Evidence for and significance of the Late Cretaceous Asteroussia event in the Gondwanan Ios basement terranes

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.

Low‐temperature thermochronology and the timing of motion on detachment faults

“…In addition, subduction (accretion) events prior to that time are also likely. Argon geochronology as well as microstructures in combination with microchemistry (Yeung et al., 2021) support the inference that garnet growth took place in the garnet‐mica‐schist slice during the eclogite facies Δ 1Β event at ~53 Ma. The open question is the relation of the garnet‐mica‐schist to the schists above the IDF since high‐angle normal fault arrays offset the IDF.…”

“…This is not the first time subduction (and overthrusting) has been suggested prior to the extensional history triggered by the operation of the South Cyclades Shear Zone (SCSZ). Yeung et al, 2021 (their figure 2a) have already shown an inferred geometry for the most recent subduction event.…”

“…In addition, subduction (accretion) events prior to that time are also likely. Argon geochronology as well as microstructures in combination with microchemistry (Yeung et al, 2021) show that 'The geometry of detachment ensures that the upper plate heats pervasively above a detachment system, even though cooling of the lower plate takes place at the same time' and 'Rapid cooling ensues only when the upper plate finds itself in the lower plate of a later-…”

“…During the last push–pull cycle (Forster & Lister, 2005; Lister & Forster, 2009), the Gondwanan Ios basement terrane was accreted. Subduction (Yeung et al., 2021, their Figure 2a) may have involved a jump in the location of the frontal megathrust, a tectonic mode switch, and the initiation of the extensional systems that began to exhume (and cool) these deeper levels. The upper plate had already been significantly exhumed by this time, evidenced by the basal levels of the upper plate suffering prograde greenschist facies contact metamorphism as the result of their juxtaposition against the hot rocks of the lower plate as it was exhumed (Forster et al., 2019).…”