DUCTILE AND BRITTLE SHEAR SENSE INDICATORS ALONG THE BUYUK MENDERES DETACHMENT SURFACE, WESTERN TURKEY

A south-dipping low-angle normal fault is well exposed along the northern margin of the Buyuk Menderes Graben, one of the major E-W trending grabens of western Turkey. The fault separates a gneissic sequence of high grade metamorphic assemblage and an Early- Miocene sedimentary rock succession in its hangingwall from the marble intercalated, mylonitized schists in its footwall rocks. It is named the Buyuk Menderes Detachment.

We have mapped in detail about 40 km² area along the detachment surface in the vicinity of the Bascayir village where the detachment is well-exposed. We have also collected footwall and hangingwall fault rocks along three different transects and conducted both macro and micro scale structural analysis, especially noting the shear-sense indicators. The fault rocks reveal the presence of structural features related to both ductile and brittle extensional deformations. In structurally lower levels, the fault rocks are ductilely deformed and contain ductile shear sense indicators such as S-C and S-C’ asymmetric structures, schistosity, asymmetric mica fish, isoclinal folds and asymmetric porphyroclasts. These features suggest top-to-north sense of shear. In more than 85 % of the analyzed samples, it is also observed that the northward directed ductile shear sense indicators are overpinted by southward directed ductile shear sense indicators, such as asymmetric porphyroclasts, mica fish, and asymmetric quartzite. During the last stage of deformation, all of these ductile features are overprinted by top-to-the-south brittle shear sense indicators such as Reidel shears. These findings suggest that the Buyuk Menderes detachment may have developed in more than one stage; an earlier ductile extension with top-to-north direction of tectonic transport, overprinted by top-to-the south tectonic transport and a later brittle extension with top to the south direction of tectonic transport.