Stretched Thin: Oligocene Extrusion and Ductile Thinning of the Basal Unit Along the Evia Shear Zone, NW Cyclades, Greece

We present field-based structural, geochronological, and petrological evidence for a major new Oligocene extensional shear zone, the Evia Shear Zone (ESZ), on the island of Evia in the NW Aegean Sea. Strain related to this structure occurs in a diffuse zone structurally underlying the nappe contact between the Cycladic Blueschist Unit and Basal Unit, within rock previously attributed to the Tsakei Unit or Basal Unit metaflysch. Structures within the ESZ record stretching in both the X and Y directions under ductile and brittle-ductile conditions, indicative of a component of oblate coaxial strain during overall top-to-E general shear. New white mica 40Ar/39Ar geochronology from the ESZ shows that the structure was accommodating ductile deformation in the late Oligocene. Thermodynamic modeling shows that rock within the ESZ records greenschist facies retrogressive conditions of 310 ± 15°C and 7 ± 1 kbar. Zircon (U-Th)/He thermochronology demonstrates that both the CBU and Basal Unit had exhumed into the brittle crust by the middle Miocene, slightly earlier than indicated by low-temperature thermochronology for the HP-LT units throughout the Aegean. We interpret these data as evidence that the Basal Unit underwent rapid exhumation via syn-orogenic processes persisting until at least the early Miocene, facilitated in part by normal-sense displacement along the ESZ. The occurrence of structures representative of coaxial deformation in the shear zone alludes to a major role for ductile thinning in controlling exhumation of HP rocks in the Cyclades, perhaps influencing the relative rates of exhumation observed across the region.