Cover Image Credits

The Aiguilles Rouges d'Arolla (Western Alps) represents the youngest oceanic core complex in the European Alps. It comprises a preserved segment of Jurassic-aged (ultra) slow-spreading Western Tethys ocean floor. From left to right: The highest peak in the background is cumulate gabbro; the reddish peak is formed of gabbro mylonite; the greenish rocks represent deformed metamorphosed basalts; the grayish-brownish rocks on the right are metamorphosed calc-schists (schistes lustrés); the rocks in the foreground (in front of the lake) are serpentinite scree. (Picture from O. Müntener).

In this issue (p. 313–372), McCarthy and others (2020) target the Pyrenees and Western and Central European Alps to highlight the diversity of subduction mechanisms as a consequence of the structure of the lithosphere prior to the initiation of subduction. The cover picture illustrates how the Jurassic Western Tethys represented an amalgamation of narrow, ultra-slow spreading oceanic domains with occasional gabbroic core complexes as well as hyperextended basins of variable width. By targeting the magmatic, metamorphic and tectonic history of the Pyrenees and Western and Central Alps, McCarthy and others (2020) explore how the formation of these orogens might not be related to typical subduction of oceanic lithosphere, or Wadati-Benioff-type subduction. With the support of numerical modeling, the authors highlight how the cycle of ultra-slow extension followed by compression was dominated by extreme deformation of continental lithosphere whereas oceanic lithosphere played a minor role only. Ultimately, McCarthy and others (2020) reintroduce the term “Ampferer-type subduction”, wherein mainly dry lithospheric mantle is driven into the convective upper mantle upon convergence whereas subduction of hydrated lithologies (marine sediments and serpentinites) is inefficient, thus inhibiting magmatism from subduction initiation until continental collision.