Fault zone architecture of the San Jacinto fault zone in Horse Canyon, southern California: implications for focused fluid flow during rupture at shallow crustal levels
Advisor Dr. Gary Girty
Stream cuts in Horse Canyon southwest of Anza, CA 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 distict 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 volumteric 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 not like 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. Furthermore, dissolution of plagioclase has resulted in a fault core highly enriched in quartz. Illite/smectite to illite conversion is also observed in the cataclasite core, suggesting elevated temperatures of at least ~150°C. The above relationships are consistent with higher water/rock ratios within the fault core than in the surrounding zones of gouge and microbreccia. These results, in conjunction with published data on the permeability behavior of cataclasite, indicate that hot pore fluids circulate upward via a narrow conduit within the San Jacinto fault zone during and after dynamic shearing episodes. Given the ~0.4 km depth of rocks studied in this investigation, such fluids reach 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).