Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20-70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit (CBU) preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution associated with Eocene subduction. Despite decades of research, the pressure-temperature-deformation history (P-T-D), and timing of subduction and exhumation, are matters of ongoing discussion. Here we show that the CBU on Syros comprises three coherent tectonic slices, and each one underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and return flow are distinguished by stretching lineations and ductile fold axis orientations: top-to-the-S (prograde-to-peak subduction), top-to-the-NE (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole chemical zonations record cooling during decompression, indicating return flow along the top of a cold subducting slab. New multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology suggest that three nappes record distinct stages of peak subduction (53-52 Ma, ~50 Ma (?), and 47-45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ~50-40 Ma and~43-20 Ma, respectively, and also young with structural depth. The datasets support a revised tectonic framework for the CBU, involving subduction of structurally distinct nappes 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 ~1.5-5 mm/yr, and accommodated ~80% of the total exhumation of this HP/LT complex.

Slickenlines are lineations thought to record slip motion and mechanical wear within shear fractures. Their formation mechanisms and effect on friction and fault rheology are poorly understood. We investigate natural slickenlines from strike-slip, normal, and low-angle detachment faults formed in volcanic, quarzitic, and mylonitized sedimentary lithologies, respectively. Slickenline surfaces exhibit non-Gaussian height distributions and anisotropic self-affine roughnesses with corresponding mean Hurst exponents in directions parallel– 0.53±0.07– and perpendicular –0.6±0.1– to slip. However, there is a significant variability in the fractal roughness descriptors obtained from multiple hand samples per fault surface. Microstructural analyses reveal that the principal slip surface is formed by a thin (≤100 µm) nanoparticulate- and phyllosilicate-rich layer, followed by a ~10 μm thick layer of increased cohesion, wherein several smaller grains coalesce into bigger aggregates. These microstructures are present in most analyzed samples suggesting that they commonly form during fault slip regardless of lithology or tectonic setting. Our results 1) suggest that deformation immediately adjacent to fault surfaces is energetic enough to comminute the rocks into nanometric grains and 2) highlight the intricacies of fault systems not fully captured by current models, which are likely to impact stress distributions and frictional responses along faults.

Accurate descriptions of natural fault surfaces and associated fault rocks are important for understanding fault zone processes and properties. Slickensides–grooved polished surfaces that record displacement and wear along faults– develop measurable roughness and characteristic microstructures during fault slip. We quantify the roughness of natural slickensides from three different fault surfaces by calculating the surfaces power spectra and height distributions and analyze the microstructures formed above and below the slickensides. Slickenside surfaces exhibit anisotropic self-affine roughness with corresponding mean Hurst exponents in directions parallel– 0.53±0.07– and perpendicular –0.6±0.1– to slip, consistent with reports from other fault surfaces. Additionally, surfaces exhibit non-Gaussian height distributions, with their skewness and kurtosis roughness parameters having noticeable dependence on the scale of observation. Below the surface, microstructural analyses reveal that S-C-C’ fabrics develop adjacent to a C-plane-parallel principal slip zone characterized by a sharp decrease in clast size and a thin (≤100 µm) nanoparticulate-rich principal slip surface (PSS). These microstructures are present in most analyzed samples suggesting they commonly form during slickenside development regardless of lithology or tectonic setting. Our results suggest that 1) PSS likely arise by progressive localization along weaker oriented fabrics 2) deformation along PSS’s is energetic enough to comminute the rocks into nanometric grains, and 3) fault geometry can be further characterized by studying the height distributions of fault surfaces, which are likely to impact stress distributions and frictional responses along faults.