Distinguishing the Contractional and Tensional Signatures of Deformation and Metamorphism on Evia Island, NW Aegean Sea

Abstract

Most of the circum-Mediterranean orogenic belts of southern Europe have accommodated dominantly extensional tectonic deformation since the early Miocene. The Aegean Sea, situated in the eastern Mediterranean Sea between Greece and Turkey, is an example of a back-arc basin formed by consequence of that extension. Trench retreat due to slab rollback starting in the Eocene subjected the Aegean region to tensional forces, resulting in the exhumation of high pressure, low temperature (HP-LT) metamorphic rocks characteristic of subduction zone metamorphism. The exhumed crustal fragments, composing the Cycladic islands, represent remnants of oceanic and continental crust situated offboard of the African margin. Those rocks were subducted beneath the southern margin of the European craton during convergence sustained between the two continents since the late Mesozoic. This thesis contributes a new island-scale study of Aegean exhumation by examining the record of unroofing on southern Evia island. Structural and petrological analyses and diverse radiometric dating techniques reveal a hitherto unrecognized major deformation corridor, whose strain geometry and geochronological record demonstrates its relevance to the extensional architecture governing exhumation of HP-LT rock units in the Aegean Sea. This structure, herein named the Evia Shear Zone, operated in tandem with the more regionally extensive North Cycladic Detachment System to exhume lowermost parts of the structural pile on southern Evia.

The constituent chapters presented herein illustrate challenges involved in discriminating among structural and petrological features developed during convergent stages of mountain building, as opposed to those related to extensional post-orogenic collapse. The combined work highlights analytical techniques that can overcome those ambiguities. Among them, the use of exploratory geochronology to inform subsequent targeted higher-resolution dating was effective at highlighting inconsistences in prior assumptions present in both the literature and first-order empirical inferences made over the course of this thesis. Overall, the data presented here demonstrate: (1) the potential morphological diversity among structures responsible for facilitating exhumation in deeply exhumed orogens; (2) the tenuous nature of some common petrological inferences as applied to subduction zone geodynamics; (3) the power of multiple geochronometers applied to tectonic problems in concert. The ultimate conclusions of this work should inform best practices for future refinement of orogenic models for the evolution and unroofing histories of HP-LT metamorphic complexes.