Understanding crustal strain and seismicity in normal faults and shear zones

The Western Anatolia–Aegean region is characterized by active extension and well-documented seismicity. Yet the relationships among fault geometries, the depth distribution of earthquakes, and crustal strain patterns remain poorly understood. In particular, the existence of two outward-dipping low-angle normal faults in the central Menderes Massif poses a challenge to current geodynamic and seismological interpretations. In this study, we investigate the evolution of low-angle ductile–brittle shear zones using high-resolution viscoplastic thermo-mechanical forward models. Employing the finite-element code ASPECT, we simulate the initiation and development of shear zones in extensional settings, explicitly coupling surface processes and syn-extensional sedimentation to assess how progressive sediment loading may influence fault evolution. The model domain spans 500 km in width and 150 km in depth, and we explore two sets of models that vary in extension velocity and crustal layering. Our results show that shear zones initiate as high-angle (50°–55°) structures and progressively rotate to lower angles (30°–35°) as deformation localizes, suggesting that low-angle fault geometries may arise through time-dependent processes rather than pre-existing configurations. The models further indicate that the brittle–ductile transition extends into the upper portions of the lower crust, consistent with observed seismicity depths of 20–25 km beneath the Gediz Graben. By integrating model predictions with regional seismicity patterns, this work provides new constraints on the mechanical stratification and fault-system evolution of extended terranes, offering improved insight into active faulting and the characterization of seismogenic zones.