Amorgos Unit: a long-lost fragment of the Pelagonian domain identified within the retro-wedge of the Oligocene Hellenic Subduction zone

The Attic- Cycladic Crystalline Complex (Aegean Sea, Greece) is characterized by a complex history emanating from the interplay of geodynamic processes acted during successive stages of subduction zone underplating, early syn-convergence, and late extensional exhumation. Extensional tectonics strongly impacted the already formed orogenic buildup, leading to intensively denudated and locally preserved upper plate rocks that constitute the hanging walls of several detachment systems. The dominant Cycladic Blueschist Unit (CBU) and the structurally lowermost Cycladic Basement and Basal Units occupy the footwalls of these detachment systems, juxtaposed against the upper plate Pelagonian-derived fragments. Amorgos Unit is considered to occupy a structurally lower position and is correlated to the Basal Unit. This study utilizes tectonostratigraphic, detrital zircon provenance, and (U-Th)/He data to shed light on the paleogeographic and tectonic position of the Amorgos Unit in the Cycladic archipelago and potentially offer valuable insights for better understanding the architecture of the Hellenic subduction orogen.

Amorgos Unit presents a low-grade metamorphosed lithostratigraphy with a basal metaconglomerate, an intermediate carbonate sequence marked by various sedimentary facies of neritic and pelagic character, and an Eocene (meta)flysch, which preserves Nummulite fossils. The basal metaconglomerate has incorporated a metabasite block showing HP metamorphic conditions higher than the country rocks, which is either an olistolith or a tectonically incorporated slice. Detrital zircon U-Pb analysis of the basal metaconglomerate revealed a dominant Ediacaran age cluster and diverse basement rocks in the source area, including recycled Cadomian and Carboniferous affinities. MDA calculations yielded a Mid- Permian and Precambrian age for the matrix and the high-grade clasts, respectively. The metaflysch yielded Triassic- Jurassic MDAs and showed DZ distributions with a dominant Carboniferous input (Variscan affinities), suggesting Pelagonian-derived source areas. The structural evolution of Amorgos includes an early stage (D_n-D_n+1) internal tectonic imbrication in response to NW-ward advancing thrust sheets in the retro-wedge of the Late Eocene – Oligocene Hellenic subduction zone. The final structural history involves extensional low- and high-angle normal faulting (D_n+2/3) with top to SE kinematics. Zircon (U-Th)/He ages revealed exhumation below ~200 °C during the Early –Mid Miocene (18-14 Ma). Significant similarities in lithostratigraphy, structural-exhumation history, and structural position between Amorgos Unit and the Pelagonian hanging wall of the Santorini Detachment System on Santorini island suggest their close spatial relationship. From all the above, we conclude that Amorgos Unit is part of the Pelagonian upper plate, structurally above the Cycladic Blueschist Unit, and paleogeographical located at the southern Pelagonian margin.