A (tectonic) window of opportunity: crustal architecture and low-temperature geochronology of the NE Attica Peninsula, Greece

Miocene extension and crustal thinning in the Aegean were largely accommodated by a bivergent detachment system. The region contains several metamorphic core complexes that have overprinted Eocene high-pressure, low-temperature (HP-LT) metamorphic rocks of the Cycladic Blueschist Unit (CBU). The Attica Peninsula, along the northern margin, hosts the lateral termination of one of the major detachments, the SW-directed West Cycladic Detachment System (WCDS). Moreover, NE Attica has long been thought to contain a large tectonic window exposing the structurally lowest unit of the Attic Cycladic Belt, the Basal Unit. Our new mapping reveals that NE Attica primarily consists of a NE-dipping tectonic nappe of greenschist-retrogressed, HP-LT units, of probable Late Triassic to Late Cretaceous origins. In the west, Upper Cretaceous low-grade or non-metamorphosed units are juxtaposed above the nappe by a NNE-SSW striking, top-to-SW detachment fault. In the east, a package of HP marbles, evinced by calcite pseudomorphs after aragonite, lie above the nappe along a newly discovered NE-dipping fault, the Marathon Thrust. The common footwall of both structures includes isoclinally folded marbles and schists resulting in an apparent map-scale repetition of units. Schists are variably quartzitic to calcitic and contain intercalations of quartzites, metabasites, marble mylonites and, near the stratigraphic top, blue-grey marble. Axial planes of D_n recumbent isoclinal folds (F­_n) develop a pervasive, gently NE-dipping S_n foliation (~320°/30°). Syn-metamorphic F_n axes have the same orientation as a NE-SW mineral and stretching lineation (L_n; from 050° to 100°) that forms along the S_n planar fabric. L_n plunges dominantly towards the NE with some variation from subsequent D_n+1 folding. The cooler D_n+1 event is recorded by SW-vergent folds with NW-SE striking F_n+1 axial planes that form an S_n+1 crenulation cleavage, locally defining the main foliation. NW-SE trending F_n+1 axes are parallel to an L_n+1 intersection lineation. Winged inclusions, flanking folds and domino boudinage of dolomite layers within calcitic marbles indicate top-to-SW sense of shear under ductile to brittle-ductile conditions. White mica ⁴⁰Ar/³⁹Ar (MAr) dates throughout the footwall are earliest Miocene and zircon (U-Th)/He (ZHe) ages are middle Miocene. A several m-thick fault gouge separates the footwall from low-grade metasandstones, limestones and serpentinite bodies in the detachment hanging wall. A narrow zone (10 m) in the immediate hanging wall contains Na-amphibole-rich schists and metabasic blocks with a different HP-LT record than the footwall. MAr dates from the hanging wall are Permian to middle Cretaceous and ZHe dates are early Eocene to middle Miocene. The geochronology from the footwall suggests coeval deformation with the CBU footwall of Mt. Hymittos and correlates the dominant top-to-SW detachment on NE Attica with the WCDS exposed at Mt. Hymittos and S Attica. Together with regional lithostratigraphic correlation, we propose the dominant nappe of NE Attica is CBU, specifically Lower Cycladic Blueschist Nappe, with Pelagonian Zone in the detachment hanging wall. Our reinterpretation is coherent with the classic Cycladic detachment architecture, and consequently limits the exposure of the Basal Unit to the easternmost marble thrust nappe on NE Attica and the Almyropotamos window on Evia.