Fracture Mechanical Properties of Damaged and Hydrothermally Altered Rocks, Dixie Valley-Stillwater Fault Zone, Nevada, USA

摘要

Damaged and hydrothermally altered rocks are ubiquitous in fault zones, with the degree of damage and type and intensity of alteration varying in space and time. The impact of damage and alteration on hydromechanical properties of fault zones is difficult to assess without characterizing the associated changes to rock and fracture mechanical parameters. To evaluate the mechanical properties of fault rocks from different alteration regimes, we conducted (1) double-torsion load-relaxation tests to measure mode-I fracture toughness (K-IC) and subcritical fracture growth index (SCI), (2) uniaxial testing to measure unconfined compressive strength (UCS) and static elastic parameters, and (3) mineralogic and textural characterization of rock from four sites in the footwall of the Dixie Valley-Stillwater fault zone. Alteration at these sites includes acid sulfate alteration and silicification associated with active fumaroles, intense silicification after calcite and chlorite alteration in an epithermal setting, quartz-kaolinite-carbonate alteration from an intermediate-depth system, and a calcite-chlorite-hematite assemblage containing abundant unhealed damage. Silicification is associated with high K-IC, SCI, UCS, and increased brittleness, and in precipitation-dominated settings produces fault cores that are as strong or stronger than adjacent damage zone material. Calcite-chlorite-hematite assemblages containing abundant unsealed microfractures are approximately 4-5 times weaker than the granodiorite protolith. Mechanical properties are not predicted by mineralogical composition alone; a key control is the accumulation of damage and degree of healing. Measures of strength increase when mineral precipitation reduces microfracture porosity to <10-15% of total microfracture area. These results show that fault-proximal weakening or strengthening is influenced by hydrothermal setting.