Wedge-confined deformation and strain partitioning in the northern Sistan suture zone (Eastern Iran) revealed by microseismicity

Mature continental collision zones are commonly characterized by complex fault networks and heterogeneous crustal rheology, yet the mechanisms by which shortening and shear are accommodated within the brittle crust remain debated. Using high-resolution microseismicity recorded by a dense temporary seismic network integrated with long-term regional observations, we resolve active deformation within a mature intracontinental suture and constrain the seismogenic architecture of the upper crust. Earthquake relocation and focal mechanism analysis show that seismicity is confined to a relatively thin seismogenic layer (∼3–16 km), indicating strong mechanical stratification and efficient localization of brittle deformation above a weaker lower crust. The systematic coexistence of strike-slip and reverse faulting demonstrates pronounced strain partitioning under oblique convergence, whereby lateral shear and crustal shortening are accommodated concurrently. Variations in focal mechanism orientations reveal that, while the regional stress field reflects large-scale plate convergence, local stress orientations are strongly modulated by fault geometry, segmentation, and interaction, producing spatially heterogeneous deformation. We propose that deformation in this setting is most consistent with a wedge-confined regime in which interacting fault systems distribute strain internally without requiring rigid block rotation or throughgoing master faults. This deformation style promotes distributed seismicity across multiple fault strands, with important implications for seismic hazard assessment in mature collision zones. Our results highlight the fundamental role of seismogenic thickness, rheological layering, and fault interaction in controlling deformation style and stress transmission within continental suture zones.