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

Abstract

We have investigated subduction‐exhumation processes in the Pelagonian zone, exposed on the Northern Sporades islands (Aegean Sea) related to successive episodes of ocean continent and continent‐continent convergence through integrating multiscale structural analysis, metamorphic petrology, and white mica 40Ar/39Ar dating. Two major progressive phases of ductile deformation are documented, which are related to distinct episodes of tectonic burial and exhumation of the Pelagonian continental margin, which was facing the Neotethys/Vardar ocean. Review of existing data sets from neighboring regions shows that both deformation phases can be correlated along strike up to the Dinarides. The first phase of tectonic burial and exhumation (D1) is characterized by NW‐SE tectonic transport, greenschist facies metamorphism, and Early Cretaceous (~105‐135Ma) 40Ar/39 Ar white mica single fusion ages. D1 is correlated with the initial closure of the Vardar ocean by top‐to‐the‐W to NW ophiolite obduction and the underthrusting of the Pelagonian margin below the oceanic upper plate. Underthrusting was followed by exhumation and the deposition of Late Cretaceous‐Paleogene sediments. The second phase of burial and exhumation (D2) is characterized by NE‐SW tectonic transport, greenschist to blueschist facies metamorphism, and latest Cretaceous‐Early Eocene 40Ar/39 Ar white mica ages of S2 fabrics. Top‐to‐the‐SW shearing is correlated with the tectonic burial of the Pelagonian zone below the Eurasian continent (Rhodopia), while top‐to‐the‐NE shearing is attributed to subsequent extensional exhumation. D2 fabrics record low‐grade P‐T conditions suggesting that the decoupled cover formations exposed on Skopelos were incorporated in an accretionary wedge that formed above the subducting Pelagonian basement during Paleogene times.

Evidence for the Late Cretaceous Asteroussia event in the Gondwanan Ios basement terranes

“…These observations also reflect on a possible distinction between European and Gondwanan terranes, with evidence mostly preserved in the Alps and in the Pelagonian zone of Greece (Pourteau et al, 2013;Porkoláb et al, 2019;Regis et al, 2014;Thöni, 2006). Brown et al (2014) reports evidence of Late Cretaceous intracontinental shear zone deformation across Africa, thus demonstrating that the ~70-80 Ma age is not limited to northern Tethys.…”

Tectono-stratigraphic correlations between Northern Evvoia, Skopelos and Alonnisos, and the postulated collision of the Pelagonian carbonate platform with the Paikon forearc basin (Pelagonian-Vardar zones, Internal Hellenides)

“…The present model (Fig, 9) infers that the carbonate platform of the forearc basin gradually overthrust the simultaneously subsiding Sporades, together with scrapped-off MORB and IAB mélange as the Almopias oceanic plate was subducting and the eastern Pelagonian plate collided with the Paikon-Paeonian back-stop. (The indications of high temperature (450°C) and pressure reported by Porkoláb et al (2019) are considered in this contribution to have been caused by tectonic compression of the shear zone rocks between the subducting Almopias plate and the arc back-stop. )…”

“…Higher grade metamorphic minerals other than greenschist facies have not been observed. (Postscript: During the terminating work on this contribution in Spring of 2019, simultaneous investigations of the geology of Skopelos had also taken place (Porkoláb et al, 2019), which indicate that temperatures between 350 and 450°C in the pressure range 1-2Gpa, had prevailed during shearing. )…”

“…The shear-zone rocks range from sheared ocean-floor mélange to sericite schist, greenschist, phyllite and phyllonite (nomenclature following Higgins 1971;Maddock 1986;Brodie et al 2007, Fossen andCavalcante, 2017), (see below and discussion). Recently, Porkoláb et al (2019) determined white mica ages (40Ar/39Ar dating) in the phyllites of south-eastern Skopelos, indicating that the deformation of the Palouki nappe took place around Palaeocene time.…”

“…3b), with the argument that the Glossa unit has been thrust south-eastwards over the Upper Triassic formations of Skopelos. Porkoláb et al (2019) have dated white mica 40Ar/39Ar ages, occurring in the quartz phyllites of the Glossa unit, showing that the mica underwent low grade heating during the Early Cretaceous. These authors reiterate the stratigraphic interpretation of Jacobshagen and Matarangas (1988) and Matarangas (1992), assuming that the Glossa unit is of Eohellenic provenience.…”