2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 9
Presentation Time: 8:00 AM-12:00 PM


MCGINNIS Jr, Ronald N., FERRILL, D.A, MORRIS, A.P. and DINWIDDIE, C.L., Department of Earth, Material, and Planetary Sciences, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238-5166, ronald.mcginnis@swri.org

Small-scale brittle deformation was studied in a cut bank exposure of poorly consolidated tuffaceous sedimentary rock located at the southern erosional boundary of the Volcanic Tableland in Owens Valley, California. This study addresses site-specific variation of fault deformation characteristics where faults cut bedded units with different textures and grain sizes. This analog study provides the structural context for permeability data that were collected at this exposure, and it was performed to evaluate potential length scales for lateral flow in tephra-fall interbeds at Yucca Mountain, Nevada. A 79-m-wide horst developed in these rocks is bounded by two faults that account for 75% of the total extension apparent in the 110-m-long exposure. Small-displacement (< 20 cm) faults and non-vertical fractures are strongly clustered and positively correlated with larger faults (displacements > 20 cm), whereas vertical fractures are present throughout the exposure. Fracture density (total fracture tracelength per unit area) is generally highest in the vicinity of larger faults. The strike trend of all faults and fractures in the exposure is consistent with an average strike azimuth of 355 degrees. Fault zones are characterized by grain size reduction and discrete slip surfaces, the number of which increases with increasing displacement. A stress analysis of this exposure, which was based on tectonic setting and reconstruction of overburden thicknesses, yields a simple history of burial and exhumation under continuous tectonic extension, comparable with that of tuffaceous rocks at Yucca Mountain. We interpret the fault and fracture development of the study area to include shear (faults), hybrid (faults, nonvertical fractures), and tensile (vertical fractures) failure modes under conditions of low overburden pressure (< 2.5 MPa). Deformation characteristics such as grain comminution, cementation, and fracture dilation have important effects on groundwater movement that depend strongly on saturation conditions. The intersecting network of faults and fractures in conjunction with stratigraphic layering generates an anisotropic permeability with maximum permeability parallel to fault and fracture strike.