Abstract
Abstract. The Late Cretaceous Asteroussia event as recorded in the Cyclades is a potential key to the tectonic evolution of Western Tethys. Microstructural analysis and 40Ar/39Ar geochronology on garnet–mica schists and the underlying granitoid basement terrane on the island of Ios demonstrates evidence of a Late Cretaceous high-pressure, medium-temperature (HP–MT) metamorphic event. This suggests that the Asteroussia crystalline nappe on Crete extended northward to include these Gondwanan tectonic slices. In this case, the northern part of the Asteroussia nappe (on Ios) is overlain by the terrane stack defined by the individual slices of the Cycladic Eclogite–Blueschist Unit, whereas in the south (in Crete) the Asteroussia slices are near the top of a nappe stack defined by the individual tectonic units of the external Hellenides. This geometry implies that accretion of the Ios basement terrane involved a significant leap of the subduction megathrust (250–300 km) southward. Accretion needs to have commenced at or about ∼38 Ma, when the already partially exhumed slices of the Cycladic Eclogite–Blueschist Unit began to thrust over the Ios basement. By ∼35–34 Ma, the subduction jump had been accomplished, and renewed rollback began the extreme extension that led to the exhumation of the Ios metamorphic core complex.
Highlights
A terrane stack accreted on the northern edge of the Tethys Ocean during the episodic closure of this ocean basin
The hypothesis that is the main contender as an opposing point of view is that this did not occur, and the Cycladic Blueschist Unit continually extruded during the long history of Alpine convergence
The effects of a high-pressure late Cretaceous event are evident in these relict fabrics in the Ios basement, so we conclude that the pre-Alpine tectonic history prior to accretion is more complicated than a single event would allow
Summary
A terrane stack accreted on the northern edge of the Tethys Ocean during the episodic closure of this ocean basin. Tectonic slices in the Cycladic Blueschist Unit were subject to high-pressure metamorphism and later juxtaposed against tectonic slices of Hercynian continental basement How this juxtaposition occurred remains controversial (Ring et al, 2007; Huet et al, 2009; Forster and Lister, 2009, and references therein). This led us to investigate the possibility that the Ios basement terrane that was made up of garnet–mica schist and augengneiss could be part of the Asteroussia nappe (see Be’eri-Shlevin et al, 2009) Previous interpretation of such age data (e.g., Andriessen et al, 1987; Baldwin and Lister, 1998) considered only the effects of “excess argon” or “mixing” and suggested that the apparent Late Cretaceous ages were the result of the Hercynian (∼ 300 Ma) argon population mixing with Cenozoic (∼ 50 Ma or younger) gas population. The high retentivity of argon in phengitic white mica (Forster and Lister, 2014) allowed these ages to survive the thermal effects of the later Alpine history
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call