Rb-Sr microchrons of synkinematic mica in mylonites: an example from the DAV fault of the Eastern Alps

A technique for texturally controlled microsampling from rock thick section s and subsequent Rb-Sr dating of mu g-sized samples has been developed. It allows direct dating of minerals unequivocally related to deformation. The technique has been applied to strictly synkinematically grown white mica pr epared from mylonites related to the Defereggen-Antholz-Vals (DAV) fault so uth of the Tauern window (Eastern Alps). Control of deformation ages is pro vided by field relationships with independently dated plutons and dykes dat ed here by single mineral Rb-Sr. The field observations and isotopic ages a re fully consistent and demonstrate that this is a reliable method for sub- mm-scale geochronology. The ages obtained establish that sinistral strike s lip shearing associated with the DAV fault lasted from at least 33 Ma to 30 Ma, followed by an abrupt change to dextral transpression related to the a djacent Periadriatic fault. (C) 2000 Elsevier Science B.V. All rights reser ved.