Exhumation of Eocene high-pressure metamorphic rock by coaxial flattening below a Miocene Cycladic-style detachment

Structures that accommodate extension during orogenic relaxation significantly modify the crustal architecture of mountain belts. Discerning the tectonic significance of superimposed structures relating to extensional overprint of initially compressional features is therefore critical to the reconstruction of an orogen, and is easiest where the large-scale mechanical interactions between different crustal domains are exposed. In the Aegean region of Greece, low-angle detachment faults of early Miocene age were partially responsible for exhuming Eocene high-pressure, low-temperature (HP-LT) metamorphic rocks of the Cycladic Blueschist Unit (CBU). Extension in the Cyclades commonly occurred along multiple detachment branches at the kilometer scale, either due to arrest of older detachment planes by late Miocene plutonism, or because strain partitioning along multiple, simultaneously active structures was rheologically favourable. We document a third plausible mechanism whereby crustal attenuation is accomplished via distributed coaxial strain in the footwall of a major detachment, described previously in the Cyclades primarily for deep crustal fabrics contemporaneous with peak HP-LT conditions. This style of deformation is recorded below the basal contact of the CBU on the island of Evia, which delineates the boundary of a major tectonic window exposing an underthrust external carbonate platform known as the Basal Unit (locally Almyropotamos Unit). New structural observations, complemented by white mica ⁴⁰Ar/³⁹Ar and zircon (U-Th)/He ages, suggest that the upper structural levels of the Basal Unit accommodated flattening strain that coincided with Oligo-Miocene extension likely related to the overlying North Cycladic Detachment System. Vertical shortening, with extension in both other principal directions, is evinced by symmetric chocolate-tablet foliation boudinage and conjugate shear bands in the Basal Unit, alongside coeval type-3 refold structures in the overlying CBU. Pseudosection modelling results from Evia further corroborate a late greenschist-facies (320 ± 40 °C, 7 ± 1 kbar) paragenesis for the fabric associated with this extension that post-dates HP-LT metamorphism. Our observations indicate extrusion of the CBU and underlying Basal Unit was accomplished at least in part by coaxial vertical shortening, in contrast to the predominantly non-coaxial strain observed in the footwalls of other major Cycladic detachments.