Ampferer-type subduction and the search for the elusive Alpine magmatic arc

Intervenant: Anders Mc Carthy

Subduction of oceanic lithosphere is a key driver of plate tectonic processes, formation of mid-ocean ridges, volcanic arcs and continental collisions. Key characteristics of subduction of oceanic lithosphere, amongst others, are the presence of high-pressure metamorphic lithologies and subduction-related magmatism. This “Benioff-type” oceanic subduction has been applied to the Alpine orogen, enabling the Alps to be used as a natural laboratory to constrain subduction zone processes. However, underlying these conceptual models is a troubling complication, namely the lack of magmatism from subduction initiation until the early stages of continental collision in the Alps. This seminar will focus on discussing the absence of magmatism during these 40-50 Myrs of subduction prior to continental collision. This amagmatic style of convergence coupled to the initiation of convergence along passive margins is similar to convergence in the Pyrenees and is distinct from Benioff-Type oceanic subduction zones. Hyper-extension of continental crust during rifting lead to the formation of variably narrow rift basins floored by exhumed mantle domains and, locally, short-lived ultra-slow spreading. This complex architecture controlled the mechanism of convergence, namely amagmatic subduction initiation along passive margins and amagmatic basin closure. The Alps and Pyrenees likely represent type localities for Ampferer-type continental subduction as opposed to Benioff-Type oceanic subductions.