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Subduction margins are the loci of a wide range of deformation processes occurring at different timescales along the plate interface and in the overriding forearc crust. Whereas long-term deformation is usually considered as stable over Myr-long periods, this vision is challenged by an increasing number of observations suggesting a long-term pulsing evolution of active margins. To appraise this emerging view of a highly dynamic subduction system and identify the driving mechanisms, detailed studies on high pressure-low temperature (HP-LT) exhumed accretionary complexes are crucial as they open a window on the deformation history affecting the whole forearc region.In this study, we combine structural and petrological observations, Raman spectroscopy on carbonaceous material, Rb/Sr multi-mineral geochronology and thermo-mechanical numerical models to unravel with an unprecedented resolution the tectono-metamorphic evolution of the Late-Cenozoic HP-LT nappe stack cropping out in western Crete (Hellenic subduction zone). A consistent decrease of peak temperatures and deformation ages toward the base of the nappe pile allows us to identify a minimum of three basal accretion episodes between ca. 24 Ma and ca. 15 Ma. On the basis of structural evidences and pressure-temperature-time-strain predictions from numerical modeling, we argue that each of these mass-flux events triggered a pulse in the strain rate, sometimes associated with a switch of the stress regime (i.e., compressional/extensional). Such accretion-controlled transient deformation episodes last at most ca. 1-2 Myr and may explain the poly-phased structural records of exhumed rocks without involving changes in far-field stress conditions. This long-term background tectonic signal controlled by deep accretionary processes plays a part in active deformations monitored at subduction margins, though it may remain blind to most of geodetic methods because of superimposed shorter-timescale transients, such as seismic-cycle-related events.

International audience; The long-lived subduction zone in the eastern Mediterranean region has been the locus of successive basal-accretion periods responsible for the formation of two main high pressure-low temperature (HP-LT) metamorphic belts recognized for a long time in the central Aegean domain (the Cycladic Blueschist Unit) and on Crete and Peloponnese (the Phyllites-Quartzites and Plattenkalk units). However, the characterization of individual tectonic slices within these metamorphic belts is only emerging and their precise timing of accretion remains enigmatic, although it is of paramount importance for constraining the protracted deformation history of the Hellenic margin.In this study, we combine structural and petrological observations, RSCM geothermometry and Rb/Sr multi-mineral geochronology to unravel with an unprecedented resolution the tectono-metamorphic evolution of the Late-Cenozoic HP-LT nappe stack cropping out in western Crete. We thus identify a minimum of six tectono-metamorphic slivers (i) bounded by major shear zones commonly reworked during exhumation and (ii) characterized by well-resolved down-stepping of the maximum recorded temperatures toward the lowermost structural levels. In addition, a consistent decrease of the accretion- and exhumation-related deformation ages, allows us to identify three slicing episodes between ca. 26 Ma and ca. 15 Ma. The subsequent fast exhumation of the nappe stack has been achieved by two detachment faults accommodating a N-S-directed, bivergent extension, which later evolved in an E-W-directed, trench-parallel extension as a consequence of the accelerating slab roll-back and the arc curvature during the middle-late Miocene.Through this multi-disciplinary approach, we finally stress that the Myr-scale cadence of slicing events resolved for the Hellenic margin sheds light on the spatial and temporal scale of deep-accretion dynamics, with implications for understanding mass fluxes in active forearc margins worldwide.

Although fundamental to the understanding of crustal dynamics in extensional setting, the relationships between the emplacement of granitic intrusions and activity of detachments still remain very elusive. Through a multi-scale approach, we here document a continuous deformation history between the monzogranitic intrusion of Naxos and the Naxos-Paros Detachment System (Cyclades, Greece). Field observations first show an early magmatic deformation followed by solid-state, ductile and then brittle deformation when approaching the detachment zone, as evidenced by the overprinting of mylonites by cataclastes and pseudotachylites. From these observations, we define six strain facies that characterize a positive strain gradient from core to rim of the Naxos monzogranite. Based on field pictures, X-ray tomography and Electron BackScatter Diffraction (EBSD) analyses along the strain gradient, we then quantify the intensity of mineralogical fabrics in 2D and 3D and better characterize the deformation mechanisms. Our measured shape variations of the strain ellipsoid corroborate the large-scale strain gradient, showing a good correlation between qualitative and quantitative studies. In addition, EBSD data indicate that dislocation creep was predominant during cooling from more than 500°C to temperature conditions of the ductile-to-brittle transition. However, 1) a weakening of quartz lattice preferred orientation with increasing strain and 2) evidence of numerous four-grain junctions in high-strain shear bands also indicate that grain boundary sliding significantly contributed to the deformation. Although the source of grain boundary sliding remains to be constrained, it provides a consistent approach to account for strain localization in Naxos.