Interactions between chemical alteration, fracture mechanics, and fluid flow in hydrothermal systems

The hydromechanical properties of fault zones reflect evolving feedback between chemical, hydrological, and mechanical processes. These processes are evident in differences in fault zone architecture and the mineralogical, textural, and mechanical properties of the constituent parts. In this study, I quantify each of these attributes and explore feedback pathways evident in the Dixie Valley-Stillwater fault zone, Nevada, USA. I conducted 1) double-torsion load-relaxation tests to measure mode-I fracture toughness (KIC) and subcritical fracture growth index (SCI) in ambient and aqueous conditions, 2) uniaxial testing to measure unconfined compressive strength (UCS) and static elastic parameters, 3) mineralogical and textural characterization of altered and damaged rock, and 4) field observations focused on the role of alteration in fault zone evolution. The first investigation explored the impact of alteration on fracture mechanical properties of exhumed alteration assemblages, including: fumarole-related acid-sulfate alteration and silicification, silicification in an epithermal environment, quartz-kaolinite-carbonate alteration in an intermediate depth system, and calcite-chlorite-hematite alteration. The second investigation examined the impact of physiochemical conditions on fracture growth in silicified rocks. Environments included: ambient air, deionized water, dilute HCl, NaOH, and NaCl solutions, and deionized water at elevated temperature. The third investigation employed field observations to assess the impact of alteration on fault evolution. The results from these complimentary investigations show that fault-proximal weakening or strengthening are strongly influenced by hydrothermal processes. Silicification is associated with increased KIC, SCI, UCS, and brittleness, producing fault cores as strong or stronger than adjacent damage zone material. Calcite-chlorite-hematite assemblages containing abundant unsealed microfractures are approximately six times weaker than silicified rocks. All measures of strength increase when sealing of microfractures surpasses ~85%. SCI in silicified rocks is reduced in aqueous environments, with >60% reduction in alkaline solutions, suggesting that physiochemical conditions in hydrothermal systems may facilitate fracture growth. Field observations support the importance of alteration and precipitation in fault zone development; silicification and precipitation-strengthening contribute to thick fault cores, whereas damage and alteration-weakening promote strain localization. Together, results from these investigations highlight the important and underappreciated role of hydrothermal processes in the development of hydromechanical properties in fault zones.