146-2 Understanding the impact of varying hydrothermal alteration around preexisting structures on local fault zone evolution and reactivation

Abstract:

The interaction of hydrothermal fluids with fault rocks throughout seismic cycles can result in hydrothermal alteration, modifying fracture networks, permeability, and ultimately influencing both subsurface fluid flow and future fault reactivation. Determining the categories and sequential relationships between preexisting and concurrent hydrothermal alteration along faults provides important insights into fault nucleation and propagation along preexisting structures. In this study, we focus on the 2019 Ridgecrest earthquake sequence, where numerous faults were activated along preexisting dike contacts. We compare surface slip with field and satellite mapping of dikes, which displays variable fault activation over the length of meters to kilometers along dike intrusion contacts. To further understand why faulting occurs on only some sections of an individual contact, we perform structural and geochemical analyses along transects orthogonal to the dike contact at two locations, one with faulting present and one without. We map out fractures through field and photogrammetric analysis, and conduct mineralogical and chemical characterization using petrographic imaging, X-ray diffraction, and X-ray fluorescence. Mineralogical analysis reveals enhanced concentrations of phyllosilicates and calcite growth along faulted dike contacts, producing a weaker zone in the granites close to the dike. In contrast, the portion of the dike that has not been reactivated as a fault shows reduced levels of hydrothermal alteration and phyllosilicate formation, and an absence of calcite. The presence of micas and sericite along the faulted section of the dike suggests localized phyllic alteration in the granite, which is frictionally weak compared to the less altered dike contact with no fault reactivation. Our results suggest that variations in hydrothermal mineral alteration along dike contacts dictate varying degrees of fault reactivation and initiate positive feedbacks wherein progressive deformation accelerates further alteration. This study leads to a better understanding of the impactful presence of fluid within fault zones and provides insights to help better predict long-term chemical evolution and changes in fault strength.

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