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.

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