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Alpine - an overview

M. Coward & D. Dietrich

SUMMARY: This overview summarizes aspects of 150 years of research in Alpine tectonics and in particular introduces the tectonic setting for the more detailed papers in this volume. The Alpine ocean, Tethys, formed as a large elongate pull-apart basin in the Jurassic, as a consequence of the opening of the Atlantic and of the movement of towards the east relative to a fixed . The NNE trending Tethys was bounded by WNW trending transforms, by the European/Iberian margin in the W and by the Adriatic promontory of Africa in the E, and its shape determined the present day configuration of the arcs of the Alpine chain. The closing of this ocean and the collision tectonics began during the , as Africa moved to the NE relative to Europe and as the N Atlantic gradually opened, to drive lberia and the southern part of the European plate to the E. of and adjacent led to high pressure of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-AIpine of the African Adriatic promontory overthrust Europe in a WNW direction. During the main Tertiary deformation the overall anticlockwise rotation of Africa led to a change-over from N to NW and WNW-directed collisional structures. The E-W striking sector of the in and is therefore a diffuse transpressive dextral shear belt, approximately reworking the northern transform boundary of Tethys, modifying it by compression related to the rotation of the African Adriatic promontory. Approximately 250 km of European lithosphere were involved in the building of the western Alps. As Alpine nappes consist largely of rock material confined to the upper crust, a large amount of lower crust and lithospheric mantle of the two continental blocks must be duplicated and/or subducted during the Alpine collision history.

The Alpine belt marks the collision zone be- the tectonics of this region. In the following, we tween the African and Eurasian plates. After do not want to present a systematic digest of all the breakup of Pangea and the opening of the this literature, but an overview of Alpine evol- Atlantic during early Mesozoic times, the area ution, with special emphasis on its still contro- between the African and Eurasian plates was versial aspects. Some themes in this introduc- occupied by oceanic crust extending from the tion and overview are based on the papers western Mediterranean to the eastern . included in this Special Publication, as well as This oceanic region has been known as 'Tethys' on others read at the Geological Society of since Neumayr (1883) and Suess (1893, 1901). London meeting, from which the volume de- The Alpine chain of western Europe (Fig. 1) rives. We must note, however, that this intro- resulted essentially from the subduction of this duction contains our own view and prejudices, Tethyan oceanic crust followed by continental sometimes supporting but occasionally at odds collision between the African and the western with other papers in the volume. part of the Eurasian plates. The chain consists therefore of slices of sediments and of con- tinental crust scraped off from both the African The main structure of the Alps and European margins, as well as of oceanic crust. These scrapings, the Alpine thrust sheets As shown in Fig. 1, the Alps consist of: or nappes (e.g. Heim 1921), are each at most a few kilometres thick, yet stacked on top of each i) Internal units, where the structures are other they have produced pronounced crustal mainly of Cretaceous-Palaeogene age and thickening. the rocks, which include relics of the This Special Publication deals with the Alpine Tethyan ocean crust, are often metamor- chain in western Europe (, Switzerland, phosed, locally up to high temperatures and and Austria), as shown in Fig. 1, with pressures. particular emphasis on the arc of the western ii) External units, where the structures are Alps. A vast number of papers in English, mainly of Oligo-Mio-Pliocene age and French, German and Italian are dedicated to the units consist of thrust and folded but

From: COWARD,M. P., DIETRICH, D. & PARK, R. G. (eds), 1989, Alpine Tectonics, Geological Society Special Publication No. 45, pp. 1-29. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

2 M. Coward & D. Dietrich

Carpathian arc

~--~ Internalunits of the Alpinechain ~__~

~7-~ Externalunits of the Alpinecbain b ~,~ ~ ,~ #>~ Mg'////./K (structuresmainlyofO/igo-Mio-Plioceneage) ~ ~ J ~

/ j tW,.t ~?~ l/h-.9 %. v~x~\\\\\

=.=o*...... =

~ Maghrebidechain ) ~a ? 300.m

FIG. 1. A generalized map of the Alpine chain in western Europe, showing the tight arcs and changes in vergence of thrusting. The internal units comprise slivers of oceanic crust and basement nappes with associated sediments from both continental margins and are affected by Alpine metamorphism. The external units consist of sedimentary d6collement nappes and were generally only slightly affected by Alpine metamorphism. The locations of the seismic sections of Fig. 4 are indicated.

weakly to non-metamorphosed basement tance of 200 km to the NW, as shown from the and Mesozoic to Cainozoic cover of the presence of windows of European plate rocks European or African plates. beneath the Austro-Alpine nappes in the Engadin and Tauern regions of Switzerland The main divisions of the western Alps are and Austria. The overthrust kinematics are shown in Fig. 2, based on Ramsay (1963, 1981), given from indicators such as stretching Debelmas & Kerckhove (1980) and Triimpy lineations (e.g. Choukroune et al. 1986, (1980). Ratschbacher & Neubauer this volume, Merle Interpretations of Alpine cross-sections have et al. this volume). In the southern part of the been controversial, as shown in the schematic , the Austro-Alpine and underlying sections in Fig. 3. In Fig. 3(a) units from the thrust sheets are steepened and cut by an southern continental margin were thrust over oblique to strike-slip shear zone known as the the northern with strong deformation Insubric Line (e.g. Schmid et al. this volume). in a part of the northern margin, the internal South of this line, in northern Italy, the Ivrea Pennine zone. The external zone is made of zone marks a part of the lower crust and upper largely autochthonous basement. In Fig. 3(b), mantle of the southern plate, deformed during the external units are represented as alloch- Palaeozoic compressive events, but uplifted thonous thrust sheets because of assumptions during Mesozoic extension and subsequent made regarding section balancing. Both figures Alpine compressive tectonics (Brodie et al. this have shortcomings suggesting a 2-D model for volume). The , to the east, form a a 3-D orogen and not considering or justifying stack of S-vergent thrusts involving basement volume balance of the lithosphere during the and Upper Palaeozoic to Mesozoic strata of the collision process. southern plate (Laubscher 1985). These struc- The plate boundary is considered to lie at the tures and their relationships to other Alpine base of the Austro-Alpine nappes, which over- structures are described by Roeder (this thrust the European plate for a minimum dis- volume). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 3

o. ,. 50 km

I

9po~Cain b d"

MESOZOIC OF EUROPEAN PLATE MESOZOIC OF FRONTAL PENNINE TURUST I~ PENN/NE NAPPES k~.iG. G~OE BASEMENT ] SUTURE PERI*ADRIATIC LINE

-~ TERTIARY BASEMENT

FIG. 2. Structural zones within the western Alps (after Ramsay 1963, Debelmas & Kerkhove 1980, Trtimpy 1980). Section lines of Fig. 3 are indicated 9SL -= Sesia-Lanzo zone, M = , B = Belledonne massif, P = Pelvoux massif, A = Argentera massif.

Along the suture zone between the two plates, and the Jura mountains in Switzerland, the there are slivers of ophiolitic material, rep- Chaines Subalpines and Provencal and resenting Mesozoic oceanic crust, metamor- thrust arcs in France, consist dominantly of non phosed to high pressures and low temperatures. to only weakly metamorphosed sediments of The underlying rocks of the European plate, the European plate. On their internal margin forming the Pennine thrust and fold nappes of there are large uplifted basement massifs: the the internal zone, represent slices of basement Aar and Gotthard in Switzerland, the Aiguilles with Mesozoic cover, which have been intensely Rouges, Mont Blanc, Belledonne, Pelvoux and deformed during Alpine compression. The Argentera massifs in France (Fig. 2). These lowest Alpine nappes emerge in the Ticino were originally considered to be uplifted but region of southern Switzerland, where they autochthonous basement (e.g. Ramsay 1963). show high temperature regional metamorphism, Boyer & Elliott (1983) and Butler et al. (1986) due to burial of the Pennine pile. The reinterpreted them as far travelled basement internal zone is limited in the W and NW by thrust sheets, but more recent interpretations what is known as the Frontal Pennine Thrust, (e.g. Lemoine et al. 1986, Gillcrist et al. 1987) along which there must have been very large show them to be strongly deformed, originally displacements, as Pennine rocks occur as Mesozoic extensional blocks, uplifted during klippen in the Pre-Alps. Upper Cretaceous and Tertiary Alpine defor- The external Alps, forming the Alps mation. The frontal regions of the Alpine folds Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

4 M. Coward & D. Dietrich

NW SE /. x 9 a ~ .. " SUTURE "" I a \ ~ ~-- --,.-..=~,.. I'

EXTERNAL ZONE INTERNAL PENNINE ZONE , ,50 km AFTER DE JONG (1973)

b .-~ -"---_ ..... SUTURE - ..... /~1 b' ..-~,-,,3 -.--:---'--"~ :...... -.%-~-.- , ' -PENNINE ZONE

j \/ .> ~/ X"'~ J.~l , ~ \ ~"~ UTURE

AFTER BUTLER e! al. (1986) "~~~ "~,~

P "- PENNINE THRUST I " INSUBRIC LINE 50 km

FIG. 3. Simplified NW-SE cross-sections through the western Alps, from (a) De Jong (1973), (b) Butler et al. (1986). Section lines are shown on Fig. 2: (a) is an unbalanced section with the external massifs shown as autochthonous or parautochthonous; (b) is a balanced section showing the Belledonne Massif as a major thrust sheet. Both sections show shortcomings; see text for discussion and Fig. 17 for comparison.

and thrusts form the Jura mountains, con- and external Alps. These deposits, together sidered to be a folded sheet of Mesozoic rocks with the early molasse, allow some of the de- which detached above Triassic evaporites. formation to be dated and hence rates of The external Alps show strongly divergent underthrusting and propagation to be thrust directions: they are northerly directed in estimated (Homewood et al. 1986). eastern Switzerland and Austria (e.g. Pfiffner 1981), change over from a northerly to a WNW direction in western Switzerland (Dietrich & Alpine characteristics - Alpine Durney 1986, Dietrich & Casey this volume), they are NW-directed in the northern French problems Alps (Gratier et al. this volume) and SW-directed The Alpine chain can be characterized by the in the southern French Alps (e.g. Fry this following main tectonic features, most of which volume). In southern France there is interfer- still pose problems with interpretation: ence between thrust and fold structures related to a Palaeogene northerly directed system, (1) Closure of the Mesozoic Tethyan basins linked to the , and these Neogene occurred at different times along the Alpine SW-directed structures. suture and involved different subduction The Alps are bordered by syn-orogenic sedi- kinematics. The timing and kinematics of ments, infilling the molassic foreland basins of subduction of Tethyan oceanic crust relative the Swiss and German plains, the Rh6ne valley to the two major continental margins is still in France and the Po basin in northern Italy. under discussion, with different models for Earlier orogenic sediments form the Cretaceous different parts of the Alpine belt being pro- and Palaeogene flysch deposits, deformed along posed (e.g. Channell & Mareschal this with the Mesozoic sediments in the internal volume; Hsii this volume; Hunziker et al. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 5

this volume). Most authors agree that in the western Alps and the Calabrian arc. These Alps sensu stricto the onset of compression tight curvatures might be explained by between the two major plates was marked (a) complex palaeogeographic outlines of by subduction of oceanic crust beneath the original continental margins leading to the African continental margin (e.g. important strike-slip movements between Laubscher's 1983 synthesis), while along the colliding , and (b) rotational the continuation of the Alpine belt in movements of the overthrusting margin Calabria, southern Italy, there is evidence relative to the underlying plate. Arcuate for early subduction of oceanic crust plate subduction (e.g. Frank 1968) and beneath the European continental margin oroclinal bending (e.g. Ries & Shackleton (e.g. Dewey et al. this volume). 1976) are not considered as valid mechan- (2) Much of the evidence for the earlier Alpine isms for the formation of the arc of the history is lost because of the overprinting western Alps. character of the compressional history and An example of the complex palaeo- because of localised middle Tertiary ex- geography of the original African conti-- tension in the western Mediterranean, to nental margin is given by the Adriatic open the Gulf of Lyons and the Ligurian promontory (Channell & Horvath 1976), Sea (Burrus et al. 1987), but particularly which indented the European plate. Such because of the fragmentation of the chain indentation in itself must lead to strike-slip by the development of later extensional motion along the plate boundaries and basins, such as the Pannonian and the within the plate (e.g. Molnar & Tapponnier Tyrrhenian basins. It is therefore still 1975). difficult to link kinematically the defor- (6) The continent-continent collision phase of mation histories postulated for individual the African and European margins has led sectors of the Alpine chain. to subduction of continental lithosphere. (3) The vergence of thrusting changes along Many Alpine geologists came to this un- the Alpine chain (Fig. 1). The main Palaeo- orthodox conclusion because of concern gene vergence, or direction of nappe regarding material balance (Ampferer emplacement in the Alps sensu stricto was 1906 and Laubscher 1969, 1970, for the on to the European continent, with partly first expression of this concept in plate tec- superimposed Tertiary backthrusting, i.e. tonics terms). In the Alpine nappe struc- thrusting towards the African hinterland tures only slices of the uppermost parts of (Argand 1916, Laubscher 1970, 1985, continental lithosphere are conserved while Merle et al. this volume, Platt et al. this huge volumes of continental lithosphere are volume). The main vergence of Cretaceous missing. Alpine geologists also found it dif- to Plio-Pleistocene nappe transport in ficult to explain HP/LT metamorphism in Calabria was on to the African plate tectonic units consisting of continental crust, (Dietrich 1988 and references therein). if they did not assume subduction and later These changes in vergence can be related to exhumation of these units (e.g. Dal Piaz et the above mentioned changes in subduction al. 1972). Subduction of continental crust kinematics. could explain the impressive isostatic dis- (4) Opposite vergences of thrusting can be equilibrium leading to the major uplift locally superimposed (e.g. in parts of the which is so apparent in today's Alpine top- Alps and northern Apennines), and out of ography (e.g. the uplift data by Hurford et sequence thrusting is an important but yet al. this volume). poorly established fact of Alpine evolution. (7) Several sectors of the Alpine chain appear Simple palinspastic restorations, based to have deep lithospheric roots, extending on the assumption that the geometrically to over 200 km. These occur, according to higher units were derived from palaeo- Panza (1984) and Mueller (this volume) in geographic belts internal relative to the the central and western Alps, the central geometrically lower units, might therefore Apennines and the Betic Cordillera. be locally impossible to construct. The subvertical roots beneath the central (5) The Alpine chain is extremely arcuate with Alps have been interpreted following tight curvatures (e.g. Goguel 1963, Laubscher's (1974) model of bivergent sub- Laubscher 1988) (Fig. 1). The sector of the duction, with participation of continental chain discussed in this volume twice lithosphere from both continents in the sub- changes strike direction of nearly 180~ duction process (Panza & Mueller 1979, through two major oroclines, the arc of the Mueller 1982, Finckh et al. 1987, Mueller Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

6 M. Coward & D. Dietrich

Front of nappes

WNW BelL*donee ESE I massif I I I 0 ~~~~i~es --_----"...... Vanoise Gran Paradtso Insubric line Po

- _.... ~ ~ "~"~."z~-~--"

Q ~m

M

- M --.... 9------".= --. M 10 km 20 (a) N Front of Penninic nappes S Hetvetic napoes f Aar massif J Pemi~ic~appes ~- ... ..-r -~-', ~ "-'.

_:.

...... :..~.:~..... - 4., -

(b) 2O FIG. 4. The deep structures of the Alps, shown by seismic sections; the locations of the sections are indicated in Fig. 1. Note the two sections have a different horizontal scale. The principal reflection layers are represented by as yet provisional line drawings and are redrawn according to Bayer et al. (1987), section a, and Pfiffner et al. (1988), section b. M: Mohorovi6i6 discontinuity, according to refraction data. TWT: two-way time-scale; 20 sec. corresponds approximately to 60 km. Section a: The ECORS-CROP line across the western Alps (from Bayer et al. 1987). The ESE-dipping reflectors suggest major lithospheric thrusting, with the African plate (s.l.) thrust over the European plate. Section b: The explosion-seismic reflection profile NFP-20 across eastern Switzerland (Mueller, this volume, Finckh et al. 1987, Pfiffner et al. 1988). A bivergent subduction zone with subduction of the decoupled lower part of the crust is assumed by Mueller (this volume) to be the best model to fit these data, though Roeder (this volume) interprets the S-verging thrusts of the southern Alps as the tip of a crustal flake back-thrust.

this volume). This interpretation is sup- (8) Alpine deformation was not confined to the ported by the N-dipping reflectors seen Alpine chain, but affected much of NW deep in the southern plate on the Swiss Europe, as far N as the Shetland Platform explosion seismic reflection data (Fig. 4, and the Atlantic margin. The European section b, and Mueller this volume). How- plate did not behave as a rigid body, but ever the deep structure of the western Alps deformed internally, reactivating Mesozoic is interpreted in a different way. The reflec- extensional faults and earlier Palaeozoic tors on the ECORS-CROP line (Fig. 4, steep strike-slip shear zones, to cause wide- section a, Bayer et al. 1987) generally dip spread tectonic basin inversion from the towards the SE, and are interpreted by Middle Cretaceous to the Neogene (Ziegler these authors as indicating lithospheric 1982, 1987, Beach 1987, Coward & Trudgill thrusting of the southern plate over the in press). Alpine deformation chronology is northern plate. This, as well as many other sometimes easier studied in these inverted interpretations of the Alps in terms of basins of the European than in the crustal accretionary wedges pose prob- main Alpine chain, where much of the evi- lems of volume balancing regarding the dence for the earlier events has been lost lower parts of the northern and southern during subsequent mountain building. lithospheres. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 7

Central and the basement massifs of the ex- Mesozoic extension in the Alpine ternal French Alps, such as the Pelvoux and domain Argentera. On the northern continent, over much of Most authors who reconstruct the plate move- western Europe, most of the Triassic deposits ments resulting from the breakup of Pangea, were continental, with the deposition of thick link the development of western Tethys to the evaporites. On the future northern margin, gradual northward opening of the Atlantic locally thick carbonate platform deposits devel- (Smith 1971, Pitman & Talwani 1972, Dewey et oped (e.g. in the Brian~onnais domain), and al. 1973, Livermore & Smith 1985, Savostin et locally basaltic volcanism occurred. On the al. 1986, Dewey et al. this volume). Extension future southern margin deep water basins were began in the Permo-Triassic, often reworking established with a very similar sedimentary earlier Palaeozoic compressional and strike-slip facies development, e.g. the Imerese- structures, and developed basins on the future Lagonegro basin in southern Italy, the Hallstatt African margin and throughout much of NW basins in Austria, the Budva-Kotor-Pindos Europe (Figs 5 and 6). Deep Triassic basins basin in Yugoslavia and Greece (Aubouin 1959, formed W of Britain on the hanging walls of Broquet et al. 1966, Scandone 1967) (Fig. 5). Caledonian and Variscan thrusts (e.g. Cheadle All these basins show basalt intrusions and et al. 1987, Gibbs 1984). Their extension direction subsided below CCD in the Liassic (Scandone was NW-SE, parallel to large steep tear faults, 1975). Lateral thick and very extensive car- which acted as lateral extensional ramps to bonate platforms developed (e.g. Ippolito et al. basins compartmentalising the extension and 1975). Middle to Upper Jurassic ocean spreading subsequent inversion (Fig. 6). These NW-SE was localised parallel to these Triassic structures were probably inherited from earlier (Scandone 1975). Variscan steeply dipping strike-slip shears and The Tethyan stretched crust was linked to the in effect controlled the main Mesozoic exten- Atlantic by a major transform system, devel- sion direction and possibly the opening direc- oped in the sinistral shear between Africa and tion of the Atlantic by tramlining relative crustal (Figs 5 and 7). The relative paths of movements (Arthaud & Matte 1977). Similar Africa and Eurasia can be obtained from recon- NW-SE trending shears occur in the Massif structing and restoring the magnetic anomaly

'o----n.n,o,~176 '... 72r ~ Jurassicplate boundary .\ 1 -- UpperTriassic deep water basins '~.,,~. Stronglysubsiding carbonate platforms Continentaldel~s#s and evar~ites

FIG. 5. The Upper Triassic palaeogeography and the Jurassic plate boundary. The plate boundary is roughly parallel to the southern front of the Hercynian deformation and does not follow closely the main sites of Middle Triassic rifting. The Germanic Triassic consists of continental deposits and evaporites (unornamented); the Alpine Triassic consists of strongly subsiding platforms and deep water basins. In the Jurassic the palaeogeography shows continuous extension of both margins (Fig. 9), and the basins and platforms of the southern margin sink partly below CCD. The restoration of the African Plate relative to the European Plate is based on the position of the continents at 175 Ma according to Dewey et al. (this volume). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

8 M. Coward & D. Dietrich

FIG. 6. Basin development in the NW foreland of the Alps; structures of Triassic-Liassic age. Regions with important Cretaceous basin inversion are shown. From Ziegler (1982) and Masson & Miles (1986), plus work by MPC.

FIG. 7. The development of Tethys at 150 Ma (Upper Jurassic), interpreted as a transform belt between the African and European continents (Laubscher & Bernoulli 1977, Dercourt et al. 1986). The more recent reconstructions of the positions of the continents (e.g. Livermore & Smith 1985, Savostin et al. 1986. Dewey et al. this volume) agree qualitatively, but not quantitatively: A-B indicates the positions of Africa at 175 and 150 Ma (interpolated) according to Livermore & Smith (1985), C-D indicates the positions of Africa at 175 and 149.2 Ma according to Dewey et al. (this volume). The location of Fig. 9 is indicated (box). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 9 data in the Atlantic; Fig. 8(b) shows the paths 1972). These ophiolites have generally been of Africa relative to stable Europe as suggested affected by Alpine metamorphism, but Jurassic by Smith (1971), Dewey et al. (1973) and ages of origin have been determined radio- Livermore & Smith (1985). These paths differ metrically (e.g. Bertrand & Delaloye 1976, from the latest paths presented by Dewey et al. Ohnenstetter et al. 1981). The Tethyan ophiolites (this volume) who suggest that the sinistral are stratigraphically overlain by a characteristic movement between the two continents was not sequence of Middle to Late Jurassic-Early followed by the pronounced dextral movement Cretaceous age, consisting of radiolarian cherts suggested by earlier models. and siliceous shales (the Radiolarite Formation), Ophiolites from the Tethyan ocean have followed by pelagic limestones (the Calpionella been found in tectonic units all along the Alpine Limestone Formation), overlain by alternations chain (e.g. the review by Knipper et al. 1986), of siliceous shales, marls and pelagic limestones from N Africa (e.g. Bouillin 1979), southern (the Argille a Palombini Formation of Decandia Italy (Dietrich & Scandone 1972, Haccard et al. & Elter 1972). These pelagic deposits have 1972, De Roever et al. 1974, Amodio Morelli been found in allochthonous units of the Alpine et al. 1976), from northern Italy (e.g. Boccaletti chain from eastern Switzerland to southern et al. 1971, Decandia & Elter 1972), from Italy. Corsica (Amaudric du Chaffaut et al. 1972, Simple reconstructions such as that shown for Ohnenstetter & Ohnenstetter 1980), from the the Kimmeridgian in Fig. 7 are based on the western Alps (e.g. Debelmas & Lemoine 1970, following considerations: Tricart & Lemoine 1983), from the central Alps (1) The width of Tethys can be estimated (e.g. Steinmann 1906, Triimpy 1960, Bearth kinematically from the relative motions of 1967, V. Dietrich 1969, 1980), from the eastern the African and European Plates. Figure 7 Alps (e.g. Bickle & Nisbet 1972, Tollman adopts the relative plate motion model of 1975), from the Carpathians (e.g. Savu 1980), Dewey et al. (this volume) and results in a from Yugoslavian Dinarides (e.g. Karamata similar reconstruction to that of Laubscher et al. 1980) and from Greece (e.g. Moores & Bernoulli (1977). As the Jurassic- 1969, Dercourt 1972, Bernoulli & Laubscher

Fro. 8. (a) Map of western Europe showing the areas affected by middle Tertiary extension and middle Tertiary compression. The Alpine displacement directions for the Neog6ne are shown. (b) Paths of Africa relative to Eurasia based on reconstructions of Atlantic opening, from Smith (1971), Dewey et al. (1973), Livermore & Smith (1985). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Io M. Coward & D. Dietrich

Cretaceous path of the African plate relative tinuity of continental crust from North to the European plate is visibly rotational, Africa to Sicily and along the Italian penin- and as the Tethyan ocean is closer to the sula to Yugoslavia and Greece, without pole of rotation, the resulting width of an oceanic connection across the prom- Tethys is slightly smaller than the contem- ontory from the western to the eastern part poraneous width of the central Atlantic. of the Tethys (Laubscher 1971, 1975, The Callovian Tethys as reconstructed by Scandone 1975, Laubscher & Bernoulli Dercourt et al. (1986, Plate 2) shows a 1977, Channell et al. 1979). A comparison much smaller width than does our recon- of the palaeomagnetic directions of Meso- struction. Since only fragments of oceanic zoic and early Tertiary rocks from the un- crust and associated sediments are pre- deformed part of the promontory (Gargano, served in the Alpine chain, there is no Istria) with the corresponding directions evidence from palinspastic reconstruction from stable Africa confirms the view that in favour of one or the other model. The the promontory moved relatively fixed to exact width of Tethys however, would de- the African plate (Lowrie 1986, and ref- pend on how much displacement is taken erences therein). Only in the post-early up by basin development on the Tethyan Tertiary did the promontory rotate counter- margins. clockwise 27 ~ around a pole near Malta (2) Tethys and the two continental margins (Van den Berg 1979) and was therefore were dissected by transform faults. Evi- decoupled from Africa. dence for the existence of a transform pat- Dercourt et al. (1986) selected a different tern in the northern part of the Tethys model, where the promontory started to domain has been presented by several decouple from the African continent at authors (e.g. Lemoine 1980, Abbate et al. 130 Ma and, according to these authors, 1980, Weissert & Bernoulli 1985), where represented an 'Apulian microcontinent.' the ophiolite breccias and ophicalcites The Adriatic promontory however was found commonly associated with the not rigid, but suffered a considerable Tethyan pillow lavas and serpentinites can amount of crustal extension throughout the be compared with breccias from present- Mesozoic. Comparable extension is not day oceanic fracture zones (Bonatti et al. recorded from the contemporaneous de- 1974). The ophiolite suite from the south- velopment of the North African crust (e.g. ern part of Tethys is petrographically very Wildi 1983) and thus active Mesozoic faults similar to that of the northern part (De had to separate the two domains, or there Roever 1972, Dietrich 1976, Lanzafame et was a major shear gradient between the two al. 1979), so that we interpret the overall blocks. Tethys, including its continental margins, (4) The width of the African-Adriatic prom- as a large pull-apart basin in a Jurassic- ontory has been constrained from palin- Cretaceous transform domain, in agree- spastic restoration of the post-Mesozoic de- ment with Abbate et al. (1980) and Dercourt formation (e.g. Bosellini 1973, Aubouin et al. (1986). The northern and southern 1973, Bernoulli & Jenkyns 1974, Ippolito et boundaries, which form the locations of the al. 1975, Laubscher & Bernoulli 1977, future Alpine and Calabrian Arcs, acted as Dewey et al. this volume). A width of about the principal transform fault margins. 1000 km is shown by all the more recent Relative to present day geographical co- reconstructions. ordinates the transforms had a NW or WNW strike (Fig. 7), parallel to the tear The reconstruction done above meets some faults which bound Triassic and Jurassic space problems. The restored Adriatic promon- basins on-shore (Fig. 6) and which often tory and unrotated Spain tend to overlap. reactivated earlier Palaeozoic strike-slip Dercourt et al. (1986) solve the problem by structures. We suggest therefore that the suggesting an extremely narrow Tethys, which Alpine transform system may have been was therefore not kinematically linked to the initially controlled by Variscan structures. Atlantic. Laubscher & Bernoulli (1977, fig. 5) (3) The African Adriatic promontory (Argand show the promontory shifted far eastwards 1924, Channell et al. 1979) was an extension to allow space for basin evolution on the of the African plate during the Mesozoic European margin and from their interpretation, and early Tertiary. Based on palinspastic the promontory had not only to rotate during reconstructions of the African continental the Mesozoic, but also to shift westwards to get margin several authors suggested the con- to its present position. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview I!

FIG. 9. The Alpine facies belts in the Upper Jurassic, near the major northern transform boundary of the Tethys, slightly modified from Laubscher & Bernoulli (1977), Bernoulli et al. (1979) and Weissert & Bernoulli (1985). The dominant palaeogeographic elements on the European margin are: the trough, interpreted by various authors as being partly oceanic (e.g. Lemoine & Tr/Jmpy 1987, Platt et al. this volume) and the Briangonnais horst. A description is given in the text and, regarding the Brianqonnais sequence, in Platt et al. (this volume). The African continental margin is dissected by extensional faults and dominated by shallow water sedimentation on the platforms and pelagic sedimentation in the basins. See e.g. Lemoine & Triimpy (1987) for an alternative palaeogeographic map of the Alpine realm with only a partly oceanic Tethys (oceanic lenses between thinned continental crust with individual microcontinents of normal crustal thickness). The simplified section illustrates the basin histories.

Figure 9 shows a rough palinspastic resto- will be of fundamental importance in deter- ration of the northern part of the Tethys and its mining the later subduction history of the margins in the realm of the future Alps. The European continent. European margin was dissected by systems of Most of the basement massifs of the western early to middle Jurassic extensional faults, Alps were probably originally extensional generally arranged in en echelon manner. fault blocks, which were later uplifted (e.g. Examples are the Crvennes, N~mes and Graciansky et al. 1979, Lemoine et al. 1981, Durance fault systems in southern France (Fig. Tricart & Lemoine 1986, Lemoine & Triimpy 10) and the faults in the Helvetic, Ultrahelvetic 1987). Furthermore, some authors (Lemoine and Pennine realms (e.g. Lemoine & Trtimpy et al. 1981, Lemoine et al. 1986) have em- 1987, and Fig. 9). Laubscher (1983) argues that phasized the greater importance of exten- such faults are responsible for the formation of sion relative to subsequent Alpine compression a large trough, the north Penninic fracture in determining Alpine tectonics and have zone, generally known as Valais trough (Fig. compared the tilted fault blocks of the external 9). This fault trough extended from Spain Alps to structures found at the Atlantic margin, to Austria and represents a branch of the as in the Bay of Biscay (Montadert et al. Pyrenean zone (Laubscher 1983). The Penninic 1979). Figure 10 comprises a region of the fracture zone is therefore interpreted by external Alps, centred at Pelvoux, where Laubscher (1983) as a subplate boundary which extensional faults are still well preserved; to Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

I2 M. Coward & D. Dietrich the N, in the Mont Blanc region, most of promontory was NE-SW, relative to today's the original extensional geometry has been coordinate system. This suggests either a dif- destroyed by subsequent compression. In the ferent extension direction on the southern part Pelvoux, Grande Rousses and Belledonne of the Adriatic promontory, relative to the original normal fault scarps have been re-eroded , or more likely, a subsequent anti- and preserved as cliffs on the sides of these clockwise rotation of the southern part of the basement massifs and the early half-graben fault Adriatic promontory during the Tertiary. geometry and sediment fill is still preserved. Figure 11 gives a diagrammatic section through the basins of the western Alps, based on Gillcrist et al. (1987), and shows the different subsidence Early compression and early plate histories. The extension direction, as deter- collision mined by the extensional lateral ramps, was NW-SE, parallel to and possibly reworking The Eoalpine phase leading to a Cretaceous Palaeozoic structures in the basement. Similar Alpine chain extensional fault geometries formed across much of NW Europe during Jurassic extension Two principal sites of early collision are easily (Fig. 6). visible from the geometry of the blocks involved The amount of extension in the external in transcurrent and rotational motion (Fig. 7). French Alps is uncertain. Gillcrist et al. (1987) Jurassic compression occurred in the Vardar argue that early normal faults were rotated to zone of the Hellenides-Dinarides, contempor- low dips during the Liassic, suggesting stretch- aneous with ocean spreading in the western part ing factors greater than two. Downthrows on of Tethys (Laubscher & Bernoulli 1977, steeper late Liassic faults are in excess of 3 km, Dercourt et al. 1986), and the component of but the stretching factors were probably low. anticlockwise rotation between the two con- Parts of the Valais zone may have had greater tinents led to consumption of the Tethyan extension, being stretched actively during the oceanic crust followed by mid-Cretaceous com- Cretaceous as well as the Jurassic, as volcanic pression between the northern and western front rocks are interlayered with Lower to Middle of the African-Adriatic promontory and the Cretaceous deep-water sediments (Antoine European continent. The following evidence 1971, 1972). Kelts (1981) and Loubat (1984) points towards the existence of Mid-Cretaceous have compared the Cretaceous lithologies and orogenic events: structures of the Valais zone to those of the In the northern calcareous Alps, formed of present-day Gulf of California pull-apart basins, Permian and Mesozoic cover to the Upper characterised by the production of small, rapidly Austro-Alpine nappes (Frank 1987), the limno- deepening basins, with thick sedimentary fluviatile to marine Gosau beds of Coniacian sequences intruded by basic sills. age unconformably transgress already formed Internal to the Dauphin6-Helvetic and thrust nappe structures. Most, if not all, of the Valais zones, the Brian~onnais zone of the Austro-Alpine basement nappes of the eastern French Alps shows a very different subsid- Alps were affected by a Cretaceous meta- ence history, with thick sediments of Triassic morphic overprint (Frank et al. 1987a), reach- age, but no or an extremely reduced Jurassic ing ->600 ~ and 6-7 kb in the Otztal-Stubai sedimentary record. The Pennine zone of the complex (Frank et al. 1987b). From shear sense Swiss Alps, however, shows initial rapid sub- indicators, Ratschbacher & Neubauer (this sidence during the Jurassic, followed by a volume and references therein) suggest W to thermal phase of slow subsidence (Homewood NW-directed movement of the Austro-Alpine & Lateltin 1988) (Fig. 11). units during this phase. Deeper parts of extensional faults are pre- In the there was also deformation served as ductile shear zones on the southern of the European continental crust during this side of the Alpine suture, along the margin of time. Part of the Penninic 'Biinderschiefer the Ivrea zone (Brodie & Rutter 1987, Brodie formation', composed of Triassic carbonates and et al. this volume). Kinematic shear indicators other intensely deformed metasediments, were suggest an E-W to NW-SE extension direc- originally deposited on thinned European con- tion of these ductile shear zones, in agreement tinental crust. These and metabasic and ultra- with the direction of extensional faulting basic rocks, representing Tethyan ophiolites affecting the NW part of the African Adriatic exposed in the Tauern , below the promontory (Fig. 9). The contemporaneous Austro-Alpine units, have been affected extension direction in the Apenninic part of the by a blueschist metamorphic event, dated Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview I3

~ . . ~ ., ,., -,..,...-,

AIGUILLES ROUGES

BLANC

9 9 E--T-] Tertiary graben fill I'] ,, molasse \ ~ flysch BELLEDONNE ~ basement massif thrust .GRAND J normal fault ROUSSES .... pre-Senonian folds 9 ..

9 . . , [

, . , . . . / .'.'." .~/ // / '/ / / ~_...,.__,._.., / // / 9

,50kin

Fie. 10. Map showing the basement massifs of the western Alps, the regions where Mesozoic normal faults are preserved, and the interference between Pyrenean and Alpine ihrusts. at 90-60 Ma. This event overprints an earlier, widespread evidence for a Cretaceous . but not dated, high pressure metamorphic event Williams & Compagnoni (1983) interpreted (Frank et al. 1987c). In summary, Cretaceous the early Alpine history of the eclogitic schists in the eastern Alps and 'Gneiss Minuti', in the lower part of the (Slapanski & Frank 1987) appears to have (see Fig. 9) as involving thrust tec- caused detachment of units from both con- tonics of Austro-Alpine basement under con- tinental margins and to have produced Eoalpine ditions of increasing pressure. Peak conditions Europe-vergent nappes. are recorded at >15 kb and 500-550 ~ and the In the central and western Alps there is also pressure peak is dated at about 110 Ma Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

14 M. Coward & D. Dietrich

~.Y=

.=_ 0 r

~.=_

.=_>

,o 0

52 Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 15

(Hunziker et al. this volume). Williams & promontory would lead to subduction of the Compagnoni (1983) suggest that the defor- ophiolites and as well to their obduction onto mation under high pressure conditions occurred the Austro-Alpine margin, and would also in an active subduction zone, located within the account for the contemporaneous high pressure Austro-Alpine crust. Underplating of the metamorphism in parts of this margin (Fig. 12). Austro-Alpine crust by its own plate margin Cenomanian Cr-spinell and garnet-bearing would conclude this deformation sequence, up- flyschs of ophiolitic and Austro-Alpine prov- lifting and folding the high grade rocks. The enance respectively are known to have been subduction zone would become inactive and a deposited on the southern margin of the Tethys new subduction zone would develop further to (e.g. review by Wildi 1985). Such flyschs, testi- the NW, within oceanic crust. Rubie (1984) monies of ophiolite obduction on the southern presents a thermal model for such an under- margin and of early erosion of Austro-Alpine plating process. crust, are interpreted by Laubscher (1970) as Hunziker et al. (this volume) review the being deposited in the foredeep of the S-vergent Cretaceous ages generally associated with high Cretaceous orogen. pressure metamorphism, which have been Cretaceous compression also affected the found in units of African, but also in units of European continental margin in the central and European continental and of Tethyan oceanic western Alps, but according to Laubscher crust. The peak P-T conditions were reached (1970) and Laubscher & Bernoulli (1982) pre- between 110 and 80 Ma (Hunziker et al. this dated continental collision. The movements volume, their Fig. 1). But it is still debated as to were mainly concentrated along the north whether all these radiometric ages represent Penninic fracture zone (Fig. 9) and produced tectonic events; Hurford et al. (this volume) the north Penninic Cretaceous flyschs. Trans- show that a well-defined Eoalpine Rb-Sr iso- pressive en 6chelon fault zones can develop chron from the Suretta nappe, considered to be characteristically double-vergent flower struc- of European central Penninic provenance, may tures. Such structures rather than Cretaceous be an artifact, i.e. could represent a 'possibly nappes are shown in Fig. 12 as a response to the Hercynian system, partially disturbed during Cretaceous compression on the western part of Alpine times' (Hurford et al. this volume). As- the European continent (e.g. Laubscher & suming that the geochronological data indicate Bernoulli 1982). In D6voluy in the external widespread Eoalpine tectonic events, and as- French Alps, the deformation can be clearly suming that the palaeogeographical reconstruc- dated as pre-Senonian, as there is a major tion of Fig. 9 is correct, it would appear that the unconformity of gently dipping Upper Eoalpine compression had to involve subduc- Cretaceous sediments resting on tight inclined tion of both continental margins, to account for NW-vergent folds involving Middle Cretaceous the high pressure metamorphism. Such a situ- rocks (e.g. Mercier 1958) (Fig. 10). The origin ation is difficult to conceive and therefore the of this deformation is not clear, since these classical palaeogeography has been questioned; compressive features cannot be linked easily to Laubscher & Bernoulli (1982) discussed the movements related to the North Penninic possibility of a different palaeogeographical fracture zone. position of the Monte Rosa nappe, generally It should be noted that an apparent discrep- assumed to represent central Penninic base- ancy exists between the model of S-vergent ment, but showing a 110-100 Ma high pressure Eoalpine ophiolite nappes and the model of event (Hunziker et al. this volume); Hunziker et W to NW-vergent Eoalpine thrusting of the al. (this volume, their fig. 2D) further suggest Austro-Alpine units. South-vergent thrusting the possibility that all the Cretaceous high of ophiolite nappes would certainly predate pressure units originated from the Austro- continental collision, while W- to NW-vergent Alpine margin of the Tethys. In this not gener- thrusting of the Austro-Alpine units of the ally accepted model, the European margin eastern Alps would be related to collision. We would end south of the Ultrahelvetic zone. Ac- therefore suggest that early ophiolite obduction cording to these authors the Eoalpine orogeny onto the southern continental margin was fol- would be related to subduction of the oceanic lowed by a diachronous continent-continent crust beneath a newly defined, tectonically collision which occurred earlier in the eastern thickened Austro-Alpine margin. Alps than in the central and western Alps. The Laubscher (1970) and Laubscher & Bernoulli concept of diachronous deformation progress- (1982) suggested a different model for the early ing from E towards W is of primary importance convergence of the three plates. Bivergent sub- when analysing the Tertiary Alpine main phase duction of the Tethys plate and of the Austro- structures. Alpine margin of the African Adriatic As already mentioned, the Middle Cretaceous Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

~6 M. Coward & D. Dietrich

Helvetic Valais Brian~onnais southvergent ophiolite Austroalpine Crust: shelf "trough" platform Tethys r~gpes ! upper . middle-

o l_~~k~

Fir. 12. The Cretaceous orogeny, according to Laubscher & Bernoulli (! 982). The ophiolite nappes form a southvergent chain which is eventually thrust over the European plate during final closure of Tethys.

continental collision in the eastern Alps pro- relative to Africa and to Europe. This move- duced NW-directed overthrusts, suggesting ment could be linked with the sea floor dextral transpressional deformation between spreading in the eastern Mediterranean as the African Adriatic promontory and the suggested by Dercourt et al. (1986) (see southern margin of the European continent also Fig. 13a). Cretaceous extension does (e.g. Ratschbacher & Neubauer this volume). affect the platform carbonate sequences of Dextral transpression also played an important the south Italian Apennines (e.g. Ippolito role during the Tertiary continental collision in et al. 1975) but the observation that the the central Alps (Dietrich & Casey this volume, facies belts of the southern Apennines con- Merle et al. this volume, Schmid et al. this tinued through Sicily to North Africa volume), as will be discussed later. Such a defor- (Scandone et al. 1974) as weIl as the palaeo- mation regime can be easily deduced from the magnetic data of Lowrie (1986) suggest that older plate models of Smith (1971) and Dewey the promontory did not become a separate et al. (1973), who postulated dextral shear plate until the early Tertiary, and argue between the African and European Plates as against the model of Fig. 13(a). a consequence of the opening of the North (3) During the Eoalpine period the N Atlantic Atlantic, but not from the new plate reconstruc- began to open and the Iberian peninsula tion by Dewey et al. (this volume), where during moved eastwards relative to the main the Cretaceous, the motion of Africa was in- European Plate (Fig. 13b). If the southern itially to the ESE relative to a fixed European part of the European margin, south of the plate, changing at about 90 Ma to an ENE to N Penninic fracture zone, was linked to the NE motion. There are several possible expla- , then these movements would nations for the apparent discrepancy between generate transtensional deformation in the the observed deformation regime and the Valais zone to the W, but transpression to model by Dewey et al. (this volume): the south-east, against the Adriatic prom- ontory (Fig. 13b). This is a preferred model (1) The recorded WNW directed motions of for the Eoalpine overthrust tectonics, as it the Austro-Alpine nappes may not have requires no extra plate generation or been the original overthrust directions motion, other than that which can be ex- during the Cretaceous, but the whole thrust plained by the gradual opening of the pile may have been rotated along with the Atlantic. Adriatic promontory during the Tertiary. This would require up to 90~ of rotation of Cretaceous compression affected the Euro- the nappe pile, a value greater than that pean crust far to the north. There was impor- recorded for the rotation of the promontory tant pre-Upper Cretaceous deformation of the (e.g. Channell et al. 1979, Van den Berg Mesozoic basins of NW Europe, S and W of 1979, Lowrie 1986). Britain and in parts of the North Sea (Ziegler (2) Several authors (e.g. Dewey et al. 1973, 1982, 1987, and Fig. 6). This may have been Dercourt et at. 1986) have assumed that the caused partially by strike-slip movements on Adriatic promontory might have broken the large NW-SE trending basin bounding tear from the African plate to form a separate faults; for example, there was probably move- Apulian plate at this time, moving NW ment on the N Pyrenean fault at this time. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview I7

(o) ~ T~r,,s (b) j AUSTROALPINE NAPPES AND OPH/OLITE 08DUCTION

FI6.13. Cartoons of the Eoalpine deformation: (a) related to opening of an E /ocean, based partly on Dercourt et al. (1986), and (b) related to the SE motion of the southern part of the European plate comprising Iberia. This motion is induced by the opening of the N Atlantic.

However, there was also regional compression N to NE directed motion of Africa relative to and uplift of Mesozoic rocks, suggesting NW-SE Europe, to close up its Lower Cretaceous basins compression across the basins (Fig. 6). We and develop a region of pronounced crustal suggest that this intraplate deformation was thickening from the Pyrenees to SE France. In related to the gradual opening of the Atlantic, southern France, north of the Penninic fracture and was formed by stresses generated at the zone, basement massifs were uplifted on spreading ridge or at the northward propagating generally northerly directed thrusts, with local tip of the ocean spreading zone. back-thrust systems (Fig. 10). Some of these The Cretaceous orogeny is also known in structures appear to have reactivated earlier Calabria. In the Polia-Copanello unit (Amodio extensional faults, related to Cretaceous Morelli et al. 1976), a nappe consisting of pre- stretching. Alpine high grade rocks of European proven- In the Pyrenees belt, there was probably no ance (Dietrich 1988), biotite ages of between true oceanic crust except in the Parentis basin 150-120 Ma have been found (Borsi & Dubois to the W of this belt, but deformation during 1968, Civetta et al. 1973). These are interpreted the Pyrenean phase considerably shortened the as the age of uplift of the high grade rocks from previously stretched crust to produce a diver- lower crust (Borsi et al. 1977). Similar high gent thrust zone. On the N side, originally S to pressure metamorphism of probably early SSW dipping extensional faults were reacti- Alpine age also affected the Castagna nappe vated in the . The distal faults (Amodio Morelli et al. 1976) as well as the were fully inverted with syn-rift sediments Calabrian ophiolites. pushed out of the half-graben, while the more Laubscher & Bernoulli (1982) have em- proximal faults were less affected and many still phasized that there already had to exist pro- show net extension. A devel- nounced changes of the strike direction of the oped ahead of the thickened crust, infilled by Cretaceous Alpine chain. From the palaeogeo- Eocene molasse. The central portion of the graphic shape of the African Adriatic prom- Pyrenees, the axial zone, consists of Hercynian ontory, these authors suggest the existence of basement, uplifted on crustal scale thrusts 'Cretaceous orogenic loops', i.e. early arc struc- (Seguret & Daignieres 1986) and also shortened tures as precursors of the actual arcs of the and thickened to develop an upright schistosity. chain. The southern margin of this axial zone forms an antiformal stack of thrusts involving basement and Mesozoic cover, with a transport direction The Pyrenean event generally to the SSW over the Ebro foreland basin (e.g. Hirst & Nichols 1986, Hossack et al. During the Upper Cretaceous and the lower 1984). Thrust analyses (Hossack et al. 1984, Tertiary, the European sub-plate south of the N Williams 1985, Munoz et al. 1986) suggest large Penninic fracture zone became caught up in the displacements, involving probably over 100 km Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

18 M. Coward & D. Dietrich shortening of the Pyrenean basins and thicken- basement gneiss and granite to Jurassic cover ing of the crust to over 50 km. Southward and overlie inclined folds and W-directed thrusts thrusting over the developed from (A. Chambers pets. comm. 1988). Figure 15 Lower Eocene to Miocene (Labaume et al. shows a simplified map of the main regions of 1985). Deep seismic profiling across the pre-Eocene deformation in the western Alps. Pyrenees (ECORS 1988) shows an apparent The exact age of this deformation is unknown, wedge of European crust driven over Spanish though it is probably co-eval with the N to NNE lower crust and mantle, with its southern tip directed Pyrenean thrusts seen further S. somewhere beneath the axial zone. The N- vergent thrusts of the N Pyrenees and the Aquitaine basin appear to be linked to a S- The Alpine main phase dipping detachment at the top of the wedge, that is they form a backthrust to the main Nappe emplacement on to the European S-directed crustal scale overthrusting and continent and anticlockwise rotation of the probably formed along a reactivated earlier direction of overthrust shear extensional detachment (Fig. 14). The ECORS section is very similar to the section proposed The main phase of Alpine deformation is here for the central and southern Alps by Roeder defined as the effect of continent-continent (this volume). collision on the already deformed margins of the Pre-Eocene deformation affected the external African Adriatic promontory and the European basement massifs of the French Alps (Gidon plate. It comprises thrusting of Austro-Alpine 1979). Much of the uplift and closure of the elements, i.e. nappes consisting of rock units of Mesozoic half-grabens occurred before the the deformed southern margin, onto the deposition of the Eocene flysch, the 'Gr~s European continent, as well as metamorphism Nummulitique'. These synorogenic sediments and thrusting of the European Pennine nappes. were derived from thickened and uplifted This was followed by the deformation of the crust to the S and SE, but along the southern Brian~onnais domain, the formation of the and eastern margins of Pelvoux, they rest major external structures, such as the Ultra- unconformably on a sequence ranging from helvetic and Helvetic units in Switzerland and

NORTH PYRENEAN SOUTH SOUTH PYRENEAN FAULT NORTH ZONE AXIAL ZONE

9-,.,., ~..,.,.,,~~ __ ,,....~. =~'~=~'l"r-'~_--r "~'~"~l"~"'~"-"-'"iJl~.,,,,,.-.--" ~ I

J~.~,,.~ --"~'-'l:'"-'"=--'-~"--~-~ ~='':------" --- "'---- -'~"X" "'- - ~ "" -"" --"

J secs (TWT) ANTIFORMAt STACK IN BASEMENT & TRIAS I UPRIGHT FOLDS TERTIARY EBRO SOUTHERN THRUST I & CLEAVAGE EDGE OF.... AQUITAINE BASIN ZONE I I BASIN ~__ -.~~ I i~..- ..-,I

..... ~ --. _....,,~'.-.--: =-

REFLECTIVE LOWER CRUST MoHO

WEDGE OF FRENCH __ ~_ (ORIGINALLY THINNED) CRUST b

I00 km

Flc. 14. The ECORS line through the east-central Pyrenees: (a) line drawing, (b) interpreted in terms of crustal scale wedge-type thrusting. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 19

of Mesozoic basins in southern Britain. Alterna-

Alpine thrust directions tively, Gillcrist et al. (1987) link the basin inver-

j~ Alpine thrusts sion and deformation of the European craton

preserved Mesozoic norrr with that of the Alps by steep strike-slip shear faults EScene flysch zones, rather than by flat detachments deeper

Mesozoic in the lithosphere. Compressive stresses causing

basement inversion in the NW European craton (Fig. 8a) may be generated within-plate or at the Atlantic extensional margin, rather than at the Alpine collisional zone. Africa's anticlockwise rotation into collision appears as a key to the understanding of the Tertiary tectonic history of the Alps. Regarding Alpine deformation, there are two principal Ylysch unconformable on folded MesozoJcs effects of an anticlockwise rotation of Africa relative to Europe: (1) A diachronous collision, occurring earlier able thrust directions end the Pelvoux fred hnverted) massff in the eastern Alps than in the central/

:lysch onlaps western Alps and related diachronous de- uplifted Combeynot basement formation, progressing from E towards W. (2) The re-occurrence of dextral transpression

I:t .' ...... , in subsequent zones of active deformation. .:.:::;~Kvobx'::::::::: b:-:.:.:.:.:.:.:.:.:.:.:.:J In the eastern Alps continent-continent col- lision is assumed to have occurred already in the early Late Cretaceous, according to Slapansky & Frank (1987). These authors suggested collision of the Austro-Alpine units Eocene flysch overlies W ver~m~ folds and thrusts m basement ano 'Hesozotcs with the Brian~onnais domain of the eastern Alps at 90 Ma, causing large scale inversion of FIG. 15. Map of the Pelvoux area showing the the frontal parts of the Austro-Alpine base- locations of partially inverted half graben and pre- ment. On the contrary, in the western Alps the Eocene deformation, the Eocene unconformity and sedimentary cover of the Brian~onnais zone the subsequent Alpine tectonics (from Gillcrist et al. ranges from Triassic to Eocene (e.g. Debelmas 1987, Chambers pers. comm. 1988, Spencer pets. comm. 1988, and work by MPC). & Lemoine 1970). Laubscher (1970) assumes continent-continent collision in the central Alps as occurring during the Eocene. the Cha~nes Subalpines in France and the The concepts of Alpine continental collision Pliocene deformation of the Jura. The time of as suggested by the various authors depend collision may be defined as the time at which all entirely on their concepts of Alpine palaeogeo- true ocean floor had disappeared. Later com- graphy. If Tethys is viewed as a complex pression, forming many of the nappe structures palaeogeographic domain consisting of small in the European crust, can be considered as oceans and intervening stretched continental post-collisional, that is, due to intra-plate blocks (e.g. Lemoine & Trtimpy 1987), the deformation. timing of continent-continent collision is less The detachment horizon, from which the clearly defined than if relatively distinct conti- main phase nappes were stripped off the nental margins are assumed, as, for example, in European lithosphere, rises from deep in the Fig. 9. Milnes (1978) suggested therefore for crust and from amphibolite facies rocks in the the central Alps mid-Cretaceous continental S, to close to the surface in the N, below the collision, not as a single subduction suture, but unmetamorphosed Jura. This upward pro- as a long period of convergence, leading to the gression of the detachment gives the Alps their deformation of the Pennine zone into a tectonic characteristically asymmetric structure, verging mega-m61ange consisting of continental and towards Europe. Bally (1981) and Beach (1987) oceanic crust and of sediments, followed by have suggested that there may be other de- post-mid- intra-plate compression. coupling horizons within the lower crust or Triimpy (1980) expressed a very similar view. lithospheric mantle, to transfer slip far into the For Platt (1986) the Tethys domain consists of European craton and cause uplift and inversion two, possibly three, oceanic zones separated by Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

20 M. Coward & D. Dietrich

microcontinents. Platt et al. (this volume) Lepontine area, the Ticino and Simplon domes therefore bracket the underthrusting of the (Merle et al. this volume). The characteristic Brian~onnais domain of the western Alps onion-shell map pattern of the Lepontine meta- beneath the Alpine accretionary wedge be- morphic isograds (Niggli 1970) reflects this tween possibly and not later than domal structure. Mid-Eocene; continental collision is assumed The deformation history of the Pennine zone by these authors as Lower Oligocene. We therefore shows an overall anticlockwise ro- prefer a simpler palaeogeographic reconstruc- tation of the direction of crustal shortening tion here (Fig. 9), with one Tethyan ocean (Steck 1987). Such a deformation pattern of which closed diachronously from E to W, foreland directed NNW to NW-wards thrusting, though there were several Triassic to Cretaceous followed by steepening of the rear part of the stretched basins on the continental margins. nappes (the formation of a 'root zone'), and Following Laubscher (1970) and Laubscher & followed by massive uplift and backthrusting Bernoulli (1982) we explain the several oceanic and by westwards 'lateral escape of rocks within slices found in the Alps as thrust sheets between the Pennine zone' (Merle et al. this volume) has units of continental crust formed by a complex been independently suggested for the sub- deformation history, as, for example, Eoalpine sequent deformation history of the Helvetic S-vergent thrusts being incorporated in the zone (e.g. Dietrich & Casey this volume). This main phase N-vergent chain, rather than by a rotational deformation sequence is therefore of complex original palaeogeographic configuration regional importance as an expression of plate involving several small sutures. convergence associated with dextral shear. The peak of metamorphism in the Pennine Anticlockwise rotation of overthrust shear led nappes of central Switzerland has been dated as to westwards structural stacking and induced ranging between 45-35 Ma (see, e.g. model A locally gravity collapse: Dietrich & Casey (this in fig. 20 of Hurford et al. this volume). The volume) interpret the Simplon normal fault Pennine nappes consist of continental basement (e.g. Mancktelow 1985, Steck 1987) as such a and Triassic-Jurassic sediments (Fig. 9); deep collapse structure, which again induced further burial of this zone has to be assumed to account westwards directed mass escape. for the metamorphic peak temperatures of Anticlockwise rotation of the thrusting direc- about 650 ~ (Frey et al. 1980). The Pennine tion in time and in space on the scale of the arc thrusting in the central Alps occurred under of the western Alps is discussed by Vialon et al. amphibolite facies conditions. The associated (this volume). These authors compare the movement directions were towards NNW in the thrust directions around the arc of the western central part of the Pennine (the Lepontine) Alps to a ring shear model leading to radial zone and towards NW in the western part compression directions in a dextral vertical (Merle et al. this volume, their fig. 4), relative simple shear band between a stable outer zone to today's coordinate system. Convergence con- and a coaxially rotating inner cylinder. tinuing during the Oligo-Miocene led to the Anticlockwise rotation of thrusting has been formation of a N-dipping steep belt in the documented in detail for the external zones of southern part of the Pennine zone (Milnes the Alps, the Helvetic zone and the Chaines 1974); this deformation corresponds to the Subalpines. Dietrich & Durney (1986) describe Insubric phase of Argand (1916, see also the rotation from northwards directed over- Schmid et al. 1987). In the early Miocene the thrust shear in the older, upper Pennine zone was uplifted more than 9 km by to NW or W directed shear in the youngest part backthrusting (thrusting towards south) along of the Helvetic nappe pile, the inverted limb of this steep belt, generating mylonites (Hurford the Morcles nappe. Gourlay (1986) describes a et al. this volume). Ridley (this volume) con- similar deformation sequence for basement and siders that it is these shears that cause juxta- cover in the Mont Blanc area. Ramsay (this position of rocks of different grade of high volume) shows how the Alpine structures de- metamorphism in the Sesia-Lanzo zone. veloped in the external Aiguilles Rouges and Movements contemporary with this uplift Mont Blanc Massifs are anticlockwise obliquely and under greenschist facies conditions over- superposed on the already emplaced Helvetic printed the earlier thrusting in the Pennine zone. nappes. Dietrich & Casey (this volume) suggest These movements were parallel to the strike of that in the belt of the Helvetic nappes, the zone the Alpine chain, i.e. towards W (Merle et al. of active thrusting moved from east towards this volume, their fig. 7). Uplift associated with west, i.e. diachronous deformation with the westwards thrusting was responsible for the youngest increment being observed only in formation of the structural domes of the western Switzerland. Gratier et al. (this volume) Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 2I deduce the Alpine displacement of the external N to NW-wards crustal shortening in the central Chaines Subalpines from a strain trajectory and eastern Alps. map, separating the displacements related to Ricou & Siddans (1986) reviewed the major the Pyrenean event from the displacements N-S trending strike-slip movements from the related to Miocene-Pliocene compression; for internal and external zones of the western Alps. the latter they find a regional anticlockwise These movements were concentrated along rotation. faults with a sinistral sense of displacement. In western Pelvoux, there are pronounced These authors showed how from the beginning local variations in thrust direction, often within of the Tertiary onwards the WSW-ENE individual thrust sheets (Beach 1981). Thus trending palaeogeographic elements of the westwards along the trace of a thrust, the trans- European margin were gradually sliced up by port directions as observed from slickensides, such faults. The resulting overall sinistral dis- shear bands, cut-off and lateral ramp data placement of the palaeogeographical zones (Fig. (Gillcrist etal. 1987, Gillcrist pers. comm. 1988), 16a) lead to an Oligocene arc configuration. change from NW to W to SW directed. How- On the other hand, the importance of dextral ever, there seems to be very little discernible E-W strike-slip movements during the evol- rotation of the Pelvoux Massif itself, as the pre- ution of the central and eastern Alps has already Alpine structures, such as the major NW-SE been mentioned. Laubscher (1971) suggested trending tear faults, have the same trend as on from regional geological reasoning 300 km of the Alpine foreland. dextral strike-slip movements between the The divergence of the direction of thrusting European foreland and his Insubric plate, part as well as the non-plane strain nature of much of the Neogene African Adriatic promontory. of the deformation make coaxial section resto- These movements would account for the domi- ration across the Alps impossible (Fig. 8a). But nantly westwards thrusting in the N-S segment it is easily visible from Alpine cross sections of the western Alps, and for the closure of the that the overall amount of displacement has to Ligurian part of the Tethys. The post-Oligocene be large. Some indication of the amount and westwards convergence of Laubscher's Insubric rate of shortening may be obtained from a study plate caused the following three major com- of the foreland basin sediments. The Swiss fore- pressional events in the western Alps (Fig. 16b): land basin sediments (e.g. Rigassi 1977, (1) The obduction of a slice of lower crust/ Mugnier & Vialon 1986, Homewood et al. upper mantle of the Insubric plate, the 1986) are of Oligocene to Upper Miocene age. Ivrea body, from below into the internal The earliest sediments are turbiditic and part of the western Alps (the Ivrea body formed in basins ahead of the encroaching being defined through characteristic seismic Alpine thrust sheets. These are followed by and gravimetric data; see the impressive molassic sediments, from early wave-dominated scaled section in fig. 8 of Schmid et al. this shoreline sands to alluvial fans and floodplain volume). and lake sediments (Homewood et al. 1986). (2) The westwards thrusting of the Pennine and From the balancing of sections and the migration of the Brian~onnais units and the particu- of sedimentary pinch-outs on restored cross larly spectacular backthrusting of the latter sections, Homewood et al. (1986) estimated an (e.g. the sections by Platt etal. this volume). average thrust propagation rate of 7.35 mm/yr, (3) The d6collement of the external units of the with a shortening rate of 3.5 mm/yr and a basin western Alps, the Chaines Subalpines (e.g. migration rate of 5 mm/yr. Assuming therefore the sections by Gratier et al. this volume). that Alpine deformation advanced at this 0.5 cm/yr time-averaged rate since final collision Laubscher (1971, 1984) presented 3-D in the Eocene, there should have been about plate models to explore the evolution of 250 km of European lithosphere involved in the the NW corner of the African Adriatic building of the western Alps. promontory and the European Plate on a lithospheric scale. The importance of strike-slip movements during The strike-slip movements laterally induced the Alpine main phase by the westwards indenting Insubric plate were It appears from the geometry of the pre- generally diffuse; they are represented by the collisional plate boundaries (Fig. 7), that post- dextral component of the Penninic Lepontine collisional plate convergence had to induce and Helvetic transpressive zones. A relatively strike-slip movements in the region of the minor part of these movements was concen- western Alps, to account kinematically for the trated along the Insubric Line. Schmid et al. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

22 M. Coward & D. Dietrich

PREAL PS ~K 2 ,~, :f

(e) CRETACEOUS/TERTIARY EOCENE/OLIGOCENE

AUSTROALPINE IVREA BODY PENNINES ...S;> EXTERNAL STRUCTURAL /:. Co :/

(b) POST- OLIGOCENE FIG. 16. The importance of strike-slip movements during the Alpine main phase. (a) Strike-slip deformation along the western edge of Tethys -- according to Ricou & Siddans (1986), modified. (b) Strike-slip movements related to indentation of the Insubric plate --according to Laubscher (1971), modified. See text for discussion.

(this volume) discuss the deformation history of The formation of the arc of the western Alps the rocks of the [nsubric Line, the amount and The following model for the formation of the timing of the displacements along it, and review arc of the western Alps emerges: the significance of this line and of its continu- ation into the eastern Alps, i.e. of the overall (1) The structures of the Alps can be explained Periadriatic lineament, during Alpine evolution. by movements generally related to the epi- Strike-slip faults which partially acted as rails sodic northward propagation of the Atlantic for plate movements are lithospheric structures. rift. The Tethyan ocean formed in a sinis- A particularly important strike-slip zone is the tral strike-slip regime between Africa and Valais or Penninic fracture zone as viewed by Europe and can be considered as a large Laubscher (1983). During the Cretaceous the pull-apart basin. On both continental fracture zone was still far from the subduction margins smaller basins developed; these zone (Fig. 12), but approached it in the Upper were active at different times and had Eocene. Compression between this crustal scale different subsidence histories. buttress and the still mobile, i.e. buried, Pennine (2) NE convergence of the African plate, nappes caused the important backfolding of the relative to the European plate, together Pennine front in the central Alps (e.g. Huber et with eastward movement of the part of al. 1980 for a section through the backfolded Europe S of the Pyrenean-North Penninic Gotthard massif). The obstacle was then during line, led to Cretaceous deformation on both the Miocene 'scraped off by the N-moving oro- continental blocks and to collision in the genic lid' (Laubscher 1983), a process which led eastern Alps by NW to WNW directed over- eventually to the tectonic extrusion of the thrusting of the Austro-Alpine part of the Helvetic nappes (Dietrich & Casey this volume). Adriatic promontory onto the European The Miocene Alps can therefore be considered plate. Cretaceous collision in the eastern as a dextrally transpressive intracontinental Alps occurred, therefore, in a dextrally mountain belt, comparable to the Pyrenees transpressive deformation regime, leading in the Eocene (Laubscher 1988). to an early arc structure (Fig. 13b). Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview z3

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o

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24 M. Coward & D. Dietrich

(3) After the consumption of Tethys, mostly by example, that during the Tertiary the African subduction, but partly by ophiolite obduc- Adriatic promontory was partially decoupled tion, the main phase of collision produced from the African plate, and considering that the general thrusting of African units over local kinematic indicators in fault zones do not Europe, as well as the northward thrusting always reflect the dominant plate motions (e.g. of slices of European crust. Since the over- Baird & Dewey 1986). Complex was as well the all movement of the African plate relative response to deformation of the extremely to the European plate was strongly ro- heterogeneous rock material being subjected to tational, with a pole somewhere near the long deformation histories under a wide range western edge of the Mediterranean (Dewey of physical conditions (Fig. 17). et al. this volume), and as there was, in the Many of the points made in this overview will Tertiary, subsequent further rotation of be discussed further in the papers given in this Apulia relative to Europe, dextral shear volume. These contributions illustrate the movements between the continental blocks range of problems on which Alpine geologists were again extremely important, especially are currently working, based on 150 years of along the NE-SW segment of the Alpine intense research. Many problems remain for deformation zone, the eastern and central future generations. We suggest three important Alps. These transpressive movements inter- lines of research for the future: fered with the sinistral strike slip move- (1) The sequential directions of extension and ments in the N-S sector of the Alps, the relative displacements are poorly known western Alps. An anticlockwise rotation of over large parts of the Alpine chain. These the direction of overthrust shear in the zone data are the basis for the establishment of a of interference produced the typical defor- kinematically consistent model for individ- mation pattern of the central-western Alps: ual sectors of the chain, from the eastern an older, northwest- or northwards shear Alps to southern Italy, North Africa and direction being overprinted by a younger Spain. fold-axis parallel stretching. The anticlock- (2) The ages of some of the movements need to wise direction of overthrust shear is viewed be found using stratigraphic and/or geo- also as an expression of the migration of chronological techniques; dating of well deformation from east towards west, the understood structures appears particularly westwards directed thrusting being younger important. than the northwards directed thrusting. (3) Quantitative restoration of Alpine palaeo- (4) Approximately 250 km of European litho- geographic belts by conventional methods sphere were involved in Alpine defor- and by unstraining techniques, particularly mation, as deduced from foreland basin in 3-D, plus further knowledge of con- migration in the central-western Alps, ditions of sedimentation and subsidence with probably more in the eastern Alps. history of these belts, should allow better Shortening was associated with lower understanding of the crustal structure, the crustal and lithospheric mantle subduction, structural history and the collisional and to form 'roots', beneath the Po plain for the post-collisional processes. western Alps, but possibly beneath the internal zones of the Alps further east. Net shortening exceeds net extension, as the ACKNOWLEDGEMENTS" MPC is grateful for long and sometimes fruitful discussions with students and Alpine basins have been fully inverted and colleagues in Britain and Switzerland. DD enjoyed the crust thickened to over 45-50 km. discussions with Daniel Bernoulli and Otto Kalin on However, it is difficult to match extension this text and with Martin Casey on individual aspects with compression, as the overthrust plate of Alpine tectonics. Thanks are due to Carolyn also suffered rotation. Holloway for typing the manuscript. The topics discussed here involve some of the controversial The overall tectonic framework given here discussions held after papers were presented during represents only a first order approach to Alpine the Geological Society meeting. DD acknowledges evolution. Detailed kinematics induced by plate financial support from Schweizerischer Nationalfonds, movements were complex considering, for project No. 2.2140.86. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021

Alpine tectonics - an overview 2 5

References

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MIKE COWARD, Geology Department, Royal School of Mines, Imperial College, London SW7 2BP, UK. DOROTHEE DIETRICH, Geologisches Institut, ETH-Zentrum, CH-8092 Ziirich, Switzerland.