Subduction of oceanic lithosphere in the Alps: Selective and archetypal from (slow-spreading) oceans

The Alps are amongst the best subduction archives in the world, with abundant blueschists and eclogites preserving fragments of mantle, gabbros, thinned continental margin and pelagic sediments partly within their pre-collisional architecture. But to what extent is the Alpine record representative of the subduction of oceanic lithosphere worldwide? What is its significance, merits and limits for understanding subduction (and exhumation) dynamics? This contribution shows that this record is neither exceptional nor atypical but rather exemplifies the fate of relatively short-lived and small, slow-spreading and slowly closing North Atlantic-type oceans (in this case a -400?700 km-wide domain closed over 60 Ma, at -1 cm/a), whose subducting slabs do not reach below the Mantle Transition Zone. Subducted fragments experienced conditions typical of mature subduction worlwide and show no sign of significant tectonic overpressure. Contrasts in rock recovery with time and space outline distinct subduction dynamics. During the first half of the subduction lifetime (-30 Ma), no subducted oceanic fragments were recovered. A marked difference is observed between metasediment-and mafic/ultramafic-dominated units (S and MUM units). Underplating of S units took place intermittently and preferentially at -30?40 km depth. The MUM units of the Western Alps deeply subducted to -80 km were only recovered late, i.e. within a few Ma at most before continental subduction and initially lied close to the margin. At the scale of the orogen, the recovery of subducted fragments allows to recognize four distinct sectors and demonstrates a strong influence of initial margin architecture and/or continental subduction. Whilst typical of subduction zone thermal regimes, the subduction archive appears to selectively preserve slow-spreading oceans and/or hyperextended margins.

Automated summary

The geological record of the Alps represents the subduction process of short-lived and slowly closing North Atlantic-type oceans, demonstrating different subduction dynamics. There are noticeable differences between metasediment-and mafic/ultramafic-dominated units, with geological structures influenced by initial margin architecture and/or continental subduction.