Faulting and Fracturing of Nonwelded Bishop Tuff, Eastern California: Deformation Mechanisms in Very Porous Materials in the Vadose Zone

摘要

Field and microstructural studies were conducted on the basal, nonwelded and partially welded portions of the rhyolite Bishop Tuff in eastern California to examine the nature and processes of brittle deformation in these units. The nonwelded tuff consists of a variable sequence of finely laminated to massive pumice-rich deposits, fine-grained ash, and pyroclastic glass erupted from the Long Valley Caldera. The deposits are experiencing east–west extension in the hanging wall of the White Mountain fault, and small-displacement faults and fractures cut the tuff. Deformation in the Bishop Tuff occurred by fracturing associated with faults, and by slip along narrow faults with smooth, often mineralized surfaces. Localization of fracturing appears to be a function of welding. Units with a greater degree of welding have a greater abundance of fractures associated with faults, whereas nonwelded portions typically have a narrow deformation band–type faults with little or no associated damage. Microstructural observations show that transgranular fractures lie along grain boundaries of pumice and feldspar phenocrysts, and these fractures are often filled with calcite. These deposits appear to have behaved as an open-cell foam with a low strength, but with a cohesion that allowed the support of a differential stress to failure that resulted in subvertical open fractures and faults. These results demonstrate how brittle deformation may be manifested in nonwelded deposits in the vadose zone, and impart an anisotropy in which flow would be enhanced vertically and impeded horizontally. The Bishop Tuff is analogous to other nonwelded tuffs in the western USA. Thus, these results have implications for understanding deformation and flow in a variety of arid regions.