Geology of the Alps

O. Adrian Pfiffner

• Publisher: Wiley-Blackwell
• Year: 2014
• ISBN:
  9781118708118
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Géologie et sciences de la Terre

The Alps, with their outstanding outcrop conditions, represent a superb natural laboratory for many geological processes, and have played a crucial role in the history of geology. This book gives an up-to-date and holistic overview of the key aspects of Alpine geology.

After a brief presentation of the plate tectonic framework, the rock suites are discussed, starting with the pre-Triassic crystalline basement, followed by Paleozoic, Mesozoic and Cenozoic sedimentary sequences. The lithological description of the rock types is supplemented by a discussion of their paleogeographic and plate tectonic contexts.

The book goes on to describe the structure of the Alps (including the Jura Mountains and the Alpine foreland to the north and south) illustrated by numerous cross-sections. The evolution of the Alps as a mountain chain incorporates a discussion of the Alpine metamorphic history and a compilation of orogenic timetables. The final sections cover the evolution of Alpine drainage patterns and the region's glacial history.

Readership: The book is essential reading for students and lecturers on Alpine courses and excursions, and all earth-scientists interested in the geology of the region.

1
The Alps in their Plate Tectonic Framework

Rocks can be found in the Alps that range in age from one billion years to present times. The rocks themselves – sedimentary, igneous, metamorphic and unconsolidated rock – cover the entire conceivable spectrum. Many of these rocks and their formation can be understood only within the context of the geological structure of Europe and the associated plate tectonic processes. In the following therefore, the plate tectonic framework for Europe, the older mountain chains and the younger Alpine mountain ranges in Europe will be considered briefly.

1.1 Older Mountain Chains in Europe

From a geological perspective, the European continent has a highly chequered history. Although the Alps are an integral component of this continent and are, essentially, a spectacular mountain chain, their origin lies in the recent geological history of the continent. In order to understand the geological structure of Europe, the individual regions need to be classified according to the age of their consolidation. In this case, the term consolidation is taken to mean the welding of continents, following on from the motion of plates. Almost all of the mountain chains in Europe originated as a result of plate movements, where an ancient ocean was swallowed up in a subduction zone and the continental blocks subsequently collided with each other. The density of continental crust is relatively low and, therefore, buoyancy acts against it sinking to greater depths once it has entered a subduction zone. As a result, continental crust remains close to the surface and is compressed. During this process, the uppermost portions of the crust are pushed upwards and gradually build a mountain chain. This process is called orogenesis or mountain-building.

Baltic Shield

A number of such collisions between continents, or orogenies, have occurred during the geological evolution of Europe. Accordingly, we distinguish between Caledonian, Variscan and Alpine orogens. The continental plates involved in these collisions were North America, Siberia, Baltica/Europe and Africa and are also called terranes. The tectonic map in Fig. 1.1 takes this division into consideration. Europe has also been subdivided into Eo-, Palaeo-, Meso- and Neo-Europe, based on the relative ages of these orogenies. It must be noted that the terranes mentioned above contain rock units that are relics of even older, fully eroded mountain chains.

► Figure 1.1 Tectonic map of Europe showing mountain ranges coloured according to their age of formation and associated terranes and continents.

Eo-Europe is a large geological structure, a welded block that experienced no further orogenies after the Precambrian. Two geological provinces are distinguished within Eo-Europe: the Baltic Shield and the Russian Platform.

The Baltic (or Fennoscandian) Shield is a convex bulge or shield covering a large area, which is composed of a highly metamorphic crystalline basement (Baltica in Fig. 1.1). Multiple, very ancient and fully eroded mountain chains can be distinguished within these series of rock formations. The oldest rocks in the Baltic Shield are three to three and a half billion years old and were encountered in a deep drill core obtained in the region of Kola, to the south of the White Sea, as well as in Lapland.

The Russian Platform is the sedimentary cover over the Baltic Shield and is composed of Neoproterozoic non-metamorphosed sediments, overlain by Cambrian rocks as well as a series of rock formations that extend into the Cenozoic. In the southeast, the platform plunges beneath the foreland of the Caucasus, to the north of the Caspian Sea, and in the east and west, beneath the forelands of the Ural and Carpathian Mountains. The internal structure of the plate contains local depressions or basins with thick sedimentary successions as well as zones with a thin sedimentary cover. The sediments of the Russian Platform reflect the later phases of mountain-building that took place at its margins. Examples are the famous Old Red Sandstone, continental fluviatile sediments of the Middle to Late Devonian that are the erosional product from the (Caledonian) mountains in Norway and Scotland, the Permo-Triassic continental lagoon sediments in the foreland of the (Variscan) Urals and the Cenozoic continental formations in the foreland of the Caucasus and Carpathians. Sediments of the Russian Platform are usually marine deposits in the centre (with the exception of the Early Carboniferous coal swamps in the area of Moscow), but the sea retreated towards the south after the Early Cretaceous and the Russian Platform became subaerial.

Russian Platform

Palaeo-Europe refers to the Caledonian orogen that extends across Scandinavia to Ireland. Other parts are found in Greenland and the Appalachians. This broad geographical distribution is sufficient to indicate that later plate movements fragmented this Early Palaeozoic mountain chain. Plate movements responsible for this were, for example, the opening up of the North Sea from the Permian onwards and the opening up of the North Atlantic starting in the Jurassic.

Meso-Europe includes the Variscan orogen that originated in the Late Palaeozoic. With the exception of the Urals, the Variscan mountain chain can be followed as a continuous range, which in Germany and France is generally completely eroded and covered with younger sediments, as illustrated by the island-like distribution of remnants of these mountains shown in Fig. 1.1.

Finally, Neo-Europe comprises a series of mountain chains that originated in the Jurassic (Turkey), in the Cretaceous (parts of the Alps and Pyrenees), but mainly in the Cenozoic. These mountain chains are often winding and arc-shaped. In addition to the Alps, good examples are the Carpathians and the Betic Cordillera–Rif–Tell–Atlas system. This arc shape is essentially due to the geometry of the plate boundaries of the different associated microplates, a point that is discussed in more detail later on. The linear mountain chains of the Pyrenees and the High and Middle Atlas share the common trait that an orogeny is mainly characterized by strike-slip motion along linear faults. In addition to the strike-slip motion, a compressive component caused a shortening of the margins of the fault lines, which was responsible for the actual ‘up-folding’ of these mountain chains.

A simplified illustration of Europe's plate tectonic evolution and the origins of the Caledonian and Variscan orogens is provided in Fig. 1.2. This figure shows how several continents were welded into a megacontinent, Pangaea, over the course of 300 million years.

Figure 1.2 Plate tectonic evolution of Europe shown in four time slices. Positions of plates are based on Blakey (2008) and Scotese & Sager (1988). A, Appalachians; K, Caledonides; E, Ellesmere orogen; V, Variscan orogen; U, Urals; NAm, North America; SAm, South America.

In the Late Cambrian (500 million years ago), the southern continent, Gondwana, unified the extant land masses of South America, Africa and parts of Asia. The continents of Baltica (approximately Sweden, Finland and Russia today), Siberia and North America were surrounded by oceanic basins, in which thick sedimentary deposits accumulated. At the northern continental margin of Baltica, 1400 metres of grey and reddish arkoses, conglomerates, limestones and shales were deposited in the shallow part of the Iapetus Ocean during the Proterozoic (about 600 million years ago). The arkoses also contain tillites, that is, fossilized diamictites (glacial deposits that indicate very ancient glaciations). The Cambrian starts with a basal conglomerate that contains alum slate, that is, a dark pelite rich in iron sulphide. The marine sedimentation continued in the Ordovician–Silurian, with clay, limestone and turbidite deposits. Greenstones with gabbro and peridotite, typical rock associations in a newly developing oceanic crust, originated in the Iapetus Ocean itself. Finally, 6000 metres of Torridonian arkoses, conglomerates, sandstones, greywackes and pelites were deposited at the North American continental margin in the Proterozoic. This was followed by quartzites in the Cambrian and then thick dolostones, which continued to be deposited into the Ordovician.

The Iapetus Ocean was gradually closed through subduction and a large mountain range was formed due to the collision of Baltica with North America: the Appalachians in North America and the Caledonian orogen in Europe (Scandinavia and the Bristish Isles).

Figure 1.3 shows two cross-sections through the Caledonian mountain chain. The cross-section through the Caledonian mountain chain in Scandinavia shows how the Baltic Shield was overthrust in an easterly direction by large thrust sheets containing the Precambrian crystalline basement of the past continental margin of Baltica and its Proterozoic–Palaeozoic sedimentary cover. These crystalline nappes were thrust onto the Baltic shield over hundreds of kilometres, as can be seen from the example of the Jotun Nappe. The thin obducted nappes of Aurdal and Synfjell are mainly composed of Early Palaeozoic sediments. At the extreme east, the Oslo Graben is visible, which is a rift within the Baltic shield that is largely filled with Permian igneous rocks. In the west, towards the North Sea, there are ophiolitic rocks overlying the Jotun Nappe, relics of the Iapetus Ocean. Fragments of this ocean were not subducted during the collision between Baltica and North America, but instead incorporated into the developing mountain chain.

◄ Figure 1.3 Geological cross-sections through the Caledonian orogen in Scandinavia and Scotland. In both cases, the crystalline basement is affected by thrusting and is involved in the nappe str...