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 (plagioclase₁) grains and the growth of very fine (<20 μm) grained secondary plagioclase neoblasts (plagioclase₂) 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 H₂O species along the plagioclase₁ and plagioclase₂ grain boundary regions, as well as incorporated into plagioclase₂ 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 plagioclase₂ layers are volumetrically dominant over dry, coarse plagioclase₁ fragments. This forms a layer capable of deforming by grain-size sensitive creep and sustaining the mobility of fluids.