Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (similar to 20-70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P-T-D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top-to-the-S-SW shear (prograde-to-peak subduction), top-to-the-NE shear (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53-52, similar to 50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span similar to 50-40 and similar to 43-20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene-Oligocene subduction channel proceeded at rates of similar to 1.5-5 mm/yr and accommodated similar to 80% of the total exhumation of this HP/LT complex.

Automated summary

Exhumed high-pressure/low-temperature metamorphic rocks on Syros Island provide insights into the subduction interface dynamics. The island consists of three coherent tectonic slices that underwent subduction, underplating, and syn-subduction return flow at progressively younger times. The metamorphic and deformation history (P-T-D) and timing of subduction and exhumation are revealed through lineations, ductile fold axis orientations, and amphibole zonations. The revised tectonic framework suggests distributed, ductile, and coaxial return flow in an Eocene-Oligocene subduction channel, accommodating a significant amount of exhumation.