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

Abstract

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.

Cretaceous‐Paleogene Tectonics of the Pelagonian Zone: Inferences From Skopelos Island (Greece)

“…(a) Correlation of deformation phases in the Pelagonian zone. The basis for correlation are geochronological and kinematic data sets compiled from published data (Gerogiannis & Xypolias, ; Kilias et al, ; Lips et al, , ; Most, ; Schermer et al, ; Walcott, ), which are comparable to our results from Skopelos. S=Skopelos.…”

“…Blueschist to greenschist facies rocks with Late Cretaceous‐Early Eocene mylonitic fabrics are found in the tectonic windows of the Olympos and Ossa regions where a more external thrust sheet of the Hellenides‐nappe‐stack crop out (Schermer et al, ; Lips et al, ; Figure ). Rocks from greenschist to blueschist facies are also found in the Pelion peninsula showing Paleogene‐Miocene fabric ages (Gerogiannis & Xypolias, ; Lips et al, ; Walcott, ). Blueschist to eclogite facies rocks with Paleogene fabrics are found in the region of the Cyclades (e.g., Jolivet et al, ; Jolivet & Brun, ; Laurent et al, ; Lister & Forster, ; Wijbrans et al, ).…”

Burial-exhumation cycles in the continental crust derived from Mediterranean field studies and numerical modelling

“…The above-mentioned ductile fabrics imply NW-SE or E-W tectonic transport and have been assigned to the Late Jurassic or Early Cretaceous ophiolite emplacement and subsequent deformation of the Pelagonian zone or alternatively to an early collision between Pelagonia and Rhodopia (Schenker et al, 2014). (Gerogiannis and Xypolias, 2017;Kilias et al, 2010;Lips et al, 1998;Lips et al, 1999;Most, 2003;Schermer et al, 1990;Walcott, 1998) which are comparable to our results from Skopelos. S=Skopelos.…”

“…Correlation of deformation phases in the Pelagonian zone. The basis for correlation are geochronological and kinematic datasets compiled from published data(Gerogiannis and Xypolias, 2017;Kilias et al, 2010;Lips et al, 1998;Lips et al, 1999;Most, 2003;Schermer et al, 1990;Walcott, 1998) which are comparable to our results from Skopelos. S=Skopelos.…”

“…Blueschist to greenschist facies rocks with Late Cretaceous -Early Eocene mylonitic fabrics are found in the tectonic windows of the Olympos and Ossa regions where a more external thrust sheet of the Hellenides-nappe-stack crop out (Lips et al, 1998;Schermer et al, 1990) (Figure 2.1). Rocks from greenschist to blueschist facies are also found in the Pelion peninsula showing Paleogene-Miocene fabric ages (Gerogiannis and Xypolias, 2017;Lips et al, 1999;Walcott, 1998). Blueschist to eclogite facies rocks with Paleogene fabrics are found in the region of the Cyclades (e.g.…”