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Description
Though saprolitization is commonly viewed as an isovolumetric process, data obtained during this investigation, along with the results of earlier studies, support the idea that significant volume change has occurred during the conversion of saprock to corestone at 10 sites scattered about the trace of the Elsinore fault. Specifically, data presented and discussed in this paper reveal that at proximal sites, i.e., those that lie between 0 and 4 km of a major strand of the Elsinore fault, dilational strains range from 25% ± 7% to 37% ± 7%. In contrast, at distal sites, i.e., those lying between 13 and 20 km to major strands of the Elsinore fault, volumetric strains range from 6% ± 3% to 12% ± 5%. A positive correlation between the orthogonal distance to the nearest strand of the Elsinore and volumetric strain yields an R2 value of 0.80. Porosity values in proximal samples range from 17% ± 9% to 26% ± 7%, and in distal samples from 10% ± 2% to 17% ± 3%. Preliminary results from a study of crack morphology suggests that most of the porosity in studied saprock samples was primarily produced by Mode I cracking, while chemical data and a linear regression model of porosity versus volumetric strain indicates that as much as 7.7% of the porosity was likely produced by partial dissolution of plagioclase and biotite. This latter process produced statistically significant but relatively small losses of Ca, Na, Sr, and Ba mass from plagioclase, and K, Fe, Mg, Mn, and Rb mass from biotite. Moreover, the intensity of chemical weathering as reflected by the Chemical Index of Alteration (CIA) at all distal sites is minor to moderate and ranges from ~52 to ~58. In contrast, the CIA values at proximal sites are either like those at distal sites or ranges from ~55 to ~72. Both sites that exhibit this latter characteristic are located on the same Cretaceous granodiorite. The clay mineralogy of the less than 4 micron fraction was studied from one of these two sites, and is composed of more than 90% kaolinite. In contrast, at all other proximal and distal sites, the clay mineralogy is dominated by mixed-layer illite/smectite and chlorite/smectite, a clay mineral assemblage that is consistent with the relatively low annual precipitation rates like those in the modern day Peninsular Ranges (< ~12 inches (~30.5 cm)/year). The clay mineralogy and intensity of the weathering at the two proximal sites lying on the ~125 Ma granodiorite, implies that these sites were influenced by annual precipitation rates that were significantly higher than those in the modern Peninsular Ranges, and as result, we infer that the regolith at these two sites may have originated in the Pleistocene. Such a conclusion is supported by data reported in the literature that shows that corestones throughout large areas of southern California have been exposed at the land surface for at least the past 10,500 years.
