The European Alps and Plate Tectonics : A rocky relationship

At the end of the 19th Century and the beginning of the 20th Century, three important discoveries were made in the European Alps that transformed our way of understanding how orogens form; namely the formation of nappes, ophiolites (Steinmann Trinity) and the underthrusting of crustal material to great depth (Verschluckung, or “subduction”). Nevertheless, the conceptual framework of Plate Tectonics would have to wait another half of a century and emerge from (ocean-)going geophysicists and not Alpine geologists. It was the geophysical identification of subducting oceanic lithosphere at convergent margins and active mid-ocean ridges at divergent margins in the mid-1900s that highlighted how the motion of rigid oceanic plates and Benioff-Type oceanic subduction were the key drivers of Plate Tectonics. Ever since, this paradigm has been systematically applied to the European Alps enabling us to decipher, amongst others, the closure of oceans, collisional magmatism, subduction-related metamorphism and exhumation of high-pressure rocks.

However, numerous characteristics of the Alpine convergence are fundamentally distinct from typical Benioff-type subduction. Benioff-type subductions have large oceanic slabs, a long-term magmatic record and particular magmatic intra-oceanic subduction-initiation signatures. Other characteristics include the minor abundances of high-pressure rocks in accretionary prism and near-absence of evidence of (ultra-)high pressure rocks. On the other hand, the Alps are characterized by a pre-collisional lithosphere comprised of rift basins characterized by thinned continental crust and exhumed subcontinental mantle. The western Tethys, from the future Pyrenees to the European Alps, was therefore not a “classical” ocean, but a series of small basins and only a sporadic, short-lived ultra-slow spreading mid-ocean ridge. More surprisingly is a 50 myr “magmatic arc-gap”, with magmatism only occurring upon collision. Moreover, extensive subduction mélanges typical of Benioff-Type convergent margins are not found in the Alps. Instead, the Alps preserve coherent imbrications of high-pressure passive margins and oceanic core complexes. Rifted passive margins remain therefore preserved, if somewhat deformed in the Alps.

The Alps were therefore formed by the forced closure of hyper-extended basins along weakened, serpentinised passive margins in a case of “Ampferer-type” continental subduction. Benioff-type oceanic subduction results in the efficient subduction of oceanic lithosphere, abundant magmatism and limited exhumation of metamorphic lithologies. On the other hand, Ampferer-type continental subduction results in the closure of hyper-extended continental basins and inefficient deep subduction of hydrated (serpentinites and oceanic sediments) lithologies, or “congested subduction”. Although the Alps are typically used as a “fossil” convergent margin akin to modern oceanic subduction zones, we will explore the geological record which suggests instead that the Alps might be better understood in terms of continental tectonics, namely extreme intracontinental extension, ultra-slow plate separation and compression of hyper-extended continental domains and we will discuss possible consequences.