Deformation style and fault rock heterogeneity along the southwestern East Anatolian Fault

The East Anatolian Fault (EAF) is a plate-boundary strike-slip fault system of the Eastern Mediterranean characterized by one of the highest seismic hazards in the region. While its present-day kinematics and seismicity are well constrained, the geological record of fault rocks that archive long-term deformation remains poorly known. Here, we integrate meso-microstructural analyses and X-ray diffraction data from key exposures along the southwestern EAF to characterize deformation mechanisms across distinct fault rocks and structural levels. Our results reveal the along-strike, present-day coexistence of different deformation styles. Calc-mylonites at ˙ Islahiye document ductile deformation at mid-crustal conditions. Block-in-matrix serpentinite shear zones at Kırıkhan preserve the interplay between distributed creep and localized brittle failure. Carbonate cataclasites and ultracataclasites at Dursunlu provide evidence of brittle failure. Mineralogical data indicate that serpentinites are dominated by lizardite and chrysotile—minerals with low frictional strength and tendency to creep—whereas carbonates are frictionally stronger and more prone to stress accumulation, thereby favoring seismic failure at shallow crustal levels. The fault rocks provide geological analogues illustrating how complex faults can represent archives of deformation representative of different crustal levels. These distinct deformation styles are consistent with the variability in slip modes observed along the modern EAF, although a direct spatio-temporal correspondence cannot be established. Therefore, we encourage future research on the long-term evolution of this plate boundary, to better link past deformation with present-day seismicity. The EAF stands as a natural laboratory bridging geological and geophysical perspectives on fault mechanics and earthquake nucleation in the Eastern Mediterranean.