Description
Stream cuts in Horse Canyon southwest of Anza, California have exposed the fault core, adjacent damage zone, and wall rocks of the San Jacinto fault at an approximate depth of 0.4 km. Here the juxtaposition of tonalitic plutons provides a unique opportunity to assess the formation of fault zone rocks with similar host-rock lithologies. We present results on the physical properties, chemistry and mineralogy of distinct zones identified within the fault zone. Samples from these zones were analyzed using bulk and grain density measurements, geochemical data, clay mineralogy, and textural and modal mineralogy. Progressive mechanical deformation is characterized by initial mode I cracking in quartz and plagioclase and subsequent shearing of fractured rock. These processes produce anastomosing seams of microbreccia and gouge that increase in frequency, thickness, and degree of grain communition towards a strongly indurated cataclasite fault core. Damage progression towards the fault core is accompanied by decreased bulk and grain density and increased porosity and dilational volumetric strain. A-CN-K plots indicate that fault zone rocks are altered along a trend from unweathered tonalitic wall rocks towards an end member that includes illite-mica. Such a trend is different from that expected from normal surface weathering. Chemical alteration along this trend is reflected in elemental mass changes that are most pronounced in the fault core where dissolution of plagioclase has resulted in a core that is highly enriched in quartz. Moreover, the illite/smectite to illite conversion is preserved in the cataclasite core, suggesting elevated temperatures that may have approached ~150°C. The above relationships are consistent with higher water /rock ratios within the fault core than in the surrounding damage zones of gouge and microbreccia. These results, in conjunction with published data on the permeability behavior of cataclasite during deformation events, indicate that hot pore fluids likely circulate upward via a narrow conduit within the San Jacinto fault zone during and after dynamic shearing episodes. Given the ~0.4 km maximum depth of rocks studied in this investigation, such fluids must have reached very shallow levels. Though difficult to constrain, the site studied during this investigation may represent the top of a series of narrow hydrothermal circulation cells that dissipate heat generated from rupture events at deeper levels (> 4 km).
