Introduction

Nicolas BELLAHSEN and Claudio L. ROSENBERG

ISTeP, Sorbonne Université, UMR 7193, CNRS, Paris, France

The European Alps represent an archetypal mountain belt, mainly because its orogenic wedge is the most studied one. It results from a period of Mesozoic divergence forming the Tethyan Ligurian oceanic realm, linking the Neo-Tethys in the east with the Central Atlantic in the west. This followed the Variscan orogenic event affecting the European lithosphere and preceded a period of convergence starting during upper Cretaceous times with the northward movement of the Adria/Africa plates. This convergence was first accommodated by oceanic subduction, and later followed by continental subduction and eventually collision sensu stricto. The transition between oceanic and continental subduction is characterized by exhumation of high-pressure (HP) nappes of oceanic origin during early Eocene times, followed by HP metamorphism and exhumation of continental units during middle to late Eocene (Briançonnais and European distal continental margin). Collision started at the Eocene–Oligocene transition and is characterized by barrovian metamorphism, crustal shortening, high topographic relief and molasse-type peri-orogenic basins. During the whole convergence history, the amount of magmatism was very small. Today, convergence has slowed down, varying from almost zero in the Western Alps to approximately 2 mm/yr in the Eastern Alps, although vertical movements, extensive and compressive earthquakes are recorded, even in the Western Alps.

In spite of the latter observations, the European Alps may be considered as simpler than many other mountain belts that also result from the closure of oceanic domains. Indeed, in many other mountain belts, subduction and collision do not follow a simple sequence. For example, in the middle Paleozoic, during the Caledonian cycle and the Iapetus closure and more recently, during Cenozoic times and the Tethys closure between India and Asia, HP metamorphism events occurred long after the onset of collision. This is possibly due to the subduction of larger oceanic domains, and to longer and more vigorous convergence dynamics, which probably did not affect the Alpine realm.

The book Geodynamics of the Alps consists of three volumes. The first includes the recent and present-day structure and tectonic setting of the Alpine chain, from lithospheric mantle to brittle crust and surface topography, but also a historical overview of Alpine research and includes two chapters covering specific Alpine regions (Corsica and Eastern Alps), through all phases of Alpine history.

The second volume presents the pre-collisional history of the Alps. It starts with the Variscan orogeny in the Alpine realm, and continues with the inferred paleogeography, structure and extensional processes affecting continental margins, and their mantle structure in the pre-orogenic Alpine realm. This volume finishes with two chapters describing oceanic and continental subduction processes.

The third volume is entirely dedicated to Alpine collision. It describes and interprets the following elements of the Alpine chain: (1) Alpine magmatism, which is mainly contemporaneous to the very first stages of collision; (2) the formation of the external massifs, which represent a peculiarity in the Alpine chain, rarely occurring in other collisional orogens; (3) the foreland (Molasse) basin, in relation to the lithospheric processes that controlled its evolution; (4) exhumation of the internal part of the chain as constrained from the thermo-chronological record of the foreland basin sediments, and finally (5) the northern deformation front of the Alps, the youngest out of the large-scale collisional structures of the orogen.

This book will not provide a single, consistent and unique view on Alpine geodynamics, but rather create a space for experts on Alpine research to present their state of the art of specific subjects, not withdrawing from proposing their own interpretations, whether or not they are entirely consistent with those suggested in other chapters.

Volume 1

The first volume starts with a chapter on the history of Alpine cross-sections. This chapter does not review the historical evolution of tectonic and geodynamic processes in the Alps, but it shows how the style and format of geological cross-sections have evolved through time in relationship and as a consequence of major tectonic discoveries and debates. It also underlines changes through time in the scientific approach of scientists, showing how the very close link to observations in the mid-19th century evolved toward more conceptual interpretations, eventually lacking an observational base at the beginning of Plate Tectonics, when entirely new concepts of Alpine orogeny were formulated. Only in the last decade of the 20th century, have these conceptual models regained a close link to structural and geophysical observable data.

Chapter 2 of Volume 1 describes the present-day deep structure of the Alps, in which several slabs and slab segments can be identified. The dip-direction and the presence or absence of break-offs in the European slab are discussed in this chapter, strongly suggesting continuous, vertical slabs. In addition, the surface of the Alpine Mohos and the first-order structures of the deep crust are reassessed. Finally, based on the integration of receiver function, Vp/Vs seismic tomography and local earthquake tomography, the geometry and orientation of the west-Alpine subduction channel is imaged.

Chapter 3 of Volume 1 provides a quantitative description of the present-day surface topography of the Alps and its inferred uplift history. Several processes affecting the Alps since the Pliocene, such as sedimentation in the foreland basins, cooling ages and geochemically inferred erosion rates are integrated in the discussion of the present-day topographic structure, suggesting that changes in both climatic and tectonic processes must be invoked to explain the uplift history of the Alps.

Chapter 4 of Volume 1 reviews stress patterns inferred by the analysis of the youngest Alpine brittle fault systems, in addition to inferred deformation fields based on seismic and GPS data. These analyses show that the topographically highest portions of the Alpine chain in the Western and Central Alps were affected by orogen-perpendicular extension during Pliocene time, and this state of deformation is still recorded by seismic activity. The areas affected by such orogen-perpendicular extension coincide with high uplift rates in the northern and central parts of the Alpine arc. The possible collisional and post-collisional large-scale geodynamic causes of this deformation and uplift patterns are discussed.

Volume 1 concludes with two chapters which are both regionally focused. Because of their broad approach, providing the geologic al history of the Alpine realm throughout its entire history and up to the present, they are included within the first volume. Chapter 5 of Volume 1 presents the geology of the Eastern Alps discussing its underlying geodynamic processes. It shows that the stacking of nappes within the upper plate of the Alpine orogen can be inferred to represent a syn-collisional Cretaceous event, hence leading to the conclusion that two distinct orogenies, each of them consisting of a subduction/collision process, affected the Eastern Alps. The younger (Cenozoic) history is also described, with its peculiar style of collision, very different from that in the Western and Central Alps. Indeed, syn-collisional extrusion-related faults accommodate a significant part of collisional convergence in the Eastern Alps.

Chapter 6 of Volume 1 gives an overview of the geology of Corsica. Because the Sardo–Corsican block rifted away from the rest of the Alpine chain at the beginning of collision, it is not affected by the latter process, and it is ideally suited to study subduction and Cenozoic extension, and in particular, to relate the Alpine to the Appeninic chain. This chapter describes and illustrates how the Eocene-aged Alpine/Corsican east-directed subduction is followed by slab break-off and the initiation of a new, west-directed subduction system, which eventually generated the Appenninic chain.

Volume 2

Chapter 1 of Volume 2 deals with the Paleozoic evolution of basement rocks in the Alps. The oldest events recorded correspond to the formation of a Neoproterozoic peri-Gondwanian arc before its accretion to Gondwana, presumably during an Ordovician collisional event. The Cambrian–Ordovician period was marked by tectonic subsidence and widespread magmatic activity, prior to the opening of the Rheic Ocean. Convergence between Laurussia and Gondwana started in the Devonian, which marked the closure of Rheic Ocean, followed by the Variscan collision during the Carboniferous. Evidence of this convergence is provided by Devonian HP-LT eclogites preserved in the Austroalpine units, and Devonian–Tournaisian arc-related magmatism in the External Crystalline Massifs (Western Alps). The main period of nappe stacking and crustal thickening took place during the Mississipian and it was followed by strike-slip tectonics and extension from Pennsylvanian to Permian times. The orogen-scale dextral strike-slip east-Variscan shear zone was active from the Late Carboniferous to the Permian. Because of the intense reorganization during the Alpine orogeny, replacing the Alpine basement in the European Variscan puzzle remains a challenge to be solved.

Chapter 2 of Volume 2 provides an overview of the continental margins involved in Alpine orogeny. The reconstructed...