Northern Apennines
The oldest and uppermost tectonostratigraphic unit of the northern Apennines is the Ligurian unit: a mix of ophiolite, pelagic sedimentary rocks and continentally-derived ‘Helminthoid’ flysch deposits accreted during Late Cretaceous to Eocene subduction of oceanic crust of the Ligurian-Piedmont ocean basin below Corsica and Sardinia (Elter, 1975; Marroni et al., 2001). The Ligurian rocks in the northern Apennines are unconformably overlain by Eocene to Pliocene sedimentary deposits known as the Epiligurian unit deposited in marine thrust-top piggyback basins (Ricci Lucchi, 1986; Barchi et al., 2001; Cibin et al., 2001). Both the Ligurian and Epiligurian units are disrupted by extensive tectonic and sedimentary mélange deposits commonly referred to as argille scagliose (Pini, 1999; Cowan and Pini, 2001; Pini et al., 2004). Immediately underlying the Ligurian unit is a tectonic unit known as the Subligurian, made up of Paleocene-Eocene shales and limestones (Bortotti et al., 2001) and thick early Oligocene siliciclastic turbidite rocks. The Subligurian unit is considered as the first and oldest Apennine foredeep deposits marking the onset of collision of the Ligurian accretionary wedge with the former continental passive margin of the Adria microplate (Catanzariti et al., 2003).
The Oligocene to Recent collisional history of the Apennine orogeny is characterized by thick and extensive syn-orogenic foredeep turbidite sedimentation. These deposits, along with their substrata of Paleozoic crystalline basement and Mesozoic-Cenozoic carbonate and evaporite rocks of the former Adria continental margin, were progressively deformed and accreted as a series of nappes into the northern Apennine orogenic wedge before being tectonically overridden by rocks of the Ligurian and Epiligurian units (Argnani and Ricci Lucchi, 2001; Fig. 6 cross section A). Two main units or domains are recognized: the Tuscan unit and the Umbria-Marche unit.
Each unit has a similar Mesozoic-Cenozoic succession of Triassic evaporites overlain by thick Jurassic to Paleogene platform and pelagic carbonates (Barchi et al., 2001; Castellarin, 2001). The two units are distinguished by the age of onset and cessation of siliciclastic turbidite deposition and their paleogeographic position in relation to the advancing orogenic front; the Tuscan unit being accreted before, and lying tectonically above the Umbria-Marche unit. The turbidites of the Tuscan unit are subdivided into the Macigno (late Oligocene-early Miocene), and Cervarola (early Miocene) Formations (Ricci Lucchi, 1986; Sestini et al., 1986).
Figure 6. Simplified geological cross sections across the Italian Apennines
Simplified geological cross sections across the Apennines. Traces are shown in Fig.1. A: Northern Apennines after Laboume (1992) redrawn and modified; B: Central Apennines after Tozer et al. (2002) redrawn and modified; C: Southern Apennines after Mazzoli et al. (2006) redrawn and modified; D: Sicily after Lentini et al. (2000) redrawn and modified (Not to scale).
Deeper metamorphosed and highly deformed Tuscan unit rocks are exposed in the Alpi Apuane tectonic window and include the Pseudomacigno (the metamorphic equivalent of the Macigno) as well as Paleozoic basement and the Mesozoic Carrara marbles. These rocks underwent late Oligocene to early Miocene HP-LT metamorphism (350-480°C; 5-9 kbar) in a stacked duplex thrust system and were then progressively exhumed from mid crustal depths before being structurally juxtaposed against overlying non-metamorphic rocks of the Tuscan unit along a major low angle detachment fault (Carmignani and Kligfield, 1990; Jolivet et al., 1998; Carmignani et al., 2001; Fellin et al., 2004).
The Umbria-Marche unit is characterized by extensive mid- to late-Miocene turbidites of the Marnoso-arenacea (MA) Formation. The MA Formation is capped by evaporite deposits marking the onset of the Messinian salinity crisis (Ricci Lucchi, 1986, 2003; Boccaletti et al., 1990; Argnani and Ricci Lucchi, 2001; Zattin et al., 2002).
Much of the syn- and post-Messinian sedimentary record of the northern Apennine accretionary wedge is currently buried beneath mid- to late-Quaternary alluvial deposits of the Po Plain. Post-Messinian deposits comprise mainly deep marine clastic turbidites (Fusignano, Porto Corsini, and Porto Garibaldi Formations). Late Pleistocene deposits record a shallowing upward trend, with marine sands (Sabbie Gialle Formation) grading upward into alluvial clastic deposits of the Po River (Castellarin, 2001).
The largely in sequence thrusting, accretion, and underplating of the Tuscan and Umbria-Marche units reached its maximum northward extent in the late Messinian-early Pliocene along the buried Monferrato, Emilia, and Ferrara-Romagna arcuate thrust fronts. The early Pliocene also marks the final NE advancement of the Ligurian unit rocks to their current position tectonically overlying the foredeep deposits of the Marnoso-Arenacea Formation. The emplacement of the Ligurian nappe is proposed to have occurred by gravitational gliding below sea-level into the foredeep depression as a process independent from thrusting in the underlying deposits (Landuzzi, 1994; Zattin et al., 2002). Several authors have proposed that since the middle Pliocene, shortening within the northern Apennines has become more complex with reactivation and out-of-sequence thrusting distributed across the more internal parts of the orogenic wedge (Castellarin, 2001; Ford, 2004) with emplacement of Macigno unit rocks over the Ligurian unit along the out-of-sequence Cervarola-Falterona thrust (Boccaletti and Sani, 1998) and shortening across the Apennine-Po Plain front (Montone and Mariucci, 1999; Argnani et al., 2003).
On the internal Tyrrhenian (SW) flank of the northern Apennines, the accretionary tectonostratigraphy is disrupted by later widespread Miocene to Recent extension. The most important mode of extension is characterized by ductile stretching and generally east-dipping low angle extensional detachment faulting that began in Burdigalian times in northeastern Corsica (Jolivet et al., 1990; Fellin et al., 2005), migrated eastward from the Tortonian to Pliocene (Carmignani et al., 1994; Keller et al., 1994; Jolivet et al., 1998) and is presently active along the Altotiberina low angle normal fault system in the internal southernmost northern Apennines (Boncio et al., 2000; Collettini et al., 2006). Low angle normal faulting and ductile thinning reflect between about 60% and 120% total extension since the early Miocene (Carmignani et al., 1994; Bartole, 1995 ). A second mode of extension is typified by high-angle brittle normal faults and the development of orogen-parallel NNW-SSE trending graben and half graben and represents total extension of about 6-7%. The age of the basal sedimentary deposits within these graben gets younger in an eastward direction, ranging from Serravallian in the offshore northern Tyrrhenian Sea, to Pleistocene in the youngest graben located close to the current Apennine crest (Bartole, 1995; Boccaletti and Sani, 1998; Martini et al., 2001). Carmignani et al. (1994; 2001) have proposed that extension occurred as two discrete events, with core complexes related to thinning of an overthickened crust followed by later development of high angle normal faults related to opening of the Tyrrhenian Sea. However, more recent analysis of the active Altotiberina low angle fault and exhumed low angle detachments in the Alpi Apuane and Elba Island to the west reveal that many of the high angle brittle normal faults and their related half graben root into the deeper low angle detachments and stretched ductile middle crust, supporting a cogenetic model for the two modes of extension (Bartole, 1995; Boncio et al., 2000; Collettini et al., 2006). To the south, the Neogene east- northeast-ward migration of extension is accompanied in its waning stages by significant magmatism that is lacking further north (Serri et al., 2001).