Post-orogenic extension in Kos Island (Greece): geodynamic implications

Despite significant differences in the early tectonic histories, rocks in the eastern Mediterranean region partly share a common Cenozoic history characterized by several tectonic events including subduction, collision and extension. The correlations between the Aegean domain and the Menderes Massif have often been proposed, but few studies have considered the geology of the central Dodecanese Islands, which are located at the transition between the Aegean and Anatolian plates. In this study, we focus on the poorly studied island of Kos and investigate the tectonic history of the central Dodecanese Islands, as well as the general correlation with the Aegean and western Anatolian. Raman Spectroscopy of Carbonaceous Material (RSCM) analyses combined with white micas ⁴⁰Ar/³⁹Ar and zircon (U-Th)/He geochronology were carried out to determine peak temperatures and the timing tectonothermal events recorded by the various units. Three different tectonic units were identified from bottom to top: (1) the Paleozoic Unit overlain by the Permo-Triassic Wildflysch Unit in which primary sedimentary structures are well preserved. The units consist of low-grade meta-sediments including bedded meta-sandstones, meta-arkosic sandstones, meta-pelites and subordinate impure marble layers, bedded meta-chert with chaotic polymictic conglomerates containing huge blocks of metavolcanites, dolomitic limestones and marbles. T_max of a graphitic phyllonite located in the Wildflysch Unit is 299 ± 14°C, confirming low-grade metamorphism. (2) The Marina Basement Unit consists of coarse-grained pure marbles, impure marbles with metachert layers, garnets, andalusite-mica schists, and quartzites, which was thrusted onto the Paleozoic and Wildflysch units with top-to-N kinematics during the Paleocene. T_max from the Marina Basement are around 565 ± 35°C, suggesting a temperature difference of over 250°C with the previous units. (3) The Marina Cover Unit consists of unmetamorphosed dolomites, dolomitic limestones, and micritic limestones. Rocks of this unit are only preserved as isolated klippen juxtaposed onto the metasediments of the Paleozoic/Wildflysch Unit along the Oligocene Kos Detachment that exhibits an overall top-to-SSE shear sense. The western part of the Paleozoic Unit was intruded by a c. 10 Ma quartz-bearing biotite-hornblende monzonite intrusion which cooled until 5 Ma. The metamorphic aureole is a few 100 meters wide, which is similar in size to metamorphic aureole of other Miocene granitoids in the Cyclades. N-S extension is recorded after the intrusion as testified by cataclasites and W-E striking high-angle faults that control the current geomorphology of the island. At the regional scale, we propose that Oligocene extension occurred along the top-S Kos-Kalymnos Detachment System, and was localized in the Pelagonian Unit. Further west and a few million years later, the deformation started to propagate to deeper structural levels within the base of the Pelagonian, affecting the rocks of the Cycladic Blueschist Unit and favoring their relatively fast exhumation. Conversely, deformation in southwestern Turkey appears to record only compressional tectonics. Here, the Lycian nappes – the Turkish equivalent of the Pelagonian – were thrust to the SE upon the Menderes Massif and the Bey Daglari platform. This implies that the subduction dynamics differ from east to west in the eastern Mediterranean region.