Resolving the chronological gap in the Olympos-Ossa-Pelion transect: New Rb-Sr constraints on Cycladic Blueschist Unit subduction and exhumation in mainland Greece

The Cycladic Blueschist Unit (CBU) records Eocene-Oligocene high-pressure/low-temperature metamorphism across the Aegean, providing critical constraints on subduction-exhumation processes during Hellenic orogenesis. While extensively studied in the Cycladic islands, the CBU's northern continuation through the Olympos-Ossa-Pelion transect in mainland Greece remains poorly constrained, resulting in conflicting tectonic models regarding nappe emplacement mechanisms, thrust transport direction, and the timing of exhumation.

Multi-mineral Rb-Sr isochrons of syn-kinematic white mica allows us to link isotopic ages to specific deformation events. To resolve the four-dimensional tectonic evolution of the Olympos-Ossa-Pelion transect, we present 34 new Rb-Sr ages integrated with detailed structural and metamorphic analysis. Our results reveal three distinct episodes of high-pressure metamorphism with systematic along-strike variations: (1) In the Olympos-Ossa region, the Ampelakia Unit (CBU equivalent) records peak metamorphism at 48–41 Ma with top-to-SW kinematics and syn-orogenic exhumation at ~40 Ma, correlating with the Middle-CBU nappe in the Cyclades; (2) The underlying Olympos-Ossa Unit (Basal Unit equivalent) experienced high-pressure metamorphism at 34–26 Ma during continued SW-wards underthrusting, with syn-orogenic exhumation starting at ~28 Ma; (3) In the Pelion region, the Pelion Blueschist Unit (also a CBU equivalent) yields preliminary high-pressure metamorphic ages of ~27 Ma, followed by (sub)greenschist-facies top-to-NE shearing at 22 Ma and 14 Ma, potentially representing the equivalent of the Bottom-CBU nappe in the Cyclades. Exhumation of the CBU in the Olympos-Ossa-Pelion transect occurred in two stages: (1) Eocene to Oligocene syn-orogenic extrusion driven by simultaneous top-to-NE normal faulting above and top-to-SW out-of-sequence thrusting below; followed by (2) Miocene post-orogenic extension accommodated by top-to-the-NE detachment faults synchronous with Aegean-wide slab rollback. These findings bridge the geochronological gap of CBU nappes between mainland Greece and the Cyclades, providing direct age constraints on the timing of underthrusting and exhumation for the northern sector of the Hellenic high-pressure metamorphic belt.