Melt Welding and Its Role in Fault Reactivation and Localization of Fracture Damage in Seismically Active Faults

Low displacement fracture damage plays an important role in influencing the behavior and mechanical evolution of faults. Fracture damage zones influence slip behavior through changing near-field stress orientations, altering fluid pathways and modifying fault structure. Here we use small displacement triaxial experiments to explore the development of fault zone damage, frictional lock-up, and the generation of new faults using samples with preground faults, oriented in 5 degrees increments between 25 degrees and 65 degrees relative to the shortening direction. With increasing reactivation angle, faults support higher peak normal stresses (104-845MPa) and behavior transitions from stable sliding to stick slip. Frictional melting occurs on surfaces where stick slip is initiated, forming micron-thick layers that locally weld asperity contacts. The extent of melt welding is correlated with normal stress and melt-welded zones increase fault cohesion. Distribution of fracture damage adjacent to the fault is spatially correlated with melt-welded zones and the corresponding concentrations of stress and elastic strain. In a process referred to as adhesive wear, fractures bypassing welded zones transfer melt-adhered material from one side of the fault to the other, forming new geometric asperities. On faults with high reactivation angles (55 degrees-60 degrees) the increase in cohesive strength resulting from melt-welded contacts drives fault lock-up after an initial slip event; subsequent slip localizes on new, favorably oriented faults. Given their size, melt-welded zones are likely to be short-lived in nature but may play a significant and previously unrecognized role in the development of fault-related damage.