Anatomy of an Orogen: the Apennines and Adjacent Mediterranean Basins

The Apennines are one of the youngest mountain chains in the world formed during Neogene and Quaternary. The adjacent southern Tyrrhenian Sea has recently developed an oceanic crust. During the last 3 million years or so the Apennines have experienced considerable foreshortening (80 to 200 km), local uplift (up to 2–2.5 km) at fast rates (about 1000 m/Ma or 1 mm/a; Apuane Alps, Calabria, Peloritani Mts), and considerable subsidence (up to 5 km) at a relatively fast rate (about 2500 m/Ma or 2.5 mm/a; Po Plain). Active tectonism renders this area prone to geological hazards such as those associated with volcanism, faulting and earthquakes, and slope instability.

The objective of this chapter is to provide a guide to what this book contains, what major conclusions the various contributors have proposed, and thus aid in the understanding of the geology of the Apennines (Fig. 2.1). To this purpose, the chapters of this book are reviewed, summarized with major points highlighted, compared and contrasted.

The Apennines are a complex arched mountain chain stretching some 1500 km from Genova in Liguria to Trapani in Sicily. The chain is a branch of the complex circum-Mediterranean Alpidic system of orogenic arcs (Fig. 3.1). It intersects on one side with the Maritime Western Alps segment, and continues with the North African Maghrebian chain on the other side. It resulted from the interplay of various microplates in the oblique collisional belt between the African and Eurasian plates during Tertiary and Quaternary time.

The scientific debate among Earth scientists on whether the Apennines thrust and fold belt originated by subduction-related processes or by different mechanisms has been ongoing for many years. One of the reasons for this has been the poor knowledge of the crust and upper mantle structure, and the lack of precise information on the occurrence of intermediate and deep earthquakes beneath the Italian peninsula. In this chapter we describe the advancement in seismological research in the last fifteen years, since the modern seismic networks became operational in Italy. In this period, millions of seismic waveforms have been recorded from mostly natural events (earthquakes) and to a less extent from artificial sources (explosions). Modern seismological tools, exploiting this huge dataset, have helped to unravel the complex structure of the lithosphere—asthenosphere system of the Mediterranean, and, thus, have contributed to a better understanding of the geodynamic evolution of the region.

Deep-seated features of the Earth’s crust can be identified and characterized by the velocities or by the reflectivity of the transmitted seismic waves generated from artificial sources near the surface and recorded by seismographs distributed along profiles or fans.

The objective of this chapter is to analyse available magnetic and gravimetric data to better understand the structure of the Apennines and adjacent marine basins (Figs 6.1, 6.2).

The temperature (T) distribution within the crust and upper mantle is a basic parameter that affects the physical properties of rocks and fluids. It also affects the deformation processes of the lithosphere. Rock densities, electrical and magnetic properties, elastic properties, mineral phase boundaries, saturation fluids and rates of chemical reactions are T dependent.

During the 1970s the most commonly accepted hypothesis for the evolution of the Alpine orogenic cycle in both the Alps and Apennines chains took into consideration the existence of one oceanic domain (the Ligurian—Piedmont basin), which opened during the Jurassic and was consumed during the Cretaceous to early Tertiary; its closure allowed the continent—continent collision of Europe and Africa, which occurred during Paleocene to early Eocene times. Laubscher (1971), Dal Piaz (1974, 1995) and Scandone (1979, 1982) are among the leading authors who put forward this hypothesis.

During the Quaternary, volcanism in Italy has been concentrated in at least five distinct settings: (1) the northernmost, extending from southern Tuscany to Basilicata, which includes the Alban Hills south of Rome, Vesuvius, the Campi Flegrei caldera, and the isolated Mt Vulture volcano; (2) the Tyrrhenian oceanic basin; (3) the Aeolian island arc; (4) eastern Sicily with Etna and the Hyblean plateau; and (5) the submarine and emerged volcanoes in the Strait of Sicily. Some of these areas are analyzed here mainly regarding their volcanic histories and character of volcanic activity, emphasizing the notable diversity of Italian volcanoes rather than giving comprehensive descriptions of all volcanic areas in Italy (Fig. 9.1). Whereas the geodynamic context of Italian magmatism is discussed in much detail by Serri et al. (this vol., Ch. 8), only brief summaries of the various recent hypotheses regarding the tectonic settings of these volcanoes will be given here.

The Italian peninsula and the bordering seas have been involved in Neogene to Recent deformation, and show a rapid crustal age variation. This is clearly expressed by the transition from the Quaternary SE Tyrrhenian oceanic crust to the possible late Proterozoic age of the Panafrican consolidated NE Apulian continental crust (Vai, 1994), which occurs in over a distance of only 170 km. Between the two, several other crustal provinces are found in this relatively small area.

Many important geological concepts and features were first recognized and described in the Apennines and Sicily. In these areas, among the most distinctive features are rock units characterized in outcrop by variably disrupted strata or blocks of diverse sizes disposed in a clay-rich matrix. The stratal disruption and the intense weathering give an overall chaotic or non-bedded appearance to these units, and various names have been used for them, including argille scagliose, argille brecciate, caotico eterogeneo, Chaotic Complex, Undifferentiated Complex, Ligurian mélange, and olistostrome. Some of these terms, such as argille scagliose and olistostrome, have been applied to mélanges in other mountain chains, even while Apennine geologists continued to debate whether local examples resulted from tectonic or gravitational processes.

The objectives of this chapter are (1) to review the complex relations between the Alps and the Northern Apennines, and (2) to outline the kinematic history in the transitional belt from the Northern Apennines front to the adjoining Po Plain foreland. The first section contains a comparative summary of the structural evolution of the two adjoining chains. The second section unravels the relations among the three main tectonic elements buried beneath the Quaternary deposits of the Po Plain; that is (1) the N-facing Northern Apennines front, (2) the S-facing Southern Alps front and (3) the Adriatic-Po Plain foreland in between. The third section discusses possible interpretations and implications.

The Northern Apennines are a fold—thrust belt formed during the Tertiary by the tectonic superposition from W to E of the Ligurides on the Tuscan nappe and on the Tuscan metamorphic complex (Boccaletti et al., 1971; Alvarez et al., 1974; Kligfield, 1979). The ophiolite-bearing Ligurides derived from the southern extension of the Ligurian—Piedmont ocean, from which similar mafic components of the Western Alps also derive (Fig. 14.1). The Tuscan units derived from the continental palaeomargin of the Adria microplate and contain a Hercynian continental basement with its upper Carboniferous Tertiary cover (Vai, this vol., Ch. 10).

The outer Northern Apennines (ONA) are an arc-shaped fold-and-thrust belt, with northeastward convexity and vergence, that plunges northwestward, extending through Romagna and Umbria—Marche to northern Latium. From the SW to the NE, it is situated between the inner Northern Apennines and the Po Plain—Adriatic foreland. To the S, the volcanic products of the Roman magmatic province cover it, while to the SE it is bounded by the Latium—Abruzzi carbonate platform. As for the Alps—ONA boundary the reader is referred to Castellarin (this vol., Ch. 13).

The Apenninic—Maghrebian orogen or Southern Apenninic (SA) arc in the central Mediterranean region developed as the product of convergence between the Europe and Africa—Adria plates, mostly during Tertiary times. The European plate margin is represented by the Sardinian block, split from the main plate during the late Oligocene following the opening of the Balearic basin (Cohen et al., 1980; Cherchi and Montadert, 1982a,b; Dewey et al., 1989; Carmignani et al.,1995). The African foreland includes the continental areas of both the Pelagian block (Burollet et al., 1978), and the Adria (Apulian) block, which have been separated, during Jurassic or earlier times, by the growing Ionian oceanic basin (Ben Avraham et al., 1992; Vai, 1994; Finetti et al., 1996). The deposits of the foreland consist of Mesozoic—Cenozoic carbonates up to 10 km thick (Burollet et al., 1978; Patacca et al., 1979; Bianchi et al., 1989; Mostardini and Merlini, 1988), whereas the oceanic Ionian crust underlies thin Mesozoic—Cenozoic, deep-sea to oceanic sediments (Finetti et al., 1996).

This chapter provides a synthesis of the past and current state of the knowledge on geological structure and evolution of the Calabria-Peloritani terrane and the northern (N) Ionian Sea. A general introduction to the study area is followed by a brief overview of past interpretations, a description of its main tectono-stratigraphic units, and a new interpretation of its evolution in terms of terrane analysis and accretion history. The term Calabria-Peloritani Arc, traditionally found in the literature, is discontinued because it refers to the present-day morphological curvature of the terrane in map view, but it is confusing if geologically defined.

Mesozoic and Tertiary limestones form the morphological backbone of the Apennines between Umbria and northern Calabria. They are also exposed in a number of tectonic windows below higher nappes along the Tyrrhenian margin of the Italian peninsula; that is, in the Tuscan nappe and in the exhumed metamorphic structural units of the Northern Apennines They further occur along the Adriatic foreland both in the subsurface and on the plateau of the Apulian peninsula. These limestones are the relics of a Mesozoic archipelago of Bahamian-type carbonate platforms, separated by deeper basins and plateaus, which originally were situated on the continental margin to the S and E of the Liguria—Piedmont segment of the (Neo-) Tethys ocean and extended over what is now the external fold-belts of the Apennines, Hellenides, Dinarides and Southern Alps and their Adriatic foreland (Figs 18.1, 18.2). Wherever the original substratum of these limestone successions is exposed or drilled, it is composed of upper Palaeozoic and Triassic elastics, limestones and evaporites, or of continental crust of Hercynian age. There exists a long-lasting discussion on whether the area was a promontory of the African continent throughout most of its Mesozoic history (Argand, 1924; Channell et al., 1979) or an independent microcontinent (Adria or Apulia, Dewey et al., 1973) that became separated from Africa in Permian (Stampfli et al.,1991; Vai, 1994), Triassic (Dewey et al., 1973) or Cretaceous time (Dercourt et al., 1986; Ricou, 1994). Nowadays there is convincing evidence that Adria was indeed separated from the North African margin by a deep oceanic basin that was connected to the Ligurian Tethys (Fig. 18.2; De Voogd et al.,1992). The Mesozoic and Tertiary limestones of Sicily were deposited along this North African margin and are now an important part of the nappes of central and southern Sicily and of their Hyblean (Iblei) foreland.

The turbidite beds of the Northern Apennines offer some of the most spectacular exposures of deep water synorogenic sediments in the world and have been often taken as representative of the typical foredeep basin fill. For the Miocene Marnoso-arenacea (MA) Formation in particular, the presence of several marker beds and of a well assessed biostratigraphy, coupled with an excellent stratal continuity, has allowed geologists to work out the shape of the basin through time, despite the tectonic slicing following the accretion of the sediments into the Apennine fold-and-thrust belt. The MA turbidites, however, represent just a time interval of a much longer story of foredeep sedimentation and migration that started in the late Oligocene and that continues at present (Ricci Lucchi, 1986b). For several reasons, the stratigraphy and geometry of the sedimentary beds of the Northern Apennine foredeep units have not always been documented in the same accurate way as the MA and the overall stratigraphie scheme is still to be refined. Within these limitations, this contribution aims at presenting a review of the stratigraphy, basin geometry and tectonic signature of the Tertiary foredeep deposits outcropping in the Northern Apennines. The Recent foredeep basin, which is located in the Po Plain and in the Adriatic Sea, is used as a modern analogue.

Imprints of past and present fast (venting) and slow (seepage) expulsion of pore fluids are scattered along the Apennine chain. This chapter reviews the problem of cold venting processes by discussing firstly, their geological, chemical and biological aspects in the Recent and, secondly, their Mediterranean counterparts from the Oligocene up to Present, with emphasis on the Apennine chain. Today, defluidization is visible on land in the form of cold springs, often enriched in hydrocarbons, such as the “salse”, mud volcanoes and hydrocarbon spills of the Emilia Apennines. World-famous Apenninic occurrences are those of Salsomaggiore, S. Andrea Bagni and Nirano where salty waters, variously enriched in sulphur, CO₂ and CH₄ are conveyed to the surface from deep reservoirs through Ligurian nappes (Zanzucchi, 1994; Bortolotti, this vol., Ch. 11). Examples of submarine defluidization occur in the Adriatic Sea and in the E Mediterranean basin.

The objective of this chapter is to review key evidence and genetic hypotheses pertaining to the Neogene—Quaternary basins of the internal (western) side of the Apennines and Calabrian arc (Fig. 22.1). The basins of the Northern Apennines will be treated first and they will be compared and contrasted with those of the Southern Apennines and Calabrian arc. Sediment terms such as clay, sand and gravel are generally used rather then rock terms, because most materials are uncemented or only slightly cemented.

The progressive time—space migration of the Apennine thrust-belt—foredeep system and the consequent diachronism of the silicoclastic flysch deposits becoming younger toward the Po Plain—Adriatic foreland are basic concepts in the current geological literature, which have been assimilated over thirty years. This migration has been interpreted as a response to the flexure-hinge retreat of the subducting foreland lithosphere according to slip vectors largely exceeding the average plate-convergence rate in the Central Mediterranean region during Neogene and Quaternary times (Malinverno and Ryan, 1986; Patacca and Scandone, 1989; Patacca et al., 1992a; Doglioni, 1991). Patacca and Scandone (1989) and Patacca et al. (1992c) have pointed out that cover detachment and nappe stacking did not proceed cylindrically along the Apennines, since adjacent segments of mountain chain, underlain by the same uninterrupted sole-thrust surface, may exhibit quite different structural architectures related to lateral changes of the thrust array.

The Italian Peninsula and Sicily along the Apenninic range form an oroclinal structure E-verging in the peninsular part, SE-verging in Calabria and S-verging in Sicily. Inside of this macrostructure, three major zones can be recognized:

The aim of this chapter is to outline the depositional and erosional processes that occurred on the coasts and shelves at the margins of the Italian mainland during the late Quaternary sea-level fluctuation. This fluctuation dramatically controlled sedimentary processes through the rapid shift of shoreline position, changes in palaeotopography, and in the size and amounts of sediment derived from the continent. The sea-level fluctuation described occurred during the last 23–25,000 years, and it has been subdivided into three main phases: (1) the final part of the last sea-level fall, the low-stand, and the beginning of the sea-level rise; (2) the sea-level rise; (3) the present highstand. Each phase will be discussed in its general aspects and with reference to some case histories. The marine data mainly derive from high-resolution seismic surveys and from grab, core, and vibracorer samples. The land data consist of stratigraphic, morphological and archaeological records. Analyses carried out on the collected sediments are mostly mineralogical, grain size, microfauna and ¹⁴C analyses.

The western side of the Apennines, from Tuscany to Campania, is characterized by the presence of many Quaternary volcanic edifices, three of which are still active: Vesuvius (last eruption in 1944), Campi Flegrei (1538) and Ischia (1301). Other active volcanoes are found in eastern Sicily (Mt Etna), in the Sicily channel where two submarine eruptions occurred in 1831 and 1891, and in the Aeolian islands of Stromboli (persistently active), Vulcano (1888–1890) and Lipari (729). Over two million people live on/or in proximity to these volcanoes, and their lives and/or properties are at risk in the case of eruption. There are two main scientific actions to be undertaken in order to protect this population: (1) to elaborate for each volcano a reliable scenario of the phenomena to be expected in case of eruption, to be used for specific and detailed Civil Protection preparedness plans, and (2) to maintain on the volcanoes a permanent and efficient monitoring network allowing for the early recognition of geophysical and geochemical precursory phenomena indicating that an eruption is impending.

The earthquake history of Italy is not just a history of ruined cities and people struggling to revive their environment; the distribution of the population, the abandonment and relocation of many settlements, the development and enforcement of modern town-planning rules, the creation of new architectural styles, and even the distribution of dialects can sometimes be blamed on the occurrence of a large earthquake. Although Italian earthquakes may not be the largest ever spawned by Earth, they contribute significantly to the development of seismology as a science. Important milestones, such as the following, in the earthquake history of the peninsula testify to this:

Gargano promontory, 1627. A catastrophic earthquake motivated the first systematic survey on the effects of earthquakes on the human environment, and led to the first isoseismic map (a map showing isoseismals, that is, lines connecting localities that experienced the same earthquake intensity level).

Hazard is one of the components that contribute to the definition of risk induced by natural disasters, and it is defined as the probability that a catastrophic phenomenon may occur in a defined area during a given period of time. Elements at risk include population, properties, buildings, transport infrastructures and economic activities. Vulnerability represents the degree of loss of an element or group of elements at risk, as a consequence of the occurrence of a natural phenomenon of a given intensity. The risk corresponds to the expected value of the loss and can be expressed as the product of three terms: hazard, vulnerability and value of the elements at risk (Varnes and IAEG Commission on Landslides, 1984).

Petroleum exploration in Italy dates back to the mid-nineteenth century and was focused initially on the Apennines and Sicily, where frequent oil and gas seeps were known since ancient times (Fig. 29.1). The complex geology of these foldand-thrust belts was then poorly known. Geologic interpretation, when and where applied, was limited to the study of local surface features. The economic results of this early stage of exploration were disappointing, and oil production reached a maximum of 30,000 metric tons (220,000 bbl) per year in 1932.