A quiet surface above a noisy slab: Eocene-Oligocene switch from shortening to extension on Astypalaia island, Aegean Sea, Greece

A correlation of the tectonostratigraphy and tectonic structures across the Attic-Cycladic belt to the Dodecanese islands and the western Menderes massif is required to reconstruct the pre-Eocene high pressure-subduction crustal architecture of the eastern Mediterranean. Notable, the transition from Cyclades to Dodecanese resides above a subduction slab tear that formed in the Miocene, which has offset the downgoing plate by >100 km, yet few tectonic structures exposed at the surface record the deeper geodynamic phenomenon. Residing along the NE-SW striking Santorini-Amorgos Fault Zone, the Astypalaia Platform exposes unmetamorphosed to weakly metamorphosed Triassic-Cretaceous neritic limestones, including Rudist- and Megalodont-bearing units. Unconformably above the limestone is an Eocene Nummulitic limestone-flysch package that contains km-scale marble and mafic volcanic olistoliths. The sequence, here named Analipsi subunit, was deformed into a series of NW-SE trending upright folds that preserves ductile top-to-N structures. The Vardia subunit, a weakly deformed Jurassic(?)-Cretaceous marble and limestone sequence capped by Eocene flysch, was thrust northward over the Analipsi subunit along the Vardia Thrust. This contact is characterized by a ductile strain gradient that increases toward the base of the hanging wall, marked by several tens of meters of marble ultramylonites on top of cataclasites. Top-to-S cataclasis overprints the earlier shortening structures localizing at the base of the Vardia Thrust. Unlike the other Dodecanese islands to the east, neither Variscan nor Paleozoic rocks are exposed on Astypalaia, indicating a higher structural level is present. Zircon (U-Th)/He dates from eight Eocene Analipsi flysch samples are partially reset, yielding single crystal ages of 130 Ma to 30 Ma, lacking a correlation to effective uranium concentrations, and exhibiting a dominant Paleocene-Early Eocene population. The overlap of the younger ZHe cooling dates and the depositional age of the Nummulite-bearing flysch suggests deposition, lithification, and subsequent deformation occurred rapidly in the Middle to Upper Eocene and under shallow crustal (<200°C) conditions. We propose that the Analipsi and Vardia subunits are part of the Pelagonian domain, which were imbricated after deposition of the Eocene flysch as the high pressure Cycladic Blueschist Unit was subducted beneath it. Although the magnitude of top-to-S extension on Astypalaia is significantly lower than the displacements recorded along the Oligocene Kalymnos and Kos detachments to the east, we correlate these events to argue that the transition from subduction to extension is constrained to 35-30 Ma, which occurred 20 Myr before the slab tear. This timing coincides with the onset of slab retreat throughout the Aegean region, mainly inferred from the migration of the volcanic arc.