THE RETREATING HELLENIC SUBDUCTION SYSTEM: HIGH-PRESSURE METAMORPHISM, EXHUMATION, NORMAL FAULTING AND LARGE-SCALE EXTENSION

The Cenozoic history of the retreating Hellenic subduction system in the eastern Mediterranean involves subduction, accretion, arc magmatism, exhumation, normal faulting and large-scale continental extension from ~60 Ma until the present day. Ages for high-pressure metamorphism in the central Aegean Sea region range from ~53 Ma in the north (Cyclades islands) to ~25-20 Ma in Crete in the south, equivalent to a decrease in age down-section. Younging of high-pressure metamorphism in a southerly direction reflects the southward retreat of the Hellenic subduction zone. The shape of pressure-temperature-time paths of high-pressure rocks is remarkably similar across all tectonic units, suggesting a steady-state thermal profile of the subduction system and persistence of deformation and exhumation styles. The high-pressure metamorphic events were caused by the underthrusting of discrete fragments of continental crust, and these short-term events were superimposed on progressive slab retreat. Most of the exhumation of high-pressure units occurred in extrusion wedges during ongoing subduction and overall lithospheric convergence. At 23-19 Ma this extrusion wedge stage was followed by large-scale lithospheric extension with the development of metamorphic core complexes and the opening of the Aegean Sea basin. This extensional stage caused limited exhumation at the margins of the Aegean Sea but accomplished the major part of the exhumation of ~21-16 Ma old high-grade rocks in the central Aegean (Naxos, Paros and Ios islands). The age pattern of extensional faults, as well as contoured maps of apatite and zircon fission-track cooling ages, do not show a simple southward progression but reflect fluctuations in regional partitioning of extensional deformation and related exhumation from 23-19 Ma to the Recent. Current data suggest that large-scale extension was accommodated by a few low-angle, large-magnitude extensional fault systems. Slip rates on these faults were of the order of 5-10 km/Myr. The pattern, magnitude and slip rates of the extensional faults in the Aegean Sea region are in marked contrast to that in the adjacent Menderes massif of west Turkey. Our review of lithologic, structural, metamorphic and geochronologic data outlines the evolution of the Hellenic subduction system, and supports a temporal link between draping of the subducted slab over the 660 km discontinuity and large-scale extension causing the opening of the Aegean Sea basin.