DIAGENESIS OF DEFORMATION-BAND FAULTS: THE RECORD AND MECHANICAL SIGNIFICANCE OF VADOSE-ZONE FLOW AND TRANSPORT IN THE BANDELIER TUFF, LOS ALAMOS, NM

Deformation-band faults in nonwelded ignimbrites of the Bandelier Tuff record unsaturated fluid flow and transport. More than two-thirds of the faults studied display preferential alteration and cementation compared to adjacent protolith. Stable isotope analyses indicate that smectite alteration and calcite cementation result from low-temperature meteoric fluid-fault interaction. Fault-zone microstructures, rare earth element signatures, and mineralogy indicate that smectite was introduced to deformation-band fault zones by a combination of translocation from the surface and in situ alteration of fault gouge. Rod-shaped calcite microcrystallites and the close association of calcite with plant roots suggest a combination of pedogenic, biologically mediated and physicochemical precipitation. Enrichment in smectite and calcite modified major oxide and trace element contents of deformation bands with respect to protolith. Collectively, these observations indicate that these faults have served as, and may still be, zones of preferential vadose-zone fluid flow and transport. These processes modify fault-rock permeability; affect the transport of solutes via processes such as dissolution, precipitation, and adsorption; and change the mechanical properties of the fault zone. Smectite and calcite result in localization of deformation, effectively halting the further development of deformation bands. Vadose-zone alteration therefore is one way to produce a clay-rich fault core at early stages in a fault's history, resulting in fault-zone weakening and localization of deformation. Progressive burial of growth faults can subsequently bring fault cores into the seismogenic zone.