INTEGRATED STRUCTURAL AND THERMOCHRONOMETRIC INVESTIGATION OF MULTI-STAGE CENOZOIC DETACHMENT FAULTING AND CRUSTAL EXTENSION IN SE SYROS, CYCLADIC ISLANDS, GREECE

In the last decades several Cordilleran type metamorphic core complexes and extensional detachments were identified in the Cyclades. They were formed during regional back-arc extension and resulted in the exhumation of lower-plate rocks and the tectonic denudation of the upper plate that is now exposed only in few areas throughout the Aegean Sea. Research on the island of Syros has largely focused on the spectacular HP/LT rocks and the ophiolitic melange of the northern part of the island. In this study, we elucidate the structural, temporal, and thermal evolution of the detachment faulting in the SE portion of Syros. Detailed mapping and deformation study combined with thermochronometric data from the upper plate Vari gneiss and Cycladic Blueschist Unity (CBU) reveal a complex deformation and exhumation history that includes at least two episodes of detachment faulting. In SE Syros, the tectonic configuration includes three tectono-metamorphic units separated by low-angle normal faults. The lower plate CBU underwent Eocene HP/LT metamorphism followed by greenschist facies overprint in Oligo-Miocene times. The tectonically overlying greenschists facies Upper Unit (UU) is wedged between CBU and the structurally highest unit, the Vari Unit (VU). The VU is dominated quartzofeldspathic gneisses that are affected by four deformation episodes: synmetamorphic tight to isoclinal folding in upper grenschist facies conditions during Late Cretaceous (D1), a mylonitic to cataclastic episode (D2), followed by open folding and kinking (D3) and brittle normal faulting (D4). While the tectonic contact between VU and UU is characterized by the mylonitic to cataclastic Vari Detachment, field relations suggest that D4 high-angle normal faulting is associated with a younger low-angle normal fault that truncates the Vari Detachment and juxtaposes both VU and UU units against the CBU in SE Syros. This relationship is corroborated by thermochronometric data showing that lower plate CBU rocks are characterized by ~8-10 Ma rapid cooling, while ages in the VU cluster at ~15 Ma, suggesting an earlier, middle Miocene phase of exhumation unrelated to late Miocene detachment faulting and revealing that crustal extension occurred in multiple stages or in a protracted fashion during Cenozoic times similar to other parts of the Cyclades.