MIOCENE DEFORMATION OF THE MENDERES MASSIF: CONCURRENT FOLDING, FAULTING AND CORE COMPLEX FORMATION CAUSED BY A LITHOSPHERE SCALE WRENCH ZONE

Within the Alpine orogenic belt the Menderes Massif in western Turkey forms a composite nappe stack that was assembled during Eocene to Oligocene crustal shortening. Whereas subduction and imbrication of continental fragments appears to have ceased in western Turkey around the Late Eocene to Early Oligocene, orogenic activity has continued in the Aegean. This discontinuity in structural evolution along strike of the Alpine orogen requires the existence of a lithosphere scale shear zone between the Aegean and Anatolia. Here we propose that a lithospheric scale wrench zone caused concurrent folding, faulting and core complex formation across the Menderes Massif in the Early Miocene. Since the Early Miocene, the Menderes Massif has been exhumed and extended in an overall N-S orientation. The most prominent extensional structures in western Turkey are Miocene to recent E-W trending grabens controlled by low angle and high angle normal faults. Two of these grabens – the Gediz graben in the north and the Büyük Menderes graben in the south – delimit the Central Menderes Metamorphic Core Complex (CMCC). The CMCC is a syncline structure forced on the Alpine nappe stack by symmetrical footwall uplift below opposite facing low-angle normal faults. The area to the north of the CMCC – the Gediz Massif – displays a pattern of NE-SW trending Miocene basins, and basement highs. South of the CMCC, in the Çine Massif, similar basins trend NNW-SSE. Recent work has suggested that the exhumation of the CMCC is driven by a counterclockwise 30 degree rotation of the Çine Massif relative to the Gediz Massif, which means that the NE-SW basins in the Gediz Massif and the NNW-SSE trending basins in the Çine Massif may have formed in a similar orientation. The NE-SW and the NNW-SSE trending basins appear to have formed contemporary with early ESE striking stages of the ongoing E–W basin formation, but are much shallower and, unlike the latter, have ceased to be active before the Pliocene. Previous studies have suggested that bounding faults or basal detachments controlled the NE-SW striking basins in the Gediz Massif, but there is little direct evidence for this. Here we propose that in the Early Miocene, transtensional folding with a wavelength of tens of kilometers has occurred concurrent with core complex formation and ESE trending normal faulting across a wide structural corridor in the Menderes Massif. Folding caused uplift of basement in the anticlines, while providing accommodation space for the Miocene basins in the synclines. The existence of large granite intruded metamorphic core complexes and widespread volcanic activity in the northern Menderes Massif during the Miocene compared to minor intrusions in the footwall of the CMCC suggest that the crust was much hotter in the north. The combination of folding and faulting, distributed across a wide NE- trending corridor is likely to amount to substantial sinistral offset between the different lithosphere domains in Anatolia and the Aegean. Our model provides an explanation for the geometry and timing of Neogene basins in the Menderes Massif, and may be applicable to marginal zones of other continental backarcs.