Deformation within the Cycladic subduction–exhumation channel: new insights from the enigmatic Makrotantalo nappe (Andros, Aegean)

New detailed geological/structural mapping coupled with microstructural investigations and amphibole chemistry analyses are used to elucidate the enigmatic tectonic evolution of the Makrotantalo nappe on northern Andros (Aegean region). On the basis of the new geological map, we re-evaluate the structural position of samples with previously published metamorphic ages to show that the Makrotantalo nappe has been affected by the early Eocene high-pressure (HP) metamorphism. Therefore, the Makrotantalo nappe is an integral part of the Cycladic Blueschist Unit (CBU). The early, main ductile deformation phase recognized in the mapped area, is associated with the emplacement of the Makrotantalo nappe over the North Cyclades nappe at deep subduction levels during early Eocene. The subsequent deformation phase occurred at the middle Eocene–Oligocene synchronous with the decompression of blueschist rocks from the stability field of glaucophane to that of actinolite. During this phase, the nappe pile was folded by map-scale folds, which were locally transposed by ductile shear zones. These shear zones were related with NE-directed oblique thrusting and, locally, either restacked the nappe pile or reworked the early nappe contact. Our results in combination with previous studies in the northern Cyclades show that the top-to-the-NE shearing was dominant at a large part of the CBU during exhumation. Opposite, SW-directed shearing was restricted at the upper structural levels of the CBU. We suggest that the observed spatial variation in shear sense is compatible with NE- rather than SW-directed ductile extrusion of the CBU.