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The Büyük Menderes Graben (BMG) is a major E-W-oriented extensional basin in Western Anatolia, which exhibits along-strike variability. Yet, the factors controlling its internal asymmetry and the dominant boundary fault remain unresolved, mostly because previous studies of the BMG lack a three-dimensional perspective. This study integrates 2D seismic reflection data, well-log information, and 3D structural modeling between Aydın and Kuyucak to reassess the kinematic evolution of the graben.Seismic reflection data reveal a distinctly asymmetric basin infill geometry, where early syn-rift deposits form clear wedge geometries and onlap patterns directed toward the southern graben-bounding fault, indicating that accommodation was primarily created along the southern margin. Overlying units show draping, subtle rollover structures, and thickening toward the south, further supporting continuous activity on the southern boundary fault throughout basin development, resembling a rift-climax system tracts described in the literature for half-grabens. The stratigraphic architecture and fault-sediment relationships observed on the N-S seismic sections are consistent with sandbox experiments and conceptual models depicting how major listric faults control the evolution of extensional basins.A key outcome of this study is the recognition of a second control on basin asymmetry: a series of transverse, consistently east-dipping normal faults with dominant fault polarity is imaged on E-W seismic lines across the BMG. These structures generate localized depocenters, divergent reflection patterns, and westward-increasing thickness trends associated with progradational sediment input. When combined with GPS and InSAR results, both of which indicate a westward increase in extension rates across the BMG, the transverse faults are interpreted as the structural response to spatially variable extension, accompanied by a delta progradation throughout basin evolution.The apparent symmetry observed on 2D seismic sections is primarily the result of the activation of high-angle normal faults along the northern margin during the Quaternary, which locally produced a more symmetrical basin infill geometry. The 3D structural model, on the other hand, further demonstrates that the basin deepens toward the southern boundary fault, whereas the northern fault retains its irregular geometry and limited subsidence. Geodetic slip-rate modeling also favors a north-dipping, active structure, aligning with the southern boundary fault. These observations suggest that the low-angle normal fault on the northern margin, commonly referred to as the Büyük Menderes Detachment Fault, is a relic structure of an earlier extensional phase, predating the formation of the current basin.Overall, stratigraphic geometries, structural characteristics, and geodetic data converge on a coherent conclusion: the BMG evolved through multi-phase extension, dominated by the southern boundary fault, while transverse east-dipping faults and delta progradation enhanced internal basin asymmetry. These results refine the current understanding of rift evolution and faulting history in Western Anatolia and emphasize the role of spatially variable extension in shaping extensional basins.Keywords: Eastern Mediterranean tectonics, Büyük Menderes Graben, extensional basin evolution, seismic reflection data, normal fault kinematics, active deformation.