Late-alpine evolution in the Eastern Mediterranean region: temporal constraints on the post-orogenic extension from the crustal-scale brittle Mykonos Detachment (Aegean Realm, Cyclades)

The Mediterranean region results from multiple tectonics, with alternating contractional (e.g., Variscan, Alpine) and extensional phases (e.g., Tethyan), that shaped the present-day structural configuration.  Since the Cretaceous, most of the Mediterranean realm experienced complex contractional tectonics, leading to the formation of a part of the Alpine-Himalayan orogeny, with different deformation styles, conditions and absolute timing, often in relation to paleogeographic and paleo-tectonics inheritance.

The Aegean Realm, located in the eastern Mediterranean region, provides an ideal setting to study the interplay between contractional and extensional tectonics, with the latter related to the late, post-orogenic extension following the onset and evolution of the Alpine contractional phase. The extension was there accommodated by crustal-scale detachments, exhuming metamorphosed rocks of the Cycladic Blueschist Unit and syn-tectonic granite bodies in the context of the Aegean Metamorphic Core Complex (AMCC). The completely brittle Mykonos Detachment (MD), together with the basal and structurally lower ductile Livada Detachment (LD), belongs to the North Cycladic Detachment System and allowed the exhumation and unroofing of the syn-tectonic Middle Miocene (14-15Ma) granite that part of the northern sector of the Aegean Sea. Despite their importance in shaping that part of the AMCC, absolute ages on the activation of the brittle MD or the ductile LD in Mikonos Island, and a detailed description of the internal architecture of the MD are still not available and/or debated, where the MD is indirectly thought to be active since 10Ma.

Aiming to fill this gap, here we present the results of a detailed architectural and geochronological study of the MD where we identified at least 7 different gouge layers that compose the core of the fault zone as it is exposed in the northeastern sector of Mykonos Island. Gouge layers are surrounded by thick SCC’ domains, reasonably representing the beginning of the fault zone formation. Brittle Structural Facies – based structural analysis with K-Ar dating on authigenic illite-smectite from 7 fault gouge(s) yielded 6 different ages spanning from the Middle Miocene (13.34±0.77 Ma), coeval to the granite, to the Late Miocene (6.37±0.21 Ma), represented by the youngest gouge that is, moreover, cut by the younger principal slip surface of MD.

These ages, coupled with a high-resolution structural analysis, constrained at least 7 Myrs of protracted deformation along the same fault zone and focused the attention on the importance of such completely brittle detachments that do not ever thus represent a late deformation phase after a former ductile, deeper shearing. Indeed, these new structural and chronologic data indicate that upper crustal brittle deformation was coeval to the lower crustal ductile deformation during a large part of the evolution of a crustal-scale detachment and during the entire exhumation of the syn-tectonic granite. Such structures, thus, potentially represent(ed) pivotal structural features in shaping the present Mediterranean configuration by allowing the exhumation of syn-tectonic granites and the formation of the AMCC.