Retroward extrusion of high-pressure rocks: An example from the Hellenides (Pelion Blueschist Nappe, NW Aegean)

Current tectonic models interpret the Hellenides as a unidirectional, SW-vergent orogenic belt. New (micro-)structural and amphibole chemistry analyses show, instead, that the exhumation of the Pelion Blueschist Nappe (PBN) of the Internal Hellenides was achieved by retroward (ENE)-directed ductile extrusion, which opposes the principal (proward) orogenic vergence. Retroward extrusion occurred between two sub-parallel, major ductile shear zones (Basal thrust and Upper detachment) with opposite shear senses, which operated simultaneously under blueschist- to greenschist-facies conditions during the late Eocene-Oligocene. Because the PBN is tectonically sandwiched between Pelagonian basement rocks, we suggest that the PBN protolith was accumulated during the Late Cretaceous in an incipient backarc basin developed above the NE-dipping subduction of the Pindos Ocean beneath the Pelagonian microcontinent. Subsequent Apulian-Pelagonian collision triggered basin inversion and the proward-dipping intracontinental subduction that caused the early/middle Eocene blueschist-facies metamorphism followed by the retroward extrusion of the PBN.