Evaluation of the potential for surface faulting at TA-63. Final report

Structure of the Tshirege Member of the Bandelier Tuff at Mesita del Buey, Technical Area 54, Los Alamos National Laboratory

“…No data are available on the age of the faults-at Mesita del Buey, other than that the faults formed since emplacement of the Tshirege Member of the Bandelier Tuff at about 1.22 million years ago. Other small magnitude faults with similar amounts of offset examined at Pajarito Mesa had ago (Kolbe et al, 1994;Reneau et al, 1995), suggesting similar long periods of inactivity at TA-54, farther away from the main Pajarito fault zone.…”

Geology of the Western Part of Los Alamos National Laboratory (TA-3 to TA-16), Rio Grande Rift, New Mexico

“…At a few localities, fractures show a strong preferred orientation with little evidence for a conjugate set (Kolbe et al 1994Vaniman and Chipera 1995). The north-northwest preferred orientation for fractures at MDA P (mean for all data: N15W ± 26°) is atypical compared to average strikes at other localities on the Pajarito Plateau but not necessarily distinct within uncertainties (e.g., Kolbe et al 1994Kolbe et al , 1995 3T, 4 *Kolbe et al (1994 measured "through-going fractures," or fractures that were logged on both walls of trenches, which biases against fractures strongly oblique to trench walls.…”

“…The basic soil-stratigraphic framework used in the surficial mapping is partially derived from previous investigations that included soil pits and trenches on the western Pajarito Plateau, including sites both inside and outside the map area (e.g., Kolbe et al 1994Kolbe et al , 1995Reneau et al 1995Reneau et al , 1996aReneau et al , 1996bReneau et al , 2002Wong et al 1995;Longmire et al 1996;McDonald et al 1996;Reneau and McDonald 1996;McCalpin 1998McCalpin , 1999Phillips et al 1998;Gardner et al 2001). Large parts of the map area have been disturbed by human activities, partially obscuring original geologic relations.…”

“…With the fracture data, we performed a statistical analysis of preferred directions, fracture density, and apertures of fractures. The results can be compared to fracture analyses done in other parts of the Laboratory (e.g., Kolbe et al 1994Kolbe et al , 1995Reneau et al 1995;Wohletz 1995;Vaniman and Chipera 1995).…”

“…Fractures at Pajarito Mesa in Unit 3 are commonly filled with clay, minor gypsum and halite, and tuff breccia (Vaniman and Chipera 1995). Kolbe et al (1995) recognized slickensides in a few fractures (generally horizontal) in Unit 3 at TA-63 but concluded that they may be due to gravitational slip toward drainages that bound the site. In the TA-67 trenches, Kolbe et al (1994) found no evidence of gouge, shear fabric, slickensides, or other evidence of faulting along fracture walls in Units 3 and 4.…”