Low temperature geochronology and lithostratigraphy of Folegandros, Cyclades, Greece: relationship between low- and high-angle faults results in crustal mosaic

Abstract

<p>Miocene extension of the Aegean region was accommodated by bivergent low-angle crustal-scale detachment systems. Folegandros island, of the southern Cyclades, lies between the SW-directed West Cycladic and S-directed Santorini detachment systems. The NW-SE oriented island is 'peanut'-shaped, consisting of a northern structural dome that exposes the structurally lower Cycladic Blueschist Unit (CBU). The CBU is characterized by coarse-grained marbles intercalated with metabasites and micaschists with a strong greenschist facies overprint. Metabasite lenses locally preserve relict HP minerals, including glaucophane and lawsonite pseudomorphs. The CBU preserves Raman spectroscopy of carbonaceous material (RSCM) peak temperatures >400°C and possesses late Eocene-early Oligocene white mica <sup>40</sup>Ar/<sup>39</sup>Ar ages. At the corset of the island, a discrete tectonic boundary separates the CBU from a less deformed, NE-dipping homocline of Early Cretaceous to early Eocene units in the south. At the base of this package of rocks, the Eleftherios unit consists of alternating low-grade, deformed fine-grained marble and subordinate quartzitic-phyllitic sequences. The uppermost Vighlitsa unit is restricted to the SE coast and is composed of ophiolite phacoids at the base of a deformed marble and quartzitic-phyllitic sequence overlain by a metaflysch with middle Cretaceous marble olistoliths. Dispersed <sup>40</sup>Ar/<sup>39</sup>Ar dates from these lower grade units are Early Cretaceous to early Eocene, consistent with RSCM temperatures <350°C. Based on the lower peak temperatures and Cretaceous to Eocene chronostratigraphy in conjunction with regional lithostratigraphic correlation, we propose the structurally higher Eleftherios and Vighlitsa units are hitherto unacknowledged exposures of the stratigraphically uppermost Pelagonian zone (Mesoautochthonous unit). A strong N-S lineation is dominant across the island, and a ductile top-to-S low-angle detachment system is overprinted by brittle-ductile top-to-N faults and shear bands. The differences in Raman temperatures together with ductile shear sense indicators and lepidoblastic muscovite <sup>40</sup>Ar/<sup>39</sup>Ar ages at the detachment between the CBU and Pelagonian rocks indicate a top-to-S extensional detachment was active during the early Miocene. Middle Miocene zircon and apatite (U-Th)/He ages are comparable from both the CBU and Pelagonian zone and likely reflect cooling (<200°C) attributed to top-to-N extension exhibited by the cooler brittle-ductile structures. A similar lithostratigraphic juxtaposition between the CBU and Pelagonian zone is observed on Thera. Unlike on Folegandros, however, the middle to late Miocene ductile top-to-S low-angle detachment is overprinted by brittle-ductile top-to-S high-angle faults. These new observations reveal persistant top-to-S low-angle extension in the western and southern Cyclades throughout the Miocene. Overprinting high-angle normal faulting preserves structurally higher tectonic units, such as the Pelagonian zone, in fault relay zones.</p>