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

The Pelagonian Zone of the Hellenides, which occurs in the northwestern Aeg ean region of the Eastern Mediterranean, contains remnants of a high-pressu re metamorphic belt which subsequently underwent localized reworking during low-temperature deformational events. Formation of Alpine deformational st ructures in the Ossa Massif (northern Pelagonian Zone) occurred at metamorp hic temperatures which failed to reset most pre-Alpine Ar-40/Ar-39 cooling ages in muscovite porphyroclasts. The low metamorphic temperatures at Alpin e times have preserved the argon isotopic signature in white mica bearing d eformational 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 se nsitive Ar-40/Ar-39 laserprobe dating has resulted in a continuous record o f the early Alpine tectonic history of the Ossa Massif from the Early Creta ceous well into the Eocene. The early Alpine history of the Ossa Massif rec ords cooling of basement thrust slices, which are mostly Hercynian granites and metamorphics, below ca. 350 degrees C by 100 Ma, while lower tectonost ratigraphic levels of the basement had already cooled below these temperatu res 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. 5 4 Ma and involved tectonic activity along blueschist facies and greenschist facies mylonites zones. Final WSW-ward transport of the metamorphic sequen ce across the structurally lowest, and supposed, autochthonous, series occu rred at around 45 Ma, and resulted in termination of the ductile deformatio n history in the studied area. This study confines the onset of high-pressu re 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 mil lion years and involving crustal shortening, high-pressure metamorphism, ex humation and subsequent crustal shortening is a key contribution to a bette r understanding of the early Alpine tectonic history of the Aegean region a nd eastern Mediterranean. (C) 1998 Elsevier Science B.V. All rights reserve d.