EVIDENCE FOR FLUID PATHWAYS ALONG DETACHMENT FAULTS FROM INFRARED SPECTRA OF GOUGE, DEATH VALLEY, CA

Major faults are clearly loci for deep fluid circulation in the crust, but opinions vary on whether this circulation is highly focused in narrow zones or broadly distributed along fault planes. To address this question, the nature and distribution of hydrous (mostly sheet) silicates in gouge along well-exposed detachment faults was evaluated using a portable infrared mineral identifier (TerraSpec Halo). Mineral distribution, as well as spectral characteristics (e.g. peak ratios, or "scalars") indicate relative temperatures of formation or alteration of gouge material. These scalars are akin to the Kübler and Árkai indices derived from XRD analysis. In particular, the illite spectral maturity (ISM) is defined by the ratio of IR absorbtion by the AlOH bond (crystallinity) to absorbtion by bound OH^-(hydration). Chlorite spectral maturity (CSM) is defined similarly. The variation of mineralogy and IR scalars along the well-studied Mormon Point detachment in Death Valley is taken as an example here, at sites identified by Haines & van der Pluijm (2012):Mormon 1 (M1) lies 100 m east of Badwater Road, Mormon 3 (M3) lies 2 km east of M1, and Mormon 2 (M2) 200 m NE of M2. The gouge has been dated as latest Pliocene (2.8 MY, Haines, et al. 2016).

Cooler fluids appear to have infiltrated at M1 & M2, with ISM and CSM in gouge distinctly smaller (less mature) than in adjacent hanging- or foot-wall rocks. Mineralogical changes consistent with this conclusion are increased abundance of smectite and iron oxide in most gouge at M1 & M2 In contrast, M3 exhibits increased ISM and CSM in gouge, and increased Illite/Smectite in gouge relative to greater smectite in adjacent hanging and footwall rocks. In addition ISM & CSM change from high to low over a horizontal distance of 70 m across the M3 site. These suggest a horizontal dimension of less than a few hundred meters for hotter upflow that yielded more mature sheet silicates and chloritic gouges along the detachment.