Description
In Blackburn Canyon, the Clark segment of the San Jacinto fault zone juxtaposes the Thomas Mountain Tonalite with the Bautista Metamorphic Complex. Rocks in the latter complex are not well exposed; hence, we focused our work on characterizing the architectural elements within the NE tonalitic block. Within the NE block, slip on an old cataclasite fault core (~1-2 m thick) appears to have been transferred across a narrow, ~70 m wide, extensional step over to a younger SW fault core (~5-15 cm thick), composed of foliated gouge. Lying between the two fault cores are, from SW to NE, an ~1-2 m thick zone of breccia and cataclasite, an ~65 m thick zone of damaged rock, and an ~2-3 m thick zone of pulverized rock. Both the old fault core and the zone of pulverized rock do not extend to the SE where they are replaced by an ~240 m thick inner damage zone. This latter architectural element then gives way NE to an ~760 m thick outer damage zone. The < 4 micron fractions extracted from the outer and inner damage zones contain mostly smectite and illite/smectite, with lesser amounts of illite. In contrast, the < 4 micron fractions derived from rocks lying between the old and young fault cores contain volumetrically more illite, although illite/smectite, vermiculite, and montmorillonitic smectite also are important constituents. On both A-CN-K and A-CNK-FM diagrams, samples analyzed from the outer damage zone to the zone of breccia and cataclasite tend to cluster in tight elliptical patterns, while specimens from the younger fault core spread toward the compositional field of illite. Changes in bulk mass vary across the fault zone architectural elements, but are generally less than ~5-6%, except in the young fault core where the change in bulk mass is 14.1% +/- 9.6%. Changes in elemental mass along with point-count data and qualitative thin section observations are consistent with moderate dissolution of plagioclase through the leaching of Na, Ca, and Ba. In addition, volumetric strains increase from ~7% in the inner damage zone to ~23 to 36% within the zone lying between and including the old and young fault cores. Increases in porosity follow a similar pattern and are likely the result of an increase in microfracture density which varies from ~3.03-mm in the inner damage zone to ~22.7-mm in the breccia and cataclasite zone. When the above data are compared to similar data derived from Horse Canyon lying 24 km to the SE, the following become evident. (1) At Blackburn Canyon, chemical alteration within the area lying between and including the old and young fault cores was not nearly as extreme as in similar zones at Horse Canyon. (2) Though discreet illite is a key volumetric component of the clay mineral assemblage lying between the zone of pulverized rock and the young fault core, the abrupt transition from largely illite/smectite to largely illite in the primary and secondary fault cores at Horse Canyon is not present at Blackburn Canyon. (3) The old and young fault cores at Blackburn Canyon yielded volumetric strains that are significantly larger than those calculated for the primary and secondary fault cores at Horse Canyon (21% and 27%, respectively). (4) The large extensional strains at Horse Canyon accumulated within an extensional bend, while those at Blackburn Canyon are the result of a narrow extensional step over. In short, though much work needs to be done, data and observations presented in this paper suggest that, given a similar exhumation depth, structural setting plays an important role in determining the intensity of the development of fluid-driven alteration and accumulated volumetric strains.
