SYN-EXTENSIONAL, NEOGENE SEDIMENTARY BASIN DEVELOPMENT ON THE MENDERES METAMORPHIC CORE COMPLEX, WESTERN TURKEY

The Menderes metamorphic massif (MM) in western Turkey is a classic core complex with exhumed high-grade crustal rocks intruded by granodioritic plutons and overlain by syn-extensional sedimentary rocks. Timing and the mechanism(s) of the initial exhumation of the MM are controversial, and different hypotheses exist in the literature. Major structural grabens (i.e. Alasehir, Büyük Menderes) within the MM that are bounded by high-angle and seismically active faults are late-stage brittle structures, which characterize the block-faulting phase in the extensional history of the core complex and are filled with Quaternary sediments. The Miocene to Plio-Pleistocene sedimentary rocks unconformably overlying the MM units in the footwalls of these range frontal faults keep a record of the early depositional history of the sedimentary basins, which developed on the detachment surfaces during the advanced stages of the MM exhumation. We examined the internal structure and stratigraphy of a supradetachment basin south of the Alasehir graben. The oldest sedimentary rocks onlapping the cataclastic shear zone of the MM here are the Middle Miocene lacustrine shale and limestone units, unconformably overlain by the Upper Miocene fluvial and alluvial fan deposits. Extensive development of these alluvial fan deposits by the Late Miocene indicates the onset of range-front faulting in the MM by this time, causing a surge of coarse clastic deposition along the northern edge of the core complex. The continued exhumation and uplift of the MM provided the necessary relief and detrital material for the Plio-Pleistocene fluvial systems in the Alasehir supradetachment basin (ASDB). A combination of rotational normal faulting & scissor faulting in the extending ASDB affected the depositional patterns and drainage systems, and produced local unconformities within the basinal stratigraphy. Horst-graben structures, half grabens, drag folds, and growth faults & folds are common in the Miocene-Pliocene strata, but no true contractional folds have been observed. There are no major interruptions in the syn-extensional depositional history of the ASDB, ruling out the pulsed-extension models suggesting a period of contractional deformation in the late Cenozoic evolution of the MM.