SYN-EXTENSIONAL FAULT GENERATIONS AND THEIR ROLE ON THE LATE CENOZOIC ALASEHIR SUPRADETACHMENT BASIN EVOLUTION IN THE MENDERES METAMORPHIC CORE COMPLEX, WESTERN TURKEY

The Menderes metamorphic massif (MM) in western Turkey is the largest core complex in the Aegean extensional province. It includes exhumed high-grade metamorphic rocks of Gondwana origin intruded by metaluminous to peraluminous granitoid plutons. Late Cenozoic structural grabens (Alasehir, Büyük Menderes) bounded by high-angle and seismically active faults crosscut the MM and are filled with Quaternary sediments. The E-W-trending Alasehir basin represents a well-preserved supradetachment basin with the N-dipping Alasehir detachment fault separating metamorphic rocks of the MM and the syn-extensional Salihli granitoid from the overlying Miocene and younger sedimentary rocks. The nearly 100-m-thick cataclastic shear zone beneath the detachment surface contains S-C fabrics, microfaults, Riedel shears, and shear bands, all consistently indicating top-to-the NNE shearing. The Miocene Salihli granitoid intruding the MM and the detachment surface displays mylonitic textures and ductile-brittle structures (microfaults, asymmetric objects, mica-fish, pressure shadows), all consistent with also top-to-the NNE extensional shearing. The oldest sedimentary rocks overlying the Alasehir detachment surface are the Lower Miocene lacustrine shale-limestone units (Gerentas, Kaypaktepe units) overlain by the Middle-to-Upper Miocene fluvial and alluvial fan deposits (Acidere and Gobekli units). Extensive occurrence of these alluvial-fluvial sedimentary rocks indicates a surge of clastic deposition along the northern edge of the core complex associated with the onset of range-front faulting in the MM by the late Miocene. Plio-Quaternary lacustrine-to-alluvial deposits (Yenipazar, Asartepe, and Erendali units) unconformably overlie the Neogene sedimentary units. The continued uplift of the MM provided the necessary relief and detrital material for the Plio-Pleistocene fluvial systems in the Alasehir supradetachment basin (ASDB). Quaternary alluvium fills the modern Alasehir graben bounded by a seismically active N-dipping, high-angle fault. We classify four different fault generations controlling the deposition, basin geometry and internal deformation in the ASDB. The Alasehir detachment fault (F1) is a N-dipping, low-angle corrugated fault plane with turtleback structures at the surface. E-W-striking, km-scale major high-angle faults (F2) crosscut and offset the detachment fault along the southern section of the basin. These faults strongly controlled the deposition of the Upper Miocene-to-Plio-Quaternary sedimentary units and resulted in significant changes in their dip angles and directions due to block rotation and tilting. The E-W-striking, low-angle normal faults (F3) mimic the geometry and kinematics of the N-dipping Alasehir detachment fault, and occur both in its footwall and hanging wall blocks during the basin evolution. Crosscutting relationships between the F2 and F3 fault generations indicate that high-angle normal faulting was a continuous mode of extension as the MCC continued its exhumation during the late Cenozoic. N-S-striking, high-angle scissor faults (F4), which crosscut the MM, the detachment surface and the basinal strata, caused differential uplift between individual rotational fault blocks showing different structural architecture. Strike orientations of sedimentary units adjacent to these scissor faults show significant changes due to rotational deformation along the N-S-striking, oblique-slip, high-angle scissor faults. The Neogene sedimentary rocks indicate higher amount of rotational deformation in the southern section of ASDB. Higher dip angles in the lower to upper Miocene units in comparison to shallower dip angles in the Plio-Pleistocene Yenipazar and Asartepe units are a result of progressive rotational deformation along F2 and F4 faults. Horst-graben structures, extensional drag folds, growth faults and folds are commonly seen in the basinal strata, but no true contractional folds or reverse faults have been observed along the southern flank of the Alasehir supradetachment basin. We interpret a few N-S-striking reverse fault planes as the extensions of range-perpendicular scissor faults. This fault kinematics and the distribution of range-parallel and range-perpendicular faults had a major role in the development of the shape and depth of the accommodation space within the ASDB, and controlled the deposition patterns and fluvial drainage systems. Local unconformities formed as a result of differential extension and uplift rates within the basin. There is no evidence for large-scale contractional deformation or major interruptions in the syn-extensional deposition history of the ASDB, and therefore we rule out the pulsed-extension models that suggest a period of contractional deformation in the late Cenozoic evolution of the MM. We conclude that the late Cenozoic extensional deformation was a continuous process controlled by four different fault generations and their kinematics affecting the syn-extensional deposition and overall basin evolution in western Turkey.