Tectonic underplating at intermediate depths in subduction zones: structural and geochronological evidence preserved in Corsican ophiolites

Peridotites from two ophiolitic units of Alpine Corsica show outcrop-scale reverse faults whose dip angles follow a bimodal distribution (weak vs. steep angles). These faults are outlined by pseudotachylyte or cataclasite, and formed concurrently at 35–70 km depths, during the subduction of the Piemonte-Ligurian oceanic lithosphere beneath Apulia in Cretaceous to Eocene times. 40 Ar/ 39 Ar dating of pseudotachylyte indicates a 38.1 Ma age of formation. The faults are interpreted as reflecting, at the outcrop-scale, similar structures formed during the propagation, along the subduction interface, of map-scale flats and ramps delimiting duplex-like thrust sheets eventually incorporated by underplating to the upper plate.