Abstract

Several generations of detachment fault systems have been recognized in the Aegean metamorphic core complex of Ios, Cyclades, Greece. Multiple strands occur in each fault system, with fault-bounded slices often characterized by distinctive lithologies. These faults accomplished significant exhumation, and telescoped the crustal section. Different slices have been deformed at different crustal levels, and then juxtaposed, so that adjacent fault slices display different deformation styles. The Aegean detachment systems differ from equivalent faults observed in the US core complexes in that they separate relatively cohesive tectonic slices. However, the Aegean detachments are dissected by the multiple systems of later high-angle normal faults that define the current geomorphology, and this raises significant difficulties in correlating detachment systems between the different islands of the Cycladic archipelago. A number of distinct extensional events can be defined on Ios. The Cycladic blueschist belt was exhumed relatively early during the history of Alpine orogeny, after it had been thrust over a Hercynian ‘basement’ terrane, with the oldest recognized detachment faults associated with Miocene N-S extension. This led to the exhumation of the ‘basement’ terrane from beneath the blueschist ‘series’ rocks. The Ios Detachment Fault system now separates the metasediments of the Cycladic blueschist belt from the mylonitized ‘basement’ complex. It was associated with the operation of the south-directed, crustal-scale, middle to upper greenschist facies, South Cyclades Shear Zone. The youngest detachment faults formed during late-stage W-directed extension, and formed a prominent sequence of low-angle normal faults termed the Coastal Fault system. These were associated with spectacular breccias, and relatively narrow ductile shear zones which formed during and subsequent to the formation of the Ios dome, under significantly lower grade conditions (lower-greenschist facies assemblages). Final exhumation of the Cycladic blueschist belt appears to have been associated with the detachment faults that led to its exposure beneath non-metamorphic sediments, and remnants of the Cyclades ‘ophiolite nappe’ as observed on adjacent islands.