Abstract: The NNE-SSW trending Gelenbe Fault Zone (GFZ), located within the Aegean extensional system, isone of the critical seismotectonic elements constituting the neotectonic structure of Western Anatolia. Extending between the Bigadiç district in Balıkesir and the Akhisar district in Manisa, this active fault system plays a key role in understanding the seismic activity and geodynamic structure of the region. The region exhibits high seismic activityin both historical and instrumental periods, making it a prominent area in terms of earthquake hazard in Türkiye.Based on geological field studies including mapping and fault surface measurements, this study provides detailed data about the surface segment geometry and kinematic character of the GFZ. Field observations and 1/25,000scale mapping studies show that the GFZ is an approximately 8-km-wide zone consisting of two nearly parallel main segments, the Eastern and Western segments, with lengths of 35 km and 36 km, respectively. The Eastern segment comprises four fault splays trending N-S and N10-20°E, while the Western segment consists of five fault splaystrending N-S to N35°E.Kinematic analyses conducted on 132 slip-plane data points collected from 12 distinct locations along thefault zone indicate local stress variations induced by fault bends—resulting from sinuous surface geometries—andthe presence of antithetic and synthetic secondary fractures developed within the shear zone. Paleostress analysis revealed that two distinct tectonic phases were effective in the evolution of the region. NE-SW compression-relatedNW-SE extension represents the Miocene-Early Pliocene compressional regime (Phase 1), while Phase 2 reflectsthe WNW-ESE compression-related NNE-SSW extensional regime effective from the Pliocene to the present. Focal mechanism solutions consistent with Phase 2 confirm that the eastern segment (DK4) of the GFZ was reactivated following the August 10, 2025, Sındırgı earthquake (Mw: 6.1). Furthermore, seismic activity concentrated on boththe primary right-lateral faults and the left-lateral antithetic secondary fractures. All obtained data indicate thatthe GFZ acts as a strategic barrier responsible for the transfer of the extensional regime, acting as a right-lateralstrike-slip transfer structure within the İzmir–Balıkesir Transfer Zone in the current deformation network of Western Anatolia and must be considered to have primary importance in regional seismic risk assessments.