Core-complex-related extension of the Aegean Hthosphere initiated at the Eocene-Oligocene transition

Core-complex-related stretching of the Aegean hthosphere was accommodated in part by the South Cyclades Shear Zone (SCSZ), which outcrops on the islands of Ios and Thera, Aegean Sea, Greece. This paper reports new microstructurally focused ⁴⁰Ar/³⁹Ar geochronology that suggests the SCSZ initiated just before the time of the Eocene-Oligocene transition (at ̃35 Ma). Therefore it is not only Miocene extension that led to the Tethyan metamorphic core complexes exposed in the Cycladic archipelago. It is shown here that pervasive top-to-the-south directed ductile shear took place in the SCSZ for ̃5 Ma during the early Oligocene, followed by initiation of overprinting north-sense shear zones at the time of the Oligocene-Miocene transition (at ̃25 Ma). This means that modern (core complex related) extension in the Aegean started much earlier than has been previously recognized and that stretching of the Aegean Hthosphere took place in a distinctly episodic fashion, not in a manner that can be described as continuous or progressive. The data also mean that the complementary shear-sense geometry of the SCSZ and the North Cyclades Shear Zone (NCSZ) cannot be interpreted as crustal-scale boudinage initiated during crustal extension in Miocene time since the opposing sense shear zones operated at distinctly different times. The paper illustrates the difficulties involved in systematically determining the age of deformation fabrics using ⁴⁰Ar/³⁹Ar geochronology and provides a case study that shows the value of carefully designed step-heating experiments on microstructurally controlled samples. In samples such as these, from complex metamorphic tectonites, laser fusion, or step-heating experiments with relatively few steps, would have led to erroneous conclusions.