Brian Rockwell, Gary H. Girty, Thomas Rockwell; San Diego State University
Clay mineralogy, non-central principal component analysis, and alteration intensity factors, NE block, Clark segment, San Jacinto fault zone, southern California, USA
The NE block of the Clark segment of the San Jacinto fault zone within Horse Canyon, SE of Anza, southern California displays a well-defined damage zone, transition zone, and primary fault core. Previous work showed that through fragmentation, slip, and chemical alteration, grain and fragment size, as well as the proportion of plagioclase, progressively diminishes from the outer damage zone inward toward the fault core. In contrast, the proportion of quartz increases.
The results of XRD work completed during this study revealed that the < 2 micron fraction is composed primarily of illite/smectite with ~15% illite in the damage zone, of illite/smectite with ~30% illite in the transition zone, and of discreet illite with very minor smectite in the fault core. In addition, PC1 derived from non-central principal component analysis explains 99.7% of the simplicial variability of the spread of A-CN-K data about a calculated compositional linear trend. Alteration intensity factors for each sample analyzed from the various architectural components of the fault zone were derived from orthogonal projection onto the calculated compositional linear trend. Results from a one-way ANOVA indicate that at least one significant difference across the group of means of alteration intensity factors are statistically different at the 95% confidence level (omnibus p = 0.0001). Post hoc routines indicate that the mean of the alteration intensity factors for the fault core are different than the means obtained from the transition and damage zones. In contrast, at the 95% confidence level, the means of the transition and damage zones are not statistically distinguishable.
Based on the above results, it is speculated that when fault zones are derived from tonalitic wall rocks at depths of ~0.4 km, the illite/smectite to illite transition will occur when alteration intensity factors exceed 0.20 ± 0.12, the average alteration intensity factor calculated for the transition zone. Under such conditions during repeated rupturing events acidic fluids with elevated temperatures (≥ ~125° C) are flushed through the fault core. Over time, the combination of shearing, fragmentation, and relatively elevated temperatures eventually overcomes the kinetic barrier for the illite/smectite to illite transition. Such settings and processes are unique to fault zones, and as a result, they represent an underappreciated setting for the development of illite from illite/smectite.
2013 SCEC Annual Meeting