The Central Mediterranean: Alps and Apennines
East of the Iberian Peninsula, the shape of the Alpine–Himalayan belt in the Central Mediterranean is extremely complex: from southeastern France, the northern branch, or European Alps, curves around through nearly 180°, describing a great arc through eastern France and Switzerland until, in the Austrian sector, it trends roughly east–west (Fig. 5.1). The chain then divides into two branches, one continuing down through the Dinarides to Greece, and the other forming the great arc of the Carpathians. Another branch continues from the west end of the Alpine chain along the western side of Italy to form the Apennines.
Figure 5.1 Mountain belts of the Central Mediterranean. Shutterstock©pio3.
To most Europeans, and certainly most mountaineers, skiers and geologists, ‘the Alps’ means the mountain chain stretching from southeastern France, through Switzerland, Italy and Austria; however, the term ‘alps’ is also applied to other mountain chains such as the Betic Alps of southern Spain and the New Zealand Alps. The word ‘alp’ itself is a Celtic word applied to any high mountain area (hence the old word ‘Alba’ for Scotland) and is also used by the Swiss to mean an area of high pasture!
This chapter deals with the sector of the Alpine–Himalayan belt that extends from the Mediterranean coast at the French–Italian border near Monaco to just south of Vienna in eastern Austria, a distance of approximately 800km. This chain includes many famous summits, including Mont Blanc, the Matterhorn, and the Eiger, which have attracted mountaineers since the earliest days of the sport, and are among around a hundred peaks over 4000m in height. Geologists generally divide the sector into the Western Alps of France and Switzerland, the Eastern or Austrian Alps, and the Southern or Italian Alps. Most of the higher peaks are in the western sector. There are distinct geological differences between the three sectors, as will be seen.
Early geological investigations
The Alpine chain was not only the first of the great mountain belts to be studied by geologists, but also the one most intensively investigated subsequently. Most of the early geologists, including both James Hutton and Charles Lyell (see chapter 2) visited the Alps and recognised that the geological structures found there signified that lateral compression as well as uplift had played a part in their development. One of the most important contributions was made in 1837 by the naturalist Louis Agassiz (Agassiz, 1840), who was the first to realise that the Alps must have been covered by ice to a much greater extent than at present, and that many of the erosional features of the mountains were due to the effects of glacial erosion, including the characteristic shapes of glacial valleys and the importance of glacial deposits such as moraines and glacial erratics. Agassiz subsequently visited Scotland and demonstrated the same features there, establishing the former existence of a great northern ice sheet covering much of Britain.
By the third decade of the twentieth century, the broad stratigraphy of the Alps was well known and its structural complexity widely recognised. Detailed studies of the stratigraphy and structure had revealed the presence of giant thrust sheets, termed ‘nappes’, enclosing complex recumbent folds, which had resulted in the same stratigraphic sequence being repeated upwards several times, accounting for considerable crustal thickening. A number of well-known geologists, including Emile Argand (1916), Albert Heim (1921), Leon Collet (1927) and Rudolf Staub (1928), had produced syntheses of Alpine geology, but the most important early attempt to understand the overall structure was made by Argand in his 1916 work Sûr l’arc des Alpes occidentales (Fig. 5.2).
The exponents of the continental drift theory believed that the overall structure of the Alps resulted from a collision between Africa and Eurasia and that Africa, or part of Africa, had effectively over-ridden Europe. In his influential 1937 work Our Wandering Continents, referred to in chapter 2, Du Toit visualises the sequence of events affecting the Alpine region as follows.
1Latest Cretaceous Period. Eurasia and Africa began moving towards each other, creating ‘geanticlinal’ systems (i.e. uplifts) along their opposing margins.
2Later Eocene Epoch. The Tethys Ocean became narrower and Western Europe became weakened by the ‘break-away’ of North America.
Figure 5.2 General section across the Alps according to Argand. Note that material from the African hinterland is thrust over the Pennine nappes and that the complex folding of the Pennine nappes directed both to the northwest and southeast is explained by the convergent movement of the African ‘indenter’. After Argand, 1916.
3Oligocene Epoch. General northwards advance of Africa resulted in squeezing of part of Africa north-westwards into the arc of the Western Alps, creating an ‘S-shaped’ line of collision.
4Pliocene Epoch. Further north–south convergence resulted in a pressing together of the arms of the ‘S’ producing overfolding both to the north and south.
In terms of the timing of events, this account is not dissimilar to the more modern versions discussed below; however, it should be noted that Du Toit and other contemporary exponents of the ‘mobilistic’ view of tectonics believed that the continental crust could behave in a much more ‘plastic’ manner than subsequent ‘rigid’ plate theory demanded.
Plate-tectonic context
One of the problems facing those attempting to explain Alpine structure by conventional plate-tectonic theory was how structures that appeared to imply movements directed north, northwest, west and southwest could be produced by the convergent motion of two rigid plates. It became clear that the overall structure of the Alps could only be explained by the relative movements of several independent terranes, as described in the previous chapter, together with changes in the convergence direction over time. Those movements were summarised in Figures 4.4 and 4.6.
The tectonic evolution of the Central Mediterranean was partly controlled by the movement path of Africa relative to Eurasia, as shown in Figure 4.6: a SE–ward motion through the Jurassic and into the early Cretaceous, parallel to the Gibraltar transform fault, followed by an abrupt change in the mid-Cretaceous to northeast, then north-northeast into the Palaeogene, when it became northwards until the Miocene. Near the end of the Miocene it changed again to northwest, which is the present-day convergence direction.
However, the formation of the Alps and Apennines was controlled more by the movement paths of the independent terranes which, although guided in a general sense by the overall convergence of Africa and Eurasia, are constrained more by the geometry of the surrounding crustal blocks. Various reconstructions have been made of the geometry and movement history of these terranes, but the details, especially regarding the shapes of the terranes, must be regarded as speculative to some extent.
According to most interpretations, Gondwana separated from the southern margin of Eurasia during the Jurassic by the opening of the Piémont and Liguria ocean basins. These were bounded to the southwest by a transform fault running through Gibraltar, which connected these movements to the opening of the Central Atlantic. It is generally recognised that five separate independent terranes were involved in the further orogenic development of this region; these are coloured pink in Figure 4.6 (see previous chapter): Adria, Alcapa, Tisza, Alkapeca and the Briançonnais Terrane.
Figure 4.6 is based on a reconstruction of the possible sequence of events through the Cretaceous and Cenozoic. By the early Cretaceous, a small western branch of the Ligurian basin had opened east of what was then the eastern margin of Iberia, isolating the small terrane known as Alkapeca. Alcapa and Adria were still attached to Africa, but Tisza had separated from Alcapa in the north (Fig. 4.6A). During the mid-Cretaceous, the most southerly and largest terrane, Adria, consisting of the piece of continental crust now underlying eastern Italy and the Adriatic Sea, became separated from Africa by the Ionian Sea basin, a branch of the Neo-Tethys Ocean, and moved north to collide with Alcapa in what is known as the Eo-Alpine orogeny (Fig. 4.6B). Meanwhile, the northernmost terrane, Tisza, moved eastwards towards what became the Carpathians. Also during this period, the Briançonnais Terrane became separated from southeastern Europe by the small Valais Ocean basin. The Briançonnais was now flanked on its northwestern side by the Valais Ocean and on its southeastern by the Piémont–Liguria Ocean.
By the end of the Cretaceous, prompted by further expansion of the Ionian Sea, Adria–Alcapa had collided with the southern European margin in the sector that subsequently became the Eastern Alps (Fig. 4.6C). To the west, Alkapeca and the small Briançonnais terrane had collided with Iberia and southeast France respectively due to the closure of the two small ocean basins on their western flanks.
The critical stages in the tectonic evolution of the Alps occurred during the Oligocene to early Miocene Epochs. The change in convergence direction between Africa and Eurasia to a more northwards direction marked the early stages of the main collision event of the Western Alps (Fig. 4.6D) and was accompanied by extension in the Western Mediterranean with the opening of the Valencia and Balearic ocean basins (Fig. 5.3). This resulted in the south-eastwards translation of the Balearic Islands and the anti-clockwise rotation of Corsica and Sardinia into their present positions. These movements were accompanied by the subduction of the Ligurian ocean basin beneath the new eastern margin of Iberia, leading to the collision of Alkapeca (now welded to Iberia) with Adria to form the Apennines. At the same time, the Ionian Basin was consumed beneath the southern margin of Adria. During the later Neogene, a further phase of extension opened up the Tyrrhenian Sea basin between Corsica–Sardinia and the Italian Peninsula.
The formation of these ocean basins has been attributed to the process of back-arc extension on the upper plate of the Ionian Sea subduction zone, and is linked to the general eastwards retreat of the subduction zone due to trench roll-back (see Fig. 3.12). This has resulted in the subduction zone moving a distance of c.775km from its original position along the east side of Iberia in the Oligocene (where it resulted in the collapse of the Alborán Basin referred to in the previous chapter) to its present location in the Calabrian Arc (Fig. 5.3). The present-day position has been determined from the loci of active earthquakes and shows a near-vertical slab between 100 and 400km depth. The subduction zone is now very much shorter and more arcuate in shape, and its former continuation along the western side of the Adriatic is now a collision zone.
Tectonic structure
The broad geological architecture of the Western Alps has been known for well over a century. By the time that Argand produced his great work in 1916 (e.g. see Fig. 5.2), much of the stratigraphic detail was already well known and the ideas of a European foreland and an ‘African’ hinterland that had converged to produce the visible complex structure were generally agreed, although many in the geological community were still opposed to continental drift. It is this western sector of the chain that is the most complex, and it is here that all the main tectonic zones are visible on a NW–SE traverse across it.
The main tectonic units of the Alpine chain, from north to south, are: the European Foreland, the Helvetic–Dauphinois (or external) Zone, the Pennine (or internal) Zone, the Austro-Alpine Nappes, the Southern Alps and the Adriatic Foreland (Fig. 5.4). At its widest, in the French-Swiss sector, the Alpine chain includes three additional zones: the Jura, the Foredeep (or Molasse) Basin and the Pre-Alps. Here the chain is nearly 300km across, although over most of its length is only about 200km across.
The European Foreland
The foreland consists of Palaeozoic basement belonging to the Eurasian Plate, which had previously experienced the Variscan orogeny, overlain by a Mesozoic sedimentary cover consisting of shallow-marine sediments dominated by shales and marls with prominent carbonate beds, consisting of both limestone and dolomite.
Figure 5.3 Opening of ocean basins in the Western Mediterranean. During the Neogene the subduction zone migrated eastwards due to trench roll-back. Note the former positions of the Alkapeca Terrane (ALK) on the upper plate of the subduction zone. Former position of islands in pale green; present positions in dark green. Note that Corsica and Sardinia have been rotated through 54° and the Italian Peninsula by a further 60°. BB, Balearic Basin; VB, Valencia Basin; C, Corsica; Sa, Sardinia; tf, transform fault. After Zeck, 1999.
Figure 5.4 Main tectonic units of the Alps. A. Map. B. Diagrammatic section taken along line AB to illustrate the main structures. DBN, Dent Blanche Nappe; GSB, Grand St. Bernard Nappe; MN, Morcles Nappe; WN, Wildhorn Nappe. C. Schematic section showing an interpretation of the deep structure. A, C, after Schmid et al., 2004; B, after Handy et al., 2010.
The Jura, the Foredeep Basin and the Pre-Alps
The Mesozoic platform sediments are involved in a foreland fold-thrust belt, the outermost part of which forms the Jura Mountains. This prominent chain is only about 300km long and is separated from the main part of the fold-thrust belt by a foredeep basin containing non-marine clastic sediments (molasse) derived from the rising mountain chain. In the Swiss sector of the belt there is a large outlier known as the Pre-Alps, consisting of folded and thrust rocks belonging to the Pennine Zone, which has become isolated from the main outcrop of that zone.
The Jura structure is the classic example of a so-called ‘thin-skinned’ fold-thrust belt, where a relatively thin sedimentary cover has been folded and moved along the basement (Fig. 5.5A). Albert Heim (1921) described this as being like a table-cloth that has been pushed from one side and rumpled into ridges and valleys. The present topography reflects this structure in that the ridges are anticlines, often box-shaped, and the valleys are synclines – a topography that is well expressed in the hilly country between Bourg en Bresse and Geneva.
Figure 5.5 A. NW–SE cross-section of the Jura and Molasse Basin. Note that the anticlines correspond to topographic ridges and the synclines to valleys. The thrusts are mostly directed westwards towards the foreland and the basal thrust follows the top of the basement for much of its length within a weak Triassic evaporite. B. The Morcles Nappe: cross-section showing the typical ductile folding of the Dauphinois nappes as shown by the variation in thickness of the layers. The Diableret Nappe above has been arched over the Morcles nappe. A, after Pfiffner, 2014; B, after Dietrich & Song, 1984.
This type of structure requires the presence of a weak layer that allows the rocks above it to glide along it. Such a layer is termed a décollement (or detachment) surface, which in the case of the Jura, is provided by a Triassic evaporite bed consisting mostly of gypsum. The Mesozoic cover has been moved up to 7km along the top of the basement, which has remained almost undeformed. The basal thrust plane passes beneath the flat-lying sediments of the Molasse Basin to link with the thrusts in the interior of the belt, indicating that the compressive stress responsible for the Jura structure has been transferred from the more highly deformed nappes of the Dauphinois Zone.
The Foredeep Basin forms the low ground of the Swiss Plain, which extends from south of Geneva to Bern and thence eastwards along the front of the Austrian Alps. Here the concept of the ‘molasse trough’ was established, as a basin collecting mostly non-marine sediments derived from the mountain chain. These sediments range from Oligocene to earliest Pliocene in age and are dominated by coarse clastic deposits – conglomerates and sandstones – but also include layers of marine sandstones and shales. Although the term ‘molasse’ is generally used to denote a continental deposit, the environment of deposition is one where occasional flooding by a shallow sea brings marine deposits into the sequence. The thickness varies widely, up to over 4km towards the mountain front. The inner parts of the zone have been deformed by thrusting and folding, as shown in Figure 5.5A.
The Pre-Alps occupy the mountainous area about 110km long and 30km across, east and northeast of Geneva. It is entirely surrounded by a thrust that is folded into a syncline and is detached from its continuation to the east in the Pennine Zone, as shown in the cross-section (Fig. 5.4B); it is composed of rocks identical to those of the Pennine Zone. Such a structure is known as a thrust outlier or klippe. The Pre-Alps are represented in Argand’s cross-section as the outermost part of the East Alpine nappe system, which overlies the Pennine Zone as a continuous sheet, as in Figure 5.2. A more modern interpretation of the structure is that the pre-Alps forms a block that has become detached from the main Pennine nappes and slid onto the foreland under gravity due to the thickening and uplift of the underlying nappe pile.
The Helvetic–Dauphinois Zone
This zone corresponds to the main part of the foreland fold-thrust belt; in the outer ‘autochthonous’ part, the sedimentary cover is still attached to its basement but the inner part is said to be ‘par-autochthonous’ in the sense that the sedimentary sequence is still recognisably part of the foreland, although now completely detached from its basement. It consists of a set of complex fold nappes underlain by ductile thrusts, or shear zones (Fig. 5.5B); the cores of the nappes are composed of European Palaeozoic basement, much of which is represented by crystalline massifs dominated by gneisses and granites such as the Mont Blanc massif (Fig. 5.6). The sedimentary cover of these massifs consists of a Mesozoic sequence including prominent carbonate beds, which can be correlated with those of the foreland, and which originated towards the margin of the European continental platform. Compared with the Jura, the Trias is much reduced in thickness, but the Jurassic sequence is much thicker, and a massive 300m-thick white Cretaceous limestone is prominent in the topography, enabling some of the complex fold structures to be easily traced out.
In the southeastern part of the zone, there are up to 2.5km of marine flysch sediments of Eocene age. This flysch sequence contains mass-flow deposits, known as turbidites, typical of the unstable environment of the continental slope. The significance of flysch deposits, first recognised in the Alps, in the historical investigation of orogenic belts was referred to in chapter 4.
The nappe structures are directed towards the foreland, as shown in Figure 5.4B, either westwards, north-westwards or northwards, depending on their position on the arc. The three main nappes in the Swiss sector are the Morcles, Dent Blanche and Wildhorn nappes: these form many of the high mountains of the Swiss Alps.
Figure 5.6 The Mont Blanc massif. These jagged peaks are carved from the Palaeozoic crystalline basement of the Helvetic-Dauphinois Zone. © Shutterstock, by Leonard Zhukovsky.
The outcrop width of the zone is greatest in the southwestern, French, sector, where it is known as the Dauphinois Zone. Here, southwest of Briançon, it reaches over 120km in width, but in the Austrian sector, the zone is almost entirely covered by the Austro-Alpine nappes except for a narrow strip along the thrust front and in two tectonic ‘windows’ where the cover has been stripped off.
The Pennine Zone
This is the internal, or ‘allochthonous’ zone, containing the metamorphic core of the mountain belt. The outcrop of the Pennine Zone is widest in the French-Swiss sector, where it occupies the high mountain belt straddling the French–Italian border from south of Briançon to Monte Rosa at the Swiss border, near Zermatt. It consists of three sub-zones: the Briançonnais Terrane, together with underlying and overlying zones of nappes containing oceanic deposits (Fig. 5.4B). The Briançonnais Terrane is only recognised in the Western Alps and must wedge out west of the Engadine Window in the Eastern Alps, where it is absent, although the other Pennine units continue eastwards. The frontal thrust of the Pennine Zone has travelled at least 40km over the External Zone in the Western Alps.
The Briançonnais Terrane consists of a Mesozoic sedimentary sequence underlain by Palaeozoic crystalline basement, which forms the core of several large nappes, including the Grand St Bernard and Monte Rosa Nappes (Fig. 5.7). The Mesozoic sequence differs significantly from that in the adjacent Helvetic–Dauphinois Zone. In the Vanoise region of eastern France, thick Permo-Carboniferous sediments, including coals, are overlain by a 300m-thick Triassic quartzite, succeeded in turn by up to 500m of carbonate. These are overlain by a much-reduced marine Jurassic–Cretaceous sequence containing evidence of several emergent episodes. The Eocene is represented by black pelagic shales followed by flysch.
The Briançonnais Terrane is underlain by a narrow zone of marine clastic deposits forming the lower Pennine nappes and overlain by the upper Pennine nappes. The latter contain a thick monotonous succession of mica-schists, known as the Schistes Lustrées, of Jurassic to Cretaceous age, which originated as calcareous shales and contain bands of basic volcanics and ophiolites.
These marked differences in stratigraphic history are explained by the sequence of events illustrated in Figure 4.6: the Briançonnais Terrane, having originated on a depressed European margin during the Triassic, broke away in the early Cretaceous, while the marine clastic sediments and ophiolites were formed in oceanic basins on each side of the terrane. The ophiolites and ocean-floor sediments of the lower Pennine nappes are thus thought to represent the remains of the Valais Ocean, and those of the upper Pennine Nappes to belong to the Piémont Ocean, which originally separated the Briançonnais Terrane from the Alcapa Terrane (see Fig. 4.6B).
Figure 5.7 Monte Rosa. This peak, above Zermatt, is composed of Palaeozoic basement in the core of one of the Pennine nappes. © Shutterstock, by mountainpix.
The complex overfolds in the Pennine Zone take the form of a fan, directed towards the foreland in the northwest, like those of the underlying zones, but directed towards the southeast at the opposite side of the zone (Figs 5.4B, 5.8A). The nappes generally rest on a décollement surface of Triassic gypsum.
The rocks of the Pennine Zone have been affected by metamorphism of late Cretaceous or Palaeogene age. The grade of metamorphism varies from greenschist facies in the outer parts of the zone to blueschist and eclogite in the inner. The presence of blueschists and eclogites indicates that these parts of the zone were subjected to high pressures and low temperatures, which is interpreted as evidence that they must have been subducted to a considerable depth before being exhumed and thrust onto the European Plate. The low-pressure metamorphic mineral assemblages were subsequently overprinted by higher-temperature, lower-pressure assemblages as the rocks were uplifted.
The Austro-Alpine Nappes
This zone consists of material that has long been interpreted as derived from the southern foreland, historically thought of as part of Africa but now assigned to the separate Alcapa Terrane. In the Eastern Alps, the Austro-Alpine Nappes form most of the outcrop area; the lower Pennine and Helvetic nappes only appear in the Engadine and Tauern Windows and in a narrow strip along the Alpine front. The Palaeozoic basement rocks of these nappes are overlain by a thick Permian to Triassic sedimentary cover dominated by massive shallow-marine limestones, which form the mountain chains known as the Northern Calcareous Alps. The oceanic material underlying these nappes thus represents the suture between Alcapa and the Eurasian plate.
In the Western Alps, tectonic units from the southern foreland only appear as high-level nappes such as the Dent Blanche Nappe, which is a klippe made of crystalline Adriatic basement overlying the Upper Pennine Nappes (Figs 5.4B, 5.8A). The Dent Blanche Nappe is considered to have rooted in the steeply-dipping Sesia Zone, which forms the northern margin of the Adria Terrane.
Figure 5.8 A. Pennine nappes: simplified cross-section through the Pennine Nappe Zone in Western Switzerland. Note: 1) the Lower Pennine nappe consisting of Valais Ocean crust is thrust over the Helvetic nappes. 2) The nappes of the Briançonnais Zone consist of a series of thrust slices overthrust towards the NW. 3) The Upper Pennine nappes consisting of Piémont Ocean crust overlie the Briançonnais Zone. 4) The Pennine nappes have been refolded and directed SE–wards. 5) The Dent Blanche Nappe consisting of Adrian crust has been thrust over the top of the Pennine Zone to form a klippe on the Dent Blanche. B. Structure of the Southern Alps: N–S section across the southern part of the Alto Adige north of Venice. Note the gentle open folding and south-directed thrusts. After Pfiffner, 2014
The Adriatic Foreland and the Southern Alps
The foreland on the south (or east) side of the Alpine Chain consists of Palaeozoic crystalline basement belonging to the Adria Terrane. The Palaeozoic basement has long been regarded as being of ‘African’ origin; that is, it records a similar geological history to the adjacent parts of northern Gondwana. As described earlier, Adria was one of several such terranes that had broken away from Pangaea in the Jurassic or early Cretaceous (see Fig. 4.6B). The Adrian basement forms a small outcrop northeast of Turin, where it is known as the Ivrea Zone, but elsewhere is obscured by a Mesozoic to Cenozoic platform cover sequence that has been deformed into a typical foreland fold-thrust belt (Fig. 5.8B). This zone of southerly-directed folded and thrust rocks is known as the Southern Alps, which include the famous Dolomites mountain range (Fig. 5.9), formed from Mesozoic platform carbonates, and well-known to rock climbers. This fold belt is absent near Turin but broadens to form a 90km-wide belt north of Verona. The structure of the belt is rather subdued compared to that of the Western and Central Alps and the strata are often quite flat-lying.
The northern boundary of the Adrian Terrane is represented now by steep faults, the Insubric Line in the west and the Peri-Adriatic Line in the east. These ‘lines’ are late-orogenic strike-slip faults that have displaced the original inter-plate suture. North of these faults, the suture is folded at the base of the Austro-Alpine Nappes, but its position at depth on the southern side has been subject to various interpretations: Figure 5.4C, which is based on geophysical surveying, shows the inter-plate boundary dipping southwards, but distorted by southerly-directed back-folding. However, more easterly cross-sections have been interpreted differently.
The Po Basin
The folded Mesozoic cover of the Southern Alps is overlain by the Po Basin, which is a foredeep basin consisting of several kilometres of Cenozoic sediments derived from the rising Alps. This basin received sediments from all three of the fold belts that surround the basin: the Apennines, Alps and Dinarides. The centre of the basin contains marine sediments of Eocene to Miocene age, but towards the Alpine margins, the marine strata give way to up to 3.6km of non-marine molasse, dominated by late Miocene to Oligocene conglomerates.
Tectonic summary
The structures of the outermost zones, the folded Jura and the Southern Alps, exhibit typical fold-thrust geometry (Figs 5.5A, 5.8B), with outward-directed thrusts and overfolds linked to a basal thrust along a weak stratigraphic horizon. The nappes of the Helvetic zone, having originated at greater depths, are more ductile, but still have an overall, foreland-directed, overthrust sense of movement (Fig. 5.5B). The more complex nature of the folding in the Pennine Zone, illustrated in Figure 5.8A, has been attributed to SSE-directed back-thrusting. This has had the effect of moving the Pennine zone upwards and backwards towards the Adrian Terrane, thus raising the metamorphosed interior of the orogenic belt to a higher level and isolating the Dent Blanche nappe (see Fig. 5.4B). This back-thrusting may have been caused by continued convergence between the opposing continental plates at a time when further overthrusting was prevented by the increased thickness of the belt.
Figure 5.9 The Dolomites. The Odle Mountains from Santa Maddalena in Val di Funes. These massive carbonate beds are a prominent feature of the Southern Alps. © Shutterstock, by Ttstudio.
Detailed analysis of the ductile structures of the Pennine zone indicate that the movement direction changed through time in an anti-clockwise sense, suggesting a link with the anti-clockwise change in the plate convergence direction noted previously. A similar pattern has been observed in the outer nappes of the Helvetic–Dauphinois Zone, with the outer, younger, thrusts showing a more northwesterly movement direction. The change in convergence direction was a response to the anti-clockwise rotation of Adria brought about by the expansion of first, the Balearic Basin, and then the Tyrrhenian Basin. Gravity gliding is thought to be responsible for the isolated position of the Pre-Alps and for many of the nappes in the Helvetic–Dauphinois Zone.
Accurate measurement of the total amount of shortening across the Alpine belt is impossible because of the complexity of the structures, especially in the interior zones, but it is likely that the shortening has been in excess of 250km across the belt. This compares with a crustal thickening to over 50km.
Tectonic history
Crustal deformation commenced during the mid-Cretaceous, as a result of the convergence between the Adrian and Alcapan terranes, resulting in the overthrusting of Adria onto the margin of the Alcapa Terrane – an event known as the Eo-Alpine Orogeny. This event was confined to the Austro-Alpine nappes. Subduction of the Piémont Ocean crust along the northern margin of Alcapa led eventually to the combined Adria–Alcapa microplate colliding with the Eurasian margin in the late Cretaceous. Alcapa was overthrust onto Eurasia to form the Austro-Alpine Nappes.
Further west, during the Palaeogene, as a result of the subduction of the Valais Ocean, the Briançonnais Terrane collided with the Eurasian margin. The main Alpine orogeny commenced during the Eocene when the Ligurian Ocean crust separating Adria from the Briançonnais Terrane, now welded to the European margin, was subducted beneath Adria. This resulted in the overthrusting of Adria onto the Briançonnais Terrane, and caused the already deformed Austro-Alpine Nappes to be emplaced over the European foreland.
The complex shape of both plate margins, together with the change in the convergence direction noted above, resulted in a significant regional variation in the orientation of the structures. The main Alpine deformation commenced in the Pennine Zone and progressed outwards to the Helvetic–Dauphinois Zone and onto the Foreland. Crustal shortening and uplift continued through the Oligocene, reaching its climax in the late Oligocene, about 25Ma ago, and continuing into the Miocene Epoch with the late back-thrusting phase. However, some convergent movement and uplift still continues today.
The Apennines
The Apennine Mountains consist of a series of mountain ranges extending along the spine of the Italian Peninsula from near Genoa in the northwest to the extreme southwest end of the ‘toe’ of Italy, in Calabria, a distance of 1200km (Fig. 5.10). The eastern slopes of the Apennines are inclined steeply down towards the Adriatic Sea while the western slopes are generally gentler and host many fertile valleys. The mountain system as a whole is relatively narrow compared with the Alps, and never more than about 250km across. It contains thirteen peaks over 2000m and is divided naturally into three sectors. The northern sector, aligned NW–SE, runs from near Genoa to east of Florence; the central sector, northeast of Rome, runs parallel to the Adriatic coast, and contains the highest peaks, including the highest, Corno Grande (2912m) in the Gran Sasso d’Italia Massif (Fig. 5.11). The northern and central sectors together form what is known geologically as the Northern Apennine Arc, ending north of the Bay of Naples. The southern sector, or Southern Apennine Arc, forms an arc convex towards the Adriatic, and curves round into the end of the Calabrian peninsula to continue across northern Sicily; these mountains are lower than those of the central sector, none being higher than 2000m.
Figure 5.10 The Apennines. Simplified tectonic map of the Apennine orogenic belt showing its relationship to the Alpine belt and the adjoining Iberian, African and Adrian plates. Note: 1) the older Palaeogene belt in the west and south has been separated from the younger Neogene belt in the east by a wide extensional region, much of which now forms a marine basin; 2) the active subduction zone in the southeast producing the Aeolian Volcanic Arc has formed a wide extensional zone on its upper plate. Cal, Calabria; Fi, Firenze (Florence); Ge, Genoa; Na, Naples; Ro, Rome. CG, Corno Grande; Et, Etna; St, Stromboli; Ve, Vesuvius. After Patacca & Scandone, 2007.
Tectonic setting
The Apennines appear to continue from the southwest end of the Alps (Fig. 5.1). However, although at first sight connected, their origin is quite different from that of the Alps. While the Alpine orogenic belt was forming along the northern boundary of Adria, the western side of Adria, where the future Apennines would form, was facing the Ligurian Ocean (see Fig. 4.6C).
The upper plate of the Apennine collisional system consists of the Alkapeca Terrane, formerly part of the Iberian Plate (see Fig. 4.6A). As we saw earlier, this terrane was separated from the Iberian Plate by the western branch of the Ligurian Ocean in the Cretaceous. This ocean began to close in the late Cretaceous by eastwards subduction beneath Alkapeca; closure was completed during the mid-Palaeogene, Alkapeca having been thrust westwards onto Corsica, creating the collisional belt of the Western, or Palaeogene Apennines to form a branch of the Alpine orogenic belt that extended around the eastern rim of Iberia (see Fig. 4.6C).
Deformation continued on the eastern side of this belt as subduction of the eastern Ligurian Ocean commenced along the eastern side of the combined Iberian–Alkapecan Plate. Adria and Iberia converged during the late Palaeogene, and during the early Miocene, around 20Ma ago, Alkapeca became sandwiched between Corsica–Sardinia, at the eastern edge of Iberia, and Adria. Adria thus formed the lower plate of a subduction zone that extended along the eastern side of the Italian Peninsula, and when the two pieces of continental crust collided, Alkapeca was backthrust onto Adria. (see Fig. 4.6D). Deformation continued until the present day with further convergence between Alkapeca and the Adrian Foreland.
The south-eastwards progression of the subduction zone is tracked by the changes in the ages of the volcanism from northwest to southeast. The volcanism in Sardinia and in the islands off the northwest coast of Italy is of late Miocene age, whereas that on the Italian mainland, including Vesuvius, is of Pliocene age. The youngest vulcanicity is in the currently active Aeolian Arc, north of Sicily. Seismic information from earthquakes associated with the subduction process can be used to trace the position of the present-day subducting slab, which outcrops in a wide arc southeast of Sicily and Calabria and reaches a depth of 450km beneath the Tyrrhenian Sea (Fig. 5.10).
Tectonic structure
The western part of the Apennine system (the Palaeogene Apennines) consists of Alkapecan crust that had been thrust over the eastern part of the Iberian Plate during the Palaeogene and is now exposed in eastern Corsica, Sicily, Calabria and the western coastal sector of the Northern Apennines (Figs 5.10, 5.12). Much of this sector is composed of Alkapecan Palaeozoic basement. The piece of Alkapecan crust outcropping in eastern Corsica has experienced high-pressure metamorphism of late Cretaceous to early Palaeogene age overprinted by low-temperature metamorphism during Neogene extension.
The eastern part of the Apennine system is a foreland fold-thrust belt, consisting of thrust slices of Adrian platform cover overthrust eastwards onto the Adrian Plate. In the northern and central Apennines, this compressional belt is separated from the western side of the orogen by a wide extensional zone, much of which is submerged beneath the Tyrrhenian Sea. Figure 5.10 shows the eastern boundary of the Apennines, now a thrust front, following the east side of the Italian Peninsula to Apulia. Here it joins the still-active part of the subduction zone, which forms the semi-circular Calabrian Arc along which Neo-Tethys Ocean crust of the Ionian Basin descends beneath Italy. This subduction zone has given rise to a volcanic arc on its upper plate that includes the three famous Italian volcanoes: Vesuvius, Etna, and Stromboli, the latter belonging to the Aeolian Islands Volcanic Arc, which mostly consists of extinct volcanoes. The Apennine thrust front reappears in Sicily, where it forms the northern margin of the African Plate, and crosses to Africa where it joins the edge of the Atlas Orogenic Belt described in the previous chapter.
Figure 5.11 The Pizzo d’intermesoli and surrounding mountains of the Gran Sasso Massif, Central Apennines. Note the uniformly inclined bedding of the Mesozoic sediments. © Shutterstock, by Eder.
In both Alkapeca and Adria, the Palaeozoic basement is overlain by a sedimentary cover of Mesozoic age, which had been deposited along the margins of the Tethys Ocean. Much of this sedimentary cover consists of a carbonate platform of Triassic to Palaeogene age that is now exposed on the Apulian Platform, the southeastern coastal plain that extends to the end of the ‘heel’ of the Italian peninsula. This cover was involved in the fold-thrust belt on the Adriatic side of the orogen together with the overlying Neogene sedimentary cover deposited on the active margin. The lower units of the thrust stack consist of Adrian Platform cover, dominated by carbonates, while the upper units are derived from deeper-water basins originating further to the west. These include flysch deposits from the ocean trench, which, together with the deformed platform strata, make up the accretionary prism. A series of foredeep basins surround the northern and eastern sides of the Northern Apennine Arc. These contain a thick succession of Pliocene to Pleistocene clastic sediments derived from the rising mountain chain.
Tectonic history
The Apennine belt differs significantly from the other parts of the Alpine–Himalayan Belt so far described, in that such a large proportion of the belt, as originally formed, is now dominated by extensional structures. While the eastern side of the Apennine chain is a compressional fold-thrust belt, the western side is dominated by extensional structures in the form of a system of fault blocks (Fig. 5.12). The extensional zone has split the older Palaeogene belt into two separate parts, as shown in Figure 5.10: one in the northwest, in Eastern Corsica, and the second in the south, extending from Calabria through Sicily to Tunisia. This system reflects the back-arc extensional regime on the upper plate of the Adrian subduction zone, which, in its central part, created the oceanic basin of the Tyrrhenian Sea during the late Miocene Epoch. There is no clear boundary between the compressional and extensional zones, and most of the original thrusts in the western part of the orogen have been re-activated in extension, as shown in Figure 5.12. It is thought that the two processes may have co-existed to some extent, so that while the lower parts of the orogen were still subject to convergence, the upper parts may have been dominated by extension.
The history of subduction in the Italian Peninsula is part of the regional process illustrated in Figure 5.3, involving the gradual eastwards retreat of the subduction zone from its original Palaeogene position around the eastern and southern sides of the combined Iberian–Alkapecan Plate to its present position in the Calabrian Arc – a distance of around 1200km. This was accommodated by the creation of the large extensional basins of the Western Mediterranean, such as the Balearic and Tyrrhenian Seas. As a consequence of this eastwards movement, Corsica–Sardinia experienced a counter-clockwise rotation of 54° between the late Eocene and the mid-Miocene, and the Italian Peninsula by a further 60° from the late Miocene to the present day.
The cause of back-arc extension is thought to lie in the process of ‘trench roll-back’, described in chapter 3 (see Fig. 3.12), where the position of the down-bend of the subducting slab (which determines the position of the trench) retreats backwards due to the downward pull of the cold, heavy, slab. It has also been suggested that a piece or pieces of lithospheric mantle may have become detached from the upper plate and sunk, leaving the crust above warmer, weaker, and more subject to extension. The precise way in which these processes may have worked in the Mediterranean context would have been governed by processes in the mantle that are not well understood, but must depend to some extent on how the descending slab was constrained by the geometry of the surrounding continental lithosphere – Eurasia to the north and Africa to the south – which were converging during this period and exerting a lateral pressure on the subducting slab.
Figure 5.12 Crustal structure of the Apennines. Simplified interpretative section from Corsica to eastern Italy showing collisional structures formed by the thrusting of Alkapeca westwards over Iberian basement and eastwards over Adria. These are cut by extensional structures associated with the opening of the Tyrrhenian Sea. After Carminati & Doglioni, 2012.
