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Alpine Tectonics - an Overview

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Alpine Tectonics - an Overview Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Alpine tectonics - 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 Mesozoic 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 Africa towards the east relative to a fixed Europe. 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 Cretaceous, 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. Subduction of oceanic crust and adjacent continental crust led to high pressure metamorphism of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-AIpine nappes 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 Alps in Switzerland and Austria 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 Himalayas. 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 (France, Switzerland, phosed, locally up to high temperatures and Italy 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 Swiss Alps, 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 continent 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 southern Alps, 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 AFRICAN PLATE FRONTAL PENNINE TURUST I~ PENN/NE NAPPES k~.iG. G~OE AUSTROALPINE NAPPES 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 = Mont Blanc massif, 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 fold 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 nappe 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 Helvetic 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 .-~ -"---_ ....
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