Quo vadis Zeus : is there a Zas shear zone on Naxos Island, Aegean Sea, Greece? A review of metamorphic history and new kinematic data

Abstract

Detecting zones of considerable early-orogenic displacement in rather monotonous rock sequences which have undergone a late and pervasive tectonometamorphic overprint is challenging. It has been proposed that the alleged Zas shear zone in the passive-margin sequence of the Cycladic Blueschist Unit (CBU) on Naxos Island, Greece, separates amphibolite-facies, non-high-P rocks (Koronos Unit) below from Eocene high-P rocks (Zas Unit) above the shear zone. We review existing pressure-temperature (P-T) data from the Koronos Unit and present new kinematic data from the anticipated Zas shear zone to evaluate the tectonic significance of this recently proposed structure. This has implications for unravelling the subduction history of rock units from tectonometamorphic data sets. Common to all P-T data from the Koronos Unit is a well-defined amphibolite-facies equilibration stage at 8-11 kbar and 600-700°C, followed by initial near-isothermal to slightly prograde decompression and subsequent pronounced cooling. This segment of the high-T P-T loop was associated with top-to-the-NNE extensional deformation in the footwall of the Miocene Naxos-Paros detachment. Little is known about metamorphism preceding the amphibolite-facies overprint in the Koronos Unit. Our review shows that it is likely that the rocks experienced a prior high-P metamorphic overprint that is typical for rocks of the CBU. Our kinematic data show that the Zas shear zone contains variably deformed rocks with dominantly top-to-the-NNE shear-sense indicators that developed under greenschist-facies metamorphism in the footwall of the Naxos-Paros detachment. No significant offset can be detected across the Zas shear zone and the geology on either side of it does not support large-scale movement across the shear zone. We discuss a model in which the Zas shear zone is considered a minor zone of deformed schist near the biotite-in isograd of Miocene high-T metamorphism. We conclude that there is no need to tectonically subdivide the CBU passive-margin sequence on Naxos.

Geometry and Kinematics of Bivergent Extension in the Southern Cycladic Archipelago: Constraining an Extensional Hinge Zone on Sikinos Island, Aegean Sea, Greece

“…However, the data by Kumerics et al (2005) from Ikaria Island also show low kinematic vorticity but the shear sense during extensional shearing at the Ikaria segment of the North Cyclades detachment system is predominantly top-N (see also Beaudoin et al, 2015). A similar, but less well constrained case can be made for the Naxos-Paros detachment system for which Peillod, Tehler, et al (2021) showed a distinct degree of coaxial deformation during extensional shearing associated with a dominant top-N sense of shear. Collectively, these data support distinct vertical flattening during large-scale ductile extension but pronounced vertical flattening does not appear a limiting factor causing the extensional hinge zone to form.…”

“…Mizera and Behrmann (2016) across the Cyclades with 47 data points and the tensor average of all data plotting in the oblate field. Peillod, Tehler, et al (2021) also described oblate-strain geometry from the CBU on Naxos and suggested that extensional shearing had a distinct pure-shear component. Xypolias (2010) reported near-plane-strain geometry from the lowermost CBU (and W m of 0.8-0.9) and prolate-strain geometry from the top of the CBU section (W m of 0.6-0.9) on Evia Island.…”

Petrochronology of polygenetic white micas (Naxos, Greece)

“…Schematic P–T–t diagram, redrawn after Peillod, Majka, et al (2021) and Peillod, Tehler, et al (2021). P–T–t paths for the top of the middle‐CBU nappe near Kalados Bay (in blue), the central section of the middle‐CBU nappe (green path).…”

“…Metamorphic zones range from the migmatitic core (approximately 700°C) to greenschist grade in the south of the island. Yellow stars: samples used by Peillod et al (2017), Peillod, Majka, et al (2021),and Peillod, Tehler, et al (2021) for P – T path analyses. Green stars: aliquots of the Peillod et al (2017) samples analysed in the present work…”

“…The metamorphic history of Naxos includes HP metamorphism (560–630°C and 15–20 kbar) in the Eocene (Cao et al, 2018; Peillod et al, 2017; Peillod, Majka, et al, 2021; Peillod, Tehler, et al, 2021; Wijbrans & McDougall, 1986, 1988). The HP rocks occur in SE Naxos, but ages related to the HP overprint occur throughout the passive margin sequence.…”