The Tectonic Significance of Multiple Episodes of High Pressure Metamorphism In The Cycladic Eclogite-blueschist Belt Aegean Sea, Greece.

We are concerned with the evolution of high-pressure metamorphic rocks in the Cyclades, Aegean Sea, Greece. Detailed studies of fabrics and microstructures on Ios, Naxos, Sifnos, and Syros has established a relative chronology that can be consistently correlated between key locations, allowing the distinction of four separate episodes of high-pressure metamorphic mineral growth, and at least two subsequent episodes in which grain growth took place under greenschist facies conditions. Previous interpretations of the metamorphic history of this region recognize several events: Eocene HP-LT metamorphism (M1), OligoMiocene Barrovian greenschist metamorphism (M2), and Miocene contact metamorphism as the result of granite emplacement (M3). A sequence of individual tectonic slices has also been identified, separated by detachment faults of several different ages. Younger detachment faults and later high-angle normal faults dissect previously formed tectonic slices. Each individual tectonic slice is dominated by particular metamorphic mineral growth events, although complex overprinting sequences have been recognized. Low-greenschist facies assemblages are locally pervasively developed, in association with overprinting faults and shear zones. Within each tectonic slice sequences of shear zones and recumbent folds can be identified, and linked to the chronology established to the metamorphic sequence. Periods of metamorphic mineral growth can be associated with individual deformation events, but a more common feature is porphyroblastic growth immediately prior to and/or synchronous with the initiation of major ductile shear zones. Many of these ductile shear zones appear to be associated with crustal extension, but in some cases km- scale recumbent folds may subsequently develop. If these microstructurally distinct episodes of metamorphic mineral growth have any tectonic significance then it must be possible to determine when they occurred. We therefore have begun to attempt to constrain the time periods during which these different metamorphic events have taken place. At the same time we consider the significance of tectonic mode switches that have been observed elsewhere along the Alpine-Himalayan chain. Such tectonic models allow explanation of the pattern of deformation and metamorphism that have been observed in the Cycladic eclogite-blueschist belt.