40Ar/ 39Ar laserprobe direct dating of discrete deformational events: a continuous record of early Alpine tectonics in the Pelagonian Zone, NW Aegean area, Greece

Abstract

The Pelagonian Zone of the Hellenides, which occurs in the northwestern Aegean region of the Eastern Mediterranean, contains remnants of a high-pressure metamorphic belt which subsequently underwent localized reworking during low-temperature deformational events. Formation of Alpine deformational structures in the Ossa Massif (northern Pelagonian Zone) occurred at metamorphic temperatures which failed to reset most pre-Alpine 40Ar/ 39Ar cooling ages in muscovite porphyroclasts. The low metamorphic temperatures at Alpine times have preserved the argon isotopic signature in white mica bearing deformational structures, which allows direct dating of these structures and refinement of the Alpine tectonic history in the region. After separation of carefully selected and characterized mica generations, application of sensitive 40Ar/ 39Ar laserprobe dating has resulted in a continuous record of the early Alpine tectonic history of the Ossa Massif from the Early Cretaceous well into the Eocene. The early Alpine history of the Ossa Massif records cooling of basement thrust slices, which are mostly Hercynian granites and metamorphics, below ca. 350°C by 100 Ma, while lower tectonostratigraphic levels of the basement had already cooled below these temperatures at around ca. 285 Ma. Blueschist facies mylonite fabrics, related to a top-to-ENE direction of tectonic transport, yield ages as old as 84.5±3.3 Ma. Exhumation of the blueschist facies sequences was initiated at ca. 54 Ma and involved tectonic activity along blueschist facies and greenschist facies mylonites zones. Final WSW-ward transport of the metamorphic sequence across the structurally lowest, and supposed, autochthonous, series occurred at around 45 Ma, and resulted in termination of the ductile deformation history in the studied area. This study confines the onset of high-pressure metamorphism in the Pelagonian Zone to an interval between 100 Ma and 85 Ma and has shown that high-pressure metamorphism had terminated by ca. 54 Ma. The recognition of an early to middle Alpine cycle, lasting over 30 million years and involving crustal shortening, high-pressure metamorphism, exhumation and subsequent crustal shortening is a key contribution to a better understanding of the early Alpine tectonic history of the Aegean region and eastern Mediterranean.