Strength Implications of Multi-Stranded Pseudotachylyte Faults within Mylonites

The strength of plate-boundary fault and shear zones is dependent on the composition and structure of the rocks within them. Earthquakes occur in faults and modify both the mineralogy and the structure of the rock, for example, by causing frictional melting. These melts quench to form fine-grained rocks called pseudotachylytes which usually have different grain size and mineralogy than the original rock. Earthquakes can also modify the structure of fault zones as multi-stranded webs of subparallel slip surfaces alter the orientation of anisotropic wall rock. Experiments designed to examine post-seismic strength of pseudotachylytes have reported both strong and weak pseudotachylytes at short timescales. Field studies have the potential to reveal the long-term strength contrasts between different rocks in shear zones under natural conditions. We report detailed field observations of the geometries of pristine and deformed pseudotachylyte veins in the Fort Foster Brittle Zone, Norumbega Shear Zone, southern Maine. We show that pseudotachylyte does not appear to impact brittle reactivation potential within a fault, but pseudotachylyte effectively focuses interseismic ductile flow. We build on the work of Price et al. (Tectonophysics, 2012), who describe how to identify deformed pseudotachylytes, and propose criteria for distinguishing the degree of deformation and recrystallization. We observe multi-stranded webs of subparallel pseudotachylyte fault veins which bound rotated wall rock blocks featuring disrupted mylonitic layering, facilitated by adhesive wear causing co-seismic slip redistribution. Rotation of mylonite blocks due to slip on anastomosing curved faults results in the discontinuity of weak layers in mylonite, causing local strengthening and partitioning of post-seismic shear into neighboring wall rock. Densely grouped reworked and fresh pseudotachylyte indicate localization of rupture along these interfaces of straight planar and disrupted mylonitic fabric. Preliminary findings are that the formation of pseudotachylyte may monotonically weaken faults, due to interseismic ductile deformation, but this weakening is limited by the geometric strengthening effects countered by block rotation of multi-stranded rupture.