EVOLUTION OF A MIDTERTIARY EXTENSIONAL SHEAR ZONE IN THE SOUTHERN MENDERES MASSIF, WESTERN TURKEY

The late Oligocene evolution of the southern Menderes Massif in wester n Turkey involves deformation in a crustal-scale, moderately dipping, ductile shear zone. The granitic rocks of the massif have been progres sively transformed to augen gneisses along the footwall of a south-dip ping extensional shear zone. The augen gneisses are strongly mylonitic and exhibit a moderately dipping foliation variably associated with a pronounced NNE- to NNW- trending mineral elongation lineation. The no n-coaxial fabrics formed during mylomitization strongly suggest an una mbiguous top-to-the-south, down-dip shear sense in a direction paralle l to the south plunging stretching lineation. The mylonitic foliation was later deformed into NNE-SSW trending open folds, with axis approxi mately parallel to the mineral elongation lineation, which suggests th at the ductile deformation was accompanied during its later stages by WNW-ESE directed compression. In the southern Menderes Massif, an aver age rate of denudation of about 0.2-0.52 cm/yr can be estimated for th e time span between 24.2 +/- 0.8 Ma (the initiation age of crustal ext ension in west Turkey) and 21 +/- 0.4 Ma (the oldest known unconformab le sediment), assuming that the mylonites were initiated at a maximum depth of 15-18 km by a fault that dips 60 degrees through the crust. T he cumulative displacement across the shear zone appears to exceed 17 km. The ages of the igneous activity and the mylonitization suggest a clear spatial and temporal relationship. It is suggested that the core complex formation in the southern Menderes Massif may be a direct res ult of, or triggered by, plutonic activity during latest Oligocene-ear ly Miocene spreading and thinning of a previously thickened continenta l crust, allowing extensional collapse of the orogen in western Turkey , due to its high thermal profile.