Mechanisms of Aqueous Fluid Infiltration and Redistribution in a Lower‐Crustal Pseudotachylyte‐Bearing Fault

Abstract Coseismic fracturing in the strong, dry, and metastable plagioclase‐rich lower‐crust is an effective mechanism for creating pathways for fluids to infiltrate the host rock, kick‐start metamorphism, and potentially lead to rheological weakening. In this study, we have characterized the damage zone flanking a lower‐crustal pseudotachylyte (solidified frictional melt produced during seismic slip) within an anorthosite to determine the mechanisms of incipient aqueous fluid infiltration and redistribution in a lower‐crustal seismogenic fault. Pulverization‐style fracturing of the host anorthosite resulted in the comminution of the host plagioclase (plagioclase 1 ) grains and the growth of very fine (<20 μm) grained secondary plagioclase neoblasts (plagioclase 2 ) filling the fractures. Fluid‐assisted grain growth accompanied surface‐ and strain‐energy minimization grain growth in the healing and sealing of the fractures. This process was not associated with the densification nor the creation of new reaction‐induced porosity. Fourier transform infrared maps transecting the damage zones show the presence of H 2 O species along the plagioclase 1 and plagioclase 2 grain boundary regions, as well as incorporated into plagioclase 2 grain interiors. Grain‐size sensitive creep of fine‐grained plagioclase localized along the pseudotachylyte margin where fracturing was most pervasive. In the absence of reaction‐induced porosity, strain localization is determined by repeated occurrences of extreme grain‐size reduction in addition to the mobilization of aqueous fluid to the grain boundary regions, to the extent in which these fine‐grained wet plagioclase 2 layers are volumetrically dominant over dry, coarse plagioclase 1 fragments. This forms a layer capable of deforming by grain‐size sensitive creep and sustaining the mobility of fluids.