Subduction‐ and exhumation‐related structures in the Cycladic Blueschists: Insights from south Evia Island (Aegean region, Greece)

Abstract

[1] Detailed geological mapping, structural investigation and amphibole chemistry analyses in southern Evia (Aegean Sea, Greece) allow us to place new constraints on the internal structural architecture and tectonic evolution of the Cycladic Blueschists. We show that the early deformation history was related to ESE directed thrusting resulting in the stacking of the Styra and Ochi nappes, which constitute the Cycladic Blueschist unit in Evia. These early thrust movements initiated just before and proceeded at peak conditions of the Eocene high-pressure metamorphism. Subsequent constrictional deformation gave rise to E-W trending upright folding accomplished at the early exhumation stage. The main ductile-stage exhumation occurred during a single deformation phase associated with the decompression of blueschist rocks from the stability field of crossite to that of actinolite. This phase was characterized by localization of ductile deformation into a series of major, tens of meters thick, ENE directed shear zones, which cut up-section in their transport direction and restack the early thrust and fold sequence, locally bringing the structurally lower Styra nappe over the higher Ochi nappe. It is suggested that these zones operated as thrusts rather than normal sense shear zones as has been previously argued and were possibly formed during the Oligocene ENE-ward extrusion of the blueschists. Brittle-ductile NE dipping normal faulting of post-early Miocene age was probably responsible for the final exhumation of rocks.Citation: Xypolias, P., I. Iliopoulos, V. Chatzaras, and S. Kokkalas (2012), Subduction-and exhumation-related structures in the Cycladic Blueschists:

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

“…() and Grujic, Hollister, and Parrish (). We conclude that the retroward extrusion/exhumation of the PBN occurred in a compressional tectonic setting (Figure d), as has also been proposed for the Cycladic Blueschists (Xypolias, Iliopoulos, Chatzaras, & Kokkalas, ; Xypolias, Spanos, Chatzaras, Kokkalas, & Koukouvelas, ), whereas its brittle exhumation was achieved by symmetric extension in the Miocene (Figure e).…”

“…This, in combination with the fact that the PBN rocks are weaker than the rocks beneath and above the channel, possibly governed the retroward ductile extrusion (Figure 7d) under a mechanism similar to that described by Grujic et al (1996) and Grujic, Hollister, and Parrish (2002). We conclude that the retroward extrusion/exhumation of the PBN occurred in a compressional tectonic setting (Figure 7d), as has also been proposed for the Cycladic Blueschists (Xypolias, Iliopoulos, Chatzaras, & Kokkalas, 2012;Xypolias, Spanos, Chatzaras, Kokkalas, & Koukouvelas, 2010), whereas its brittle exhumation was achieved by symmetric extension in the Miocene (Figure 7e).…”

New Detrital Zircon Geochronology From the Cycladic Basement (Greece): Implications for the Paleozoic Accretion of Peri‐Gondwanan Terranes to Laurussia

“…The provenance specifications of the Cycladic Cover Series (Cycladic Blueschist Unit; Lower Unit) bear implications also on the identity of the Mesozoic ocean basin in which the Cycladic protolith was deposited. In previous studies (e.g., Channell & Kozur, 1997;Okay et al, 2006;Robertson, 2002;Robertson et al, 1991;Smith, 1993) two different versions were suggested: (1) that the Cyclades cover section was deposited in the Triassic-Jurassic oceanic basin located to the southwest of the Pelagonian terrane, that is, within the External domain, and called the Pindos Ocean (e.g., Bonneau, 1984), and (2) the Cyclades section was deposited to the northeast of the Pelagonian terrane, that is, in a more internal position (e.g., Gerogiannis & Xypolias, 2017;Xypolias et al, 2003Xypolias et al, , 2012. The latter basin was referred to in various studies either as Vardar, Maliac, Axios, Almopias, Eastern Neotethys, or Vardar-Izmir Oceanic Unit (e.g., Bortolotti et al, 2013, and references therein;Ferriere et al, 2016;Papanikolaou, 2009;Ring et al, 2010;Robertson et al, 2012;Sharp & Robertson, 2006;Smith, 1993).…”

The Tectonostratigraphy of Cycladic Blueschist Unit on Sikinos and Sifnos: Implication for the Mesozoic Tectonic Setting

“…In any case, the results presented here do not support the occurrence of a mature oceanic basin as the potential site for the formation of the CBU on Sikinos and Sifnos. However, it should be emphasized that in other islands the CBU shows evidence for larger oceanic component expressed by the presence of metacherts-rich marbles (e.g., Evia; Xypolias et al, 2012) and ultrabasic rocks (e.g., Syros and Tinos; Bulle et al, 2010). Therefore, it is reasonable to assume that the original protolithic sequence of CBU on Sikinos and Sifnos were formed along the margin of the rift basin.…”

“…Subduction-related deformation is mainly manifested by ductile thrusting that led to the formation of individual nappe stacks within the CBU (e.g, Aravadinou et al, 2016). Ductile deformation during the exhumation is represented by thrust-sense shear zones (Xypolias et al, 2012), kilometre-scale folds (e.g., Xypolias and Alsop, 2014) and large-scale extensional detachments (Jolivet and Brun, 2010). Therefore, it is clear, that these large-scale syn-metamorphic structures have significantly modified the premetamorphic stratigraphy of the Permo-Mesozoic protolithic sequence of the CBU, which was dominated by carbonate and pelitic rocks as well as volcanic rocks (Papanikolaou, 2013).…”