Gypsum/bassanite in a major southern California fault zone: observations from the Clark segment, San Jacinto fault zone, USA
Advisor: Dr. Gary Girty
Thursday, May 6th, 2021
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Fault zones are heavily fractured, leaving them with the capacity to be hydraulic conduits for fluids. We document the presence of basanite (CaSO4×0.5H2O), a relatively rare mineral, within the ~30-40 cm thick black fault core associated with the Clark segment of the San Jacinto fault in southern California, USA. At the study site, the Clark segment has been exhumed from depth of no more than ~250 m, and juxtaposes alluvial sandstones of the Pleistocene Bautista beds against Mesozoic gneiss of the Burnt Valley complex (BVC) in the hanging wall block. Along the NE side of the black fault core is an ~20-cm-thick strip of Bautista-derived cataclasites, and along the SW side are BVC-derived cataclasites (~5 m) and breccias (~10 m). XRD and thin section study indicate that bassanite fibers and prisms partially to completely replace gypsum in the black fault core. Similar, but less frequent, occurrences of gypsum/bassanite occur in the Bautista-derived and BVC-derived cataclasites where they are present along cracks and infill pores. Using newly obtained SO4 concentrations, we corrected 41 analyzed samples for the presence of gypsum/bassanite, and for the non-silicate minerals calcite and apatite. Mass balance calculations showed that statistically significant losses in Ca*, along with increases in Mg and LOI* mass, occurred in Bautista-derived cataclasites. In BVC-derived cataclasites, statistically significant increases in Fe, Mg, Ti, and LOI* mass are evident. In the black fault core, statistically significant increases in Ca*, K, P, and LOI* mass were calculated. Based on these results, molar A-CN-K and A-CNK-FM compositional linear trends were calculated. The calculated A-CN-K compositional linear trend for Bautista-derived cataclasites is directed toward the idealized composition of illite, a result that reflects the partial destruction of biotite and release of K+ during formation of Bautista-derived cataclasites, combined with the loss of Ca* mass. In contrast, in A-CNK-FM space, data from the Bautista-derived and BVC-derived cataclasites yielded Mg- or Fe-Mg enrichment compositional linear trends that pass near or include the idealized composition of smectite and chlorite. The source of Mg, or Fe, Mg, and Ti is likely biotite located upslope of the study area, either laterally or along strike. Calculated compositional linear trends for the black fault core, in both A-CN-K and A-CNK-FM space form Ca-enrichment trends. The above data suggest that saline Ca- and SO4-rich fluids were episodically focused within the black fault core. In contrast, fluids responsible for the enrichments in Mg, and Mg, Fe, and Ti in the Bautista- and BVC-derived cataclasites respectively did not enter the black fault core. Hence, compartmentalization of fluid flow within the various architectural components is indicated.