Cadenced basal-accretion events along the hellenic margin: insights from the high-pressure metamorphic complex in western Crete
Résumé
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.