Evolution of Nanocavities to Ductile Fractures in Crustal‐Scale Faults at the Base of the Seismogenic Zone

Abstract To understand the fracturing processes at the base of the seismogenic zone, the evolution of creep cavities to ductile fractures was examined within an exhumed ultramylonite zone. This ultramylonite zone records deformation at the base of the seismogenic zone. Ductile fractures are recognized phenomena in metallurgy, occurring during ductile deformation as deformation‐induced cavities grow and coalesce, leading to fracture formation. Using a heterogeneously deformed ultramylonite, we establish a correlation between cavity density and the quartz recrystallized fraction, which served as a general indicator for strain. Prior to fracturing, the cavity density increases linearly with the recrystallized fraction. The empirically determined area cavity density critical for ductile failure to occur is ∼7.5%. This trend indicates that ductile fracture results from the evolution of creep cavities. Ductile fractures are ubiquitous along the length of the exposed ultramylonite zones, spanning over 7 km. Similar microstructures of ductile fractures have previously been documented in various major shear zones. These suggest that ductile fracture represents a commonplace occurrence at the base of the seismogenic zone of continental crustal faults, and possibly controls various phenomena occurring below the seismogenic zone along these faults.