Strain Partitioning in a Flattening Shear Zone: Re-Evaluation of a Cycladic Style Detachment

The Almyropotamos tectonic window on southern Evia island in the NW Aegean Sea divides two high pressure-low temperature metamorphic units, representing distinct Hellenic thrust sheets. Ductile thinning along the major low-angle Evia Shear Zone has closely juxtaposed the lower (Basal Unit) marble-flysch sequence structurally below Styra marbles (Cycladic Blueschist Unit). The partially attenuated flysch comprises a matrix dominated by pelitic schist, with dispersed cm- to hm-scale blocks of marble, carbonate schist, quartzite, and metabasite. Structural investigation of the different lithotypes in the flysch reveals tectonic fabrics related to general flattening strain are developed most strongly in the pelitic matrix, whereas the compositionally diverse blocks exhibit differential preservation of older structures. Quartz c-axis distributions from quartz veins in the schists reflect an early, moderate temperature plane strain deformation. Colder deformation is evident in some pelitic schists, capturing Z-centered girdles consistent with the oblate finite strain ellipsoid inferred from macroscopic structures. New in situ ⁴⁰Ar/³⁹Ar and ⁸⁷Rb/⁸⁷Sr geochronology delineate the timing of the two deformation events. Geochronological data reaffirm the first-order observations of strain partitioning behavior at the scale of the shear zone, and confirm that the structure records two resolvable tectonometamorphic events: an early Oligocene HP-LT event, and a late Oligocene-early Miocene greenschist facies overprint coinciding with ductile thinning. The diffuse and discontinuous style of deformation recorded within the shear zone is unusual for major structures facilitating exhumation in the Aegean Sea, and may represent an analogue to mélange-hosted shear zones that accommodate progressive strain during subduction.