DUCTILE-BRITTLE TRANSITION ALONG THE ALASEHIR AND SIMAV SHEAR ZONES AND THEIR STRUCTURAL RELATIONSHIPS, MENDERES MASSIF, WESTERN TURKEY

The northern Menderes massif contains two well-developed shear zones (Simav and Alasehir shear zones) and associated detachment faults (Simav and Alasehir detachments). The Alasehir shear zone is developed in metamorphic rocks and in a syn-extensional granitoid (Salihli granitoid). The granitoid shows, structurally upward, a gradual change from the undeformed isotropic granodiorite to the deformed granodiorite. The metamorphic rocks are also mylonitized structurally upward. The mylonites contain protomylonite, mylonite and ultramylonite with mylonitic foliation and lineation. Both mylonitic metamorphites and granitoid, in turn, grades into a cataclastic zone whose uppermost part of is the Alasehir detachment surface. This gradual upward change suggests that the Cenozoic extension resulted in a ductile deformation at depth and as the crust isostatically adjusted to the removal of the rocks in the hanging wall, the ductilely deformed granitoids were brought to shallower depths where they were brittlely deformed. A similar transition is present along the Simav shear zone whose upper part is also a cataclastic zone leading to the Simav detachment surface which separates the low-grade metamorphic rocks and/or non-metamorphic rocks in its hanging wall from mainly high-grade metamorphic rocks and syn-tectonic granitoids in its footwall. During the ductile extension, greenschist-amphibolite facies-grade mylonitic deformation developed in the metamorphic rocks and in the granitoids. The mylonites contain foliations that include NE trending mineral lineations, defined by stretched quartz and feldspar grains plus grain shape preferred orientations of mica, kyanite and amphibole. The mylonitic rocks of the footwall grades structurally upward into several meters thick cataclastic zone.

Shear sense indicators along the two shear zones show top to N-NE sense of shear, consistent with the regional Cenozoic extension direction in western Turkey. Available radiometric ages from the granitoids in the footwall of the two detachment faults suggest that the extension in western Turkey was initiated in Oligocene to Early Miocene and the Simav shear zone is older than the Alasehir shear zone.