A CASE OF AMPFERER-TYPE SUBDUCTION AND CONSEQUENCES FOR THE ALPS AND THE PYRENEES

In this contribution, we review aspects of the petrological, metamorphic and sedimentological characteristics of Neotethyan and Pacific subduction zones and compare them to the Western and Central Alps and the Pyrenees. We argue that the current models of formation of the Western and Central Alps, which invoke the subduction of significant volumes of oceanic lithosphere and spontaneous subduction initiation are unable to explain fundamental characteristics unique to the Pyrenees and European Alps orogens. Their characteristic geodynamic features are surprisingly distinct from features of Wadati-Benioff-type subduction, as the latter typically have large subducted oceanic slabs, near-continuous magmatism and specific subduction initiation characteristics. The Pyrenees and the Western to Central Alps however, are characterized by subduction initiation without magmatism at rifted margins and inefficient subduction of hydrated lithologies into the convective upper mantle. The pre-collisional lithosphere from the Pyrenees to the Central Alps, or Western Tethys sensu lato comprised several sub-basins characterized by extremely thinned continental crust, exhumed subcontinental mantle and, locally, minor volumes of embryonic ultra-slow spreading ocean crust. This allows us to distinguish Benioff-type oceanic subduction resulting from the efficient subduction of hydrated oceanic lithosphere from Ampferer-type continental subduction. The latter records the closure of hyper-extended continental basins with minor volumes of oceanic crust, and subduction of predominantly dry lithosphere into the convective upper mantle.