No abstract

Current models for the Oligo-Miocene postorogenic back-arc extension of the Aegean domain suggest that stretching is accommodated by two bivergent detachment systems of opposing shear sense. The coexistence in the Eocene of a top-to-the-south thrust at the base of the Cycladic Blueschists unit and top-to-the-north extensional shear zones at the roof raises the problem of differentiating synorogenic and postorogenic deformations with similar directions and shear senses. Based on structural field data, this study shows that the postorogenic deformation recorded in the Southern Cyclades is extremely asymmetric as the Cycladic Blueschists unit is pervasively affected by top-to-the-north shearing deformation distributed on four main shear zones. All activated in greenschist-facies conditions, some of these shear zones operated in the brittle regime during the final part of the exhumation. The Cycladic Blueschists/Cycladic Basement contact displays clear polyphased deformation with the preservation of top-to-the-south thrust kinematics. Thermal structure of the Cycladic Blueschists unit with regard to position of ductile shear zones was retrieved using the Raman Spectroscopy of Carbonaceous Material peak-metamorphic temperatures. This study shows a series of major metamorphic gaps accommodating an upward and stepwise decrease of more than 200°C within the Cycladic Blueschists unit. Pressure-temperature estimates show that only lower parts of the Cycladic Blueschists unit recorded approximately 18-20 kbar for 530°C peak conditions. While flanking the West Cycladic Detachment System, which shows a top-to-the-south shear sense, the Southern Cyclades are dominated by a top-to-the-north noncoaxial shearing. Deformation is therefore genuinely asymmetric in the center of the Aegean domain.

The Shimanto Belt in SW Japan is commonly described as a paleo-accretionary prism, whose structure is explained by continuous accretion like in modern accretionary prisms such as Nankai. We carried out a structural study of the Cretaceous to Miocene part of the Shimanto Belt on Kyushu to test this hypothesis of continuous accretion. Most deformation structures observed on the field are top-to-the-SE thrusts, fitting well the scheme of accretionary wedge growth by frontal accretion or underplating. In particular, the tectonic mélange at the top of the Hyuga Group records a penetrative deformation reflecting burial within the subduction channel. In contrast, we documented two stages of extension that require modifying the traditional model of the Belt as a "simple" giant accretionary wedge. The first one, in the early Middle Eocene, is mostly ductile and localized in the foliated bases of the Morotsuka and Kitagawa Groups. The second one, postdating the Middle Miocene, is a brittle deformation spread over the whole belt on Kyushu. Integrating these new tectonic features to existing data, we propose 2-D reconstructions of the belt evolution, leading to the following conclusions: (1) Erosion and extension of the margin in the early Middle Eocene resulted from the subduction of a trench-parallel ridge. (2) The Late Eocene to Early Miocene evolution is characterized by rapid growth of the prism, followed by a Middle Miocene stage where large displacements occurred along low-angle out-of-sequence thrusts such as the Nobeoka Tectonic Line. (3) From middle Miocene, the strain regime was extensional.