An Oligocene extrusion wedge of blueschist-facies nappes on Evia, Aegean Sea, Greece: implications for the early exhumation of high-pressure rocks

Abstract

We show that the Styra Nappe of the Cycladic Blueschist Unit on Evia constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The nappe pile on Evia was assembled during D 2 top-to-the-SSW-directed thrusting (in restored Oligocene coordinates), which emplaced the Styra and Ochi nappes of the Cycladic Blueschist Unit above the Almyropotamos Nappe between c . 33 and 21 Ma. Peak metamorphism of the Almyropotamos Nappe at c . 23 Ma occurred at lower metamorphic pressure, showing exhumation of the Styra Nappe during underthrusting and burial of the Almyropotamos Nappe. This exhumation was largely accomplished by the D 2 top-to-the-NNE-displacing Mt. Ochi normal-sense shear zone. Normal shearing commenced at c . 33 Ma under peak high-pressure metamorphism in the Styra and Ochi nappes. Fission-track dating indicates slow cooling after D 2 in the Styra Nappe. Subsequently, the former thrust contact between the Almyropotamos Nappe and the Styra Nappe was reactivated as a D 3 top-to-the-ESE extensional shear zone and this extensional phase led to the formation of a number of Middle to Late Miocene graben. Our main conclusion is that there is strong evidence for an Oligocene extrusion wedge accomplishing the early exhumation of the Styra Nappe, which demonstrates the importance of extrusion wedges for the initial exhumation of the Cycladic Blueschist Unit.