Miocene south directed low‐angle normal fault evolution on Kea Island (West Cycladic Detachment System, Greece)

Abstract

New structural, petrologic, and thermochronologic data from Kea, West Cyclades, define a crustal‐scale ductile shear zone and ductile/brittle low‐angle normal fault (LANF) system. Both the greenschist‐facies shear zone forming the footwall and the overlying LANF zone formed during constrictional strains, with a consistent top‐to‐SW‐S shear sense, with increasing finite strains toward higher structural levels but decreasing temperatures from footwall to hanging wall. The tectonostratigraphy comprises a ∼450 m thick footwall of shallowly dipping schists and calcite marbles representing the Intermediate Unit of the Attic‐Cycladic Crystalline (ACC). Above the footwall is a ∼60 m thick highly strained LANF zone, consisting of phyllonites, cataclastic schists, ultramylonitic calcite marbles, (proto) mylonitic calcite marbles, and cohesive cataclasites that arch over the whole island. These fault rocks exhibit multistage LANFs, evolving from ductile to brittle conditions. An up to ∼50 m thick brecciated limestone and dolostone sequence forms the unmetamorphosed hanging wall which is most probably part of the Upper Unit of the ACC. Multiequilibrium P‐T estimates on chlorite‐white mica pairs in the footwall yield 7–5.5 kbar/360°C–450°C for inclusions in albite and epidote, 5.5–3 kbar/400°C–350°C for the main foliation, and 3–2 kbar/350°C–280°C in localized shear bands (C and C′ foliations). The 40Ar/39Ar white mica footwall cooling ages demonstrate that greenschist‐facies retrogression occurred between ∼21 and 17 Ma. Localized, late decimeter thick shear zones were active and dynamically recrystallized before ∼15 to 13 Ma. The LANF on Kea, together with similar structures in South Attica and the West Cyclades define the West Cycladic Detachment System, characterized by ductile to brittle top‐SW‐S shear sense.