the loading of the adjacent thrust belt and related to subduction processes, such as the flexural retreat of the subducting lithosphere (Royden et al. 1987). This process was particularly severe during Plio-Pleistocene times as evidenced by the presence, in the external part of the Apenninic chain, of a foredeep-basin system which contains up to 8 km of Pliocene-Quaternary sedimentary rocks (Royden et al. 1987).
Figure 3. Simplified geological map of Northern Apennines.
Extension on the Northern Tyrrhenian sector was coeval with thrust emplacement in the external Umbria-Marche-Romagna chain, with both extensional and compressional fronts migrating toward the Adriatic foreland from middle Miocene up to Pleistocene (e.g., Elter et al. 1975). Extensional tectonics dissected the already formed Apennines chain and generated new NW-SE trending extensional basins filled by ‘neoautochthonous’ marine and continental sequences (Jolivet et al. 1998; Collettini et al. 2006 and references therein). Moreover, crustal thinning, high heat flow and upraise of magmatic bodies accompanied extension along the Tyrrhenian margin. Today, active tectonics is represented by NW-SE normal faults, with a well documented historical and recent seismicity, mostly located in the internal sector of the Umbria-Marche-Romagna region, at the edges of intramontane basins (among others, Chiaraluce et al. 2004).
The tectonic history of the Northern Apennines has been largely investigated by paleomagnetic studies in the last 30 years (Fig. 3). The Umbria-Marche region is actually one of the most studied areas in the world, being the late Cretaceous-early Tertiary Scaglia Formation the most studied sedimentary sequence in Italy for paleomagnetism and magnetostratigraphy, from where the concept of counterclockwise rotation of the Italian peninsula arose (e.g., Channell and Tarling 1975; Klootwijk and Van den Berg 1975; Lowrie and Alvarez 1975; Vandenberg et al. 1978).
Early paleomagnetic studies on the Umbria-Marche sequences were used to interpret this area as autochthonous and to infer a 30°-40° counterclockwise rotation of the Italian Peninsula during Cretaceous, followed by a 25° post-Eocene rotation (Lowrie and Alvarez 1975; Vandenberg et al. 1978). Later paleomagnetic studies recognized the allochthonous character of the Umbria-Marche region, measuring different amount of counterclockwise rotation in the northern and southern parts of the Umbria region (e.g., Channell et al. 1978; Channell et al. 1992; Van der Voo 1993). On the base of these data these Authors suggested that results from the Umbria arc could not be extrapolated to the entire Italian peninsula. In the following years, several Authors have documented widespread vertical-axis rotations associated with thrusting and folding in Northern Apennines and today the timing of the tectonic rotations has been nicely constrained. Besides Mesozoic and early Tertiary pelagic deposits of the Umbria-Marche region, paleomagnetic data were gained from Messinian to Pleistocene sediments of the external front of the chain (e.g., Dela Pierre et al. 1992; Lanci and Wezel 1995; Speranza et al. 1997). These data indicate that Plio-Pleistocene rotations affected the external part of the arc contributing to the present-day geometry. These rotations occurred with different senses and variable amount along the arc. In particular, the post-Messinian counterclockwise 20° rotation measured in the Marche-Romagna area are of the same amplitude of those calculated for Mesozoic sequences in northern Umbria, suggesting a Plio-Pleistocene age for the rotations measured in older sequences (Speranza et al. 1997; Sagnotti et al. 2000).
Moving northwestward along the chain, paleomagnetic data are available for the more internal sector of the chain. Paleomagnetic study of the upper Oligocene–middle Miocene Epiligurian units revealed a 52° counterclockwise rotation with respect to Africa (Muttoni et al. 1998). Further data by Muttoni et al. (2000) showed that ~24° of this 52° rotation was Oligocene-Miocene in age, and likely related to the drift (and counterclockwise rotation) of the Corsica-Sardinia block (e.g., Montigny et al. 1981; Speranza et al. 2002; Gattacceca et al. 2007). Conversely, the remaining 28° counterclockwise rotation, observed in upper Miocene to Pliocene sediments, was due to Pliocene shortening episodes occurring at the Apennine chain front, which may have (re)activated thrust planes in the Apennines structures below the Ligurian wedge. The latter paleomagnetic data confirmed the above described results from Speranza et al. (1997), who found a post Messinian counterclockwise rotation of ~20° in the northern part of the studied area. These data suggest that the deformation of the Apennines continued after the end of the rotational motion of the Corsica–Sardinia block.
Paleomagnetic data at the edge with Western Alps indicate that the sedimentary sequences of the Tertiary Piedmont Basin rotated about 50° CCW during Aquitanian-Serravallian times (Thio 1987; Carrapa et al. 2003; Maffione et al. 2008), an amount and timing of rotation very similar to those registered for the Sardinia-Corsica block.
During Neogene, thrusting and bending in the external Northern Apennines were accompanied by progressive collapse of the internal sector of the belt, related to the Tyrrhenian basin opening with the formation of several syn-rift basins along the Tyrrhenian margin. The late-orogenic collapse was, however, irrotational (Lowrie and Alvarez 1979; Sagnotti et al. 1994; Mattei et al. 1996). Paleomagnetic data indicate, in fact, that the Messinian-Pleistocene extensional Tyrrhenian basins did not underwent tectonic rotations, differentiating the Northern Apennines and the Tyrrhenian margin as two different rotational domains (Mattei et al. 1996).
In Northern Apennines a mechanism of oroclinal bending was first confirmed (Channell et al. 1978; Eldredge et al. 1985) and then rejected (Van der Voo and Channell 1980; Lowrie and Hirt 1986; Hirt and Lowrie 1988) on the base of different data sets from Mesozoic-lower Tertiary sediments in the internal domain of the chain. Channell et al. (1978) proposed that the Umbrian Apennines underwent oroclinal bending, suggested by the curvature of the mountain belt and its fold axes from north (fold axes strike = 315°) to south (fold axes strike = 350°) and the change in paleomagnetic direction from north (declination = 316°) to south (declination = 338°). Eldredge et al. (1985) applied the method proposed by Schwartz and Van der Voo (1983) to determine a possible relationship between fold axes and declinations, confirming the hypothesis of an orocline for the Umbrian orogen. Hirt and Lowrie (1988) found some weaknesses in the analysis of the Eldredge et al. (1985), such as the scarce accuracy in determine fold axis strike and paleomagnetic declinations, and the use of a single reference declination for the whole Scaglia formation where the sites came from, whose age covers a too long time span. From the study of the Maiolica formation (upper Jurassic-lower Cretaceous), Hirt and Lowrie (1988) proposed that the changing of declination values along the arc was not related to the orientation of fold but, in turn, to rotations associated with the deformation of the sedimentary cover. From the study of Messinian sediments of the external domain, Speranza et al. (1997) indicated that the present shape of the northern Apennine arc is related to the oroclinal bending of an originally N320° trending straight belt. These Authors suggested that vertical axis rotations accompanied the migration of the main thrust front toward the Adriatic foreland and characterized also the development of second-order arcuate thrust fronts in the Apennines.