Timescales and mechanisms for subduction and exhumation of a passive continental margin: A high-resolution study of the Cycladic Blueschist Unit on Syros, Greece

Subduction zones recycle crustal rocks into the mantle, generating earthquakes and arc volcanism. Subduction of passive continental margins plays a pivotal role in the evolution of convergent plate boundaries, as subduction of thin, mafic and dense oceanic crust transitions to subduction of thicker, felsic and lighter continental crust. This may lead to either an orogenic cycle of continental collision, slab breakoff, sinking of the dense oceanic slab, and subsequent orogenic collapse or, in case of subduction of continental ribbons of limited width or thickness, accretion of crustal rock units and continued subduction through backstepping of the subduction interface. High-pressure (HP) metamorphic rocks exposed at the Earth’s surface record the timing (t) and pressure and temperature (PT) conditions of their subduction and their exhumation to the surface, information that can be used to constrain the evolution of former convergent plate boundaries. Syros Island in the Cyclades Archipelago, Greece, exposes excellently preserved HP metamorphic rocks, reflecting their Eocene burial in the Hellenic subduction zone, followed by initial exhumation from subduction depths and subsequent Oligocene-Miocene exhumation from mid crustal depths to the Earth’s surface. The lithostratigraphic sequence of Syros consists of a Variscan continental basement, a Mesozoic volcano-sedimentary passive margin sequence and rocks of Late Cretaceous oceanic crustal provenance. While studies on Syros provided important advances in modern understanding of subduction zone processes, discussion remains on potential heterogeneity in timing and conditions for metamorphism, on the geological significance of white mica-based isotopic ages in the partially thermally overprinted sequence and on the mechanisms for exhumation from subduction depths to the surface. Available data on the PT conditions, timing and duration of metamorphism and structures recorded on Syros provide complex, non-systematic variation and spawned contrasting interpretations on the tectonometamorphic history of the island. This thesis describes a high-resolution study of the timescales and mechanisms for subduction and exhumation of the Cycladic Blueschist Unit (CBU) on Syros, as a record of the transition from oceanic to continental subduction and associated changes in plate boundary geometry. The timing and conditions for triggering rock exhumation are constrained by first mapping lithological domains of oceanic, passive margin and continental crustal provenance on Syros and Sifnos islands. High resolution Lu-Hf garnet dating shows that exhumation of the oceanic domain started at 52.0 ± 0.6 Ma, for the passive margin domain at 48.6 ± 0.6 Ma and 45.8 ± 1.0 Ma and for the continental domain at 43.1 ± 0.6 Ma. Thermodynamic modelling yields similar maximum burial conditions of 19-22 kbar during a phase of isobaric heating from 400°C to 500°C for all three domains, indicating rocks of the different domains were exhumed from similar depths despite their highly contrasting lithologies. Consistent exhumation from depths of ~70 km is either interpreted to reflect thermal weakening at the depth of coupling between the subducting slab and mantle wedge corner flow, or an increase in relative buoyancy of subducting rocks as they subducted below a depressed crust-mantle boundary. Similar exhumation for ~10-20 Myr periods from consistent depths recorded in the Alps and Chinese Tian Shan suggest that steady state detachment and exhumation may reflect local equilibrium conditions that may be characteristic for the transition from oceanic to continental subduction. Using the novel approach of multiple single grain fusion phengite 40Ar/39Ar dating at the sample, outcrop and regional scale, the intricate interplay of diffusion, recrystallisation and inheritance governing age preservation during retrograde metamorphism is resolved. Peak metamorphic crystallisation ages of 55-45 Ma roughly equal to ~52 Ma Lu/Hf crystallization ages for the local garnet are preserved in northernmost Syros, while in central Syros isotopic closure of white micas occurred at ~40 Ma, younger