[1] Core-complex-related stretching of the Aegean lithosphere 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 40 Ar/ 39 Ar 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 lithosphere 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 40 Ar/ 39 Ar 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.

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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.

Metamorphic rocks in West Sarawak are poorly exposed and studied. They were previously assumed to be pre-Carboniferous basement but had never been dated. New 40 Ar/ 39 Ar ages from white mica in quartz-mica schists reveal metamorphism between c. 216 to 220 Ma. The metamorphic rocks are associated with Triassic acid and basic igneous rocks, which indicate widespread magmatism. New U-Pb dating of zircons from the Jagoi Granodiorite indicate Triassic magmatism at c. 208 Ma and c. 240 Ma. U-Pb dating of zircons from volcaniclastic sediments of the Sadong and Kuching Formations confirm contemporaneous volcanism. The magmatic activity is interpreted to represent a Triassic subduction margin in westernmost West Sarawak with sediments deposited in a forearc basin derived from the magmatic arc at the Sundaland-Pacific margin. West Sarawak and NW Kalimantan are underlain by continental crust that was already part of Sundaland or accreted to Sundaland in the Triassic. One metabasite sample, also previously assumed to be pre-Carboniferous basement, yielded Early Cretaceous 40 Ar/ 39 Ar ages. They are interpreted to indicate resumption of subduction which led to deposition of volcaniclastic sediments and widespread magmatism. U-Pb ages from detrital zircons in the Cretaceous Pedawan Formation are similar to those from the Schwaner granites of NW Kalimantan, and the Pedawan Formation is interpreted as part of a Cretaceous forearc basin containing material eroded from a magmatic arc that extended from Vietnam to west Borneo. The youngest U-Pb ages from zircons in a tuff layer from the uppermost part of the Pedawan Formation indicate volcanic activity continued until c. 86 to 88 Ma when subduction terminated.

New geochronology from Syros in the Cycladic eclogite-blueschist belt, Aegean Sea, Greece, shows that 40 Ar/ 39 Ar geochronology consistently dates microstructural events in metamorphic rocks. We demonstrate that the age spectra depend on microstructure in a predictable and systematic way. Ages can be inferred by applying the method of asymptotes and limits to data from the step-heating experiments. The results are consistent with previously published estimates for the timing of a sequence of distinct and discrete episodes of high-P metamorphic mineral growth observed regionally across this belt. Arrhenius plots from these experiments imply that phengitic white mica is highly retentive of argon, and therefore (if these data can be extrapolated to the natural environment) the ages can be interpreted as recording the timing of episodic deformation and metamorphism. Porphyroblastic growth begins: (i) for omphacite-jadeite-eclogite facies parageneses at c. 53 Ma; and (ii) for garnetglaucophane facies parageneses at c. 47 Ma. The Kini Shear Zone started as an extensional postepidote-albite-transitional-blueschist facies shear zone that had completed operation by c. 31 Ma. The scatter in ages is due to the effect of deformation, recrystallization and multiple growth events in shear zones that continued operating for 3-6 million years from the start of each episode.