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_Journal of Structural Geology 32 (2010) 766e780_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Choices controlling fault damage zone width, structure, and symmetry in the Bandelier Tuff, New Mexico_ _Paul R. Riley a,*, Laurel B. Goodwin a, Claudia J. Lewis b_ _a University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, USA_ _b Los Alamos National Laboratory, EES-13, MS D452 Los Alamos, NM 875445, USA_ _article info_ _Article history: Received 10 March 2009; Received in revised form 1 May 2010; Accepted 12 May 2010; Available online 20 May 2010_ _Keywords: Fault zone Rio Grande rift Damage zone Fracture density Bandelier Tuff_ _abstract_ _We studied welded and glassy nonwelded ignimbrites of the Bandelier Tuff cut by the Pajarito fault system to examine the influence of primary lithology and structure on fault damage-zone characteristics. Our work supports previous studies that indicate welding and resulting rock strength are first-order controls on the type of fault-zone structure that forms in high porosity ignimbrites. However, inherited mechanical anisotropy is the most significant control on spatial variations in fault-zone width and orientation of structures for a given throw. Cooling joints in welded ignimbrite localize strain, producing a narrower damage zone than that in glassy nonwelded ignimbrite. The joints also control the orientations of discrete fractures formed during faulting, so fractures show the same patterns inside and outside damage zones, which we attribute to local reorientation of stresses adjacent to joints. In contrast, deformation bands formed in relatively isotropic, glassy nonwelded ignimbrite exhibit conjugate sets oblique to the Pajarito fault, consistent with left-lateral extension across a pre-existing structure. Where footwall and hanging wall damage-zone widths can be compared in welded ignimbrite, they reflect greater hanging wall deformation, consistent with near-surface faulting. These observations collectively record 3D strain of a physically heterogeneous system._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Upper crustal faults are most accurately described as 3-D volumes rather than 2-D surfaces. Within these volumes, the majority of slip accumulates within a fault core, which is surrounded by a less deformed damage zone (e.g., Chester et al., 1993; Caine et al., 1996; Shipton et al., 2006). The implications of this architecture are diverse. For example, fault-related deformation can change rock mechanical properties (Faulkner et al., 2008), causing stress orientations to rotate in fault damage zones, and affecting seismicity over time (Faulkner et al., 2006). The types, densities, and orientations of structures in fault zones also exert a first-order control on fault-zone permeability structure, permeability anisotropy, and flow pathways (e.g., Knipe, 1993; Haneberg, 1995; Caine et al., 1996; Zhang and Sanderson, 1995; Rawling et al., 2001)._ _Rawling et al. (2001) suggested that understanding petrophysical controls on fault-zone deformation processes could facilitate prediction of fault-zone structure from knowledge of faulted materials; i.e., faults in granite develop different structures than those in poorly lithified sediments. Likewise, the addition of clay to sandstone results in narrower deformation band fault zones (Antonellini and Aydin, 1995), and large-displacement fault zones in poorly lithified clastic sediments are narrowest where numerous juxtaposed clay layers are present, and are widest where juxtaposed beds are sand and gravel (Heynekamp et al., 1999; Minor and Hudson, 2007). Similarly, Shipton et al. (2006) proposed that lithologic controls may affect the degree to which damage zone width increases with displacement._ _To further explore the effects of lithology on damage zone evolution in normal faults, we have documented variations in damage zone structure and width of the Pajarito fault system in the 1.6e1.2 Ma Bandelier Tuff of New Mexico (Smith and Bailey, 1966; Izett and Obradovich, 1994; Fig. 1). Earlier work identified a mechanical stratigraphy within the ignimbrites that compose the Bandelier Tuff. Glassy nonwelded ignimbrites in which volcanic clasts are not fused together form deformation bands, whereas ignimbrites in which clasts are welded exhibit extensive damage-zone fractures (Wilson et al., 2003) (Fig. 2)._ _The Pajarito fault system is seismically active, with numerous fault scarps across which throw has been well documented (Lewis et al., 2009). In addition, the exposures we studied have been above the water table since deposition, such that variations in pore fluid pressure cannot have played a role in deformation. Thus, we have examined the effect of the same deformation, at the same conditions of pressure, temperature, and pore fluid pressure, on mechanically distinct units, so the structures should reflect the influence of mechanical variation alone. We use these data to evaluate:_ _the north (e.g., Rendija Canyon and Guaje Mt. faults; Lewis et al., 2009); and in the south, it tips out along one main segment. Associated with the major faults that constitute the fault system are numerous small-scale faults and fractures that have been documented within the Bandelier Tuff (Carter and Winter, 1995; Gardner et al., 1999; Wilson et al., 2003; Wilson, 2004)._ _The Bandelier Tuff comprises a series of ignimbrite deposits erupted from the Toledo and Valles calderas, and is the primary stratigraphic unit that forms the Pajarito Plateau (Smith and Bailey, 1966; Gardner and Goff, 1984). It includes two members, the 1.6 Ma Otowi Member (Qbo) and the 1.2 Ma Tshirege Member (Qbt) (Fig. 3; Izett and Obradovich, 1994). They are primarily composed of silicic ash with varying amounts of pumice, lithic fragments, and phenocrysts (Broxton et al., 1995; Gardner et al., 1999; Wilson et al., 2003). Separating the Tshirege Member from the Otowi Member is the 1.6e1.3 Ma Cerro Toledo interval (Qct), a reworked rhyolitic volcaniclastic unit erupted from domes within the Toledo Caldera (Fig. 3) (Smith et al., 1970; Spell et al., 1996)._ _The Bandelier Tuff includes both nonwelded and welded ignimbrite (Fig. 3). Welding refers to the processes of compaction and fusion of fragments that takes place at the time of deposition in sufficiently hot deposits (Broxton et al., 1995; Broxton and Reneau, 1995). High porosity portions of cooling units are typically nonwelded and are variably affected by vapor-phase crystallization. Nonwelded cooling units may be glassy or crystallized, depending on the degree of post-depositional devitrification of glass and/or vapor-phase crystallization in pores (Ross and Smith, 1961; Stimac et al., 1996). Glassy units typically have >60 wt. % glass, and crystallized units have <10 wt. % glass (Wilson, 2004)._ _The Otowi Member is a single cooling unit with a glassy nonwelded ignimbrite (Broxton et al., 1995; Gardner et al., 1999). The Tshirege Member mostly contains cooling units that are either welded ignimbrite or crystallized nonwelded ignimbrite (Broxton and Reneau, 1995; Wilson, 2004). Portions of Qbt1 and Qbt4, the lowermost and uppermost cooling units of the Tshirege Member, are glassy nonwelded ignimbrite, closely resembling the Otowi Member._ _Welding zonation gives rise to predictable outcrop characteristics. Glassy, nonwelded ignimbrite weathers easily, producing broad debris aprons at the angle of repose and limiting outcrop. Welded ignimbrite forms cliffs. The welded ignimbrite outcrops are characterized by columnar cooling joints, which typically form pentagonal or hexagonal patterns on rare pavement surfaces (Fig. 4)._ _2.1 Mechanical stratigraphy of the Bandelier Tuff_ _(1) the nature of lithologic controls on damage zone structure, (2) potential lithologic and primary structural controls on damage zone width, (3) lithologic controls on spacing of damage zone structures, and (4) symmetry of fault-zone architecture._ _2. Geologic setting_ _The Pajarito fault zone is a tectonically active, northerly trending normal fault zone that bounds the western side of the Española Basin of the Rio Grande rift (Fig. 1) (Kelley, 1979; Baldridge et al., 1995; Wolff and Gardner, 1995; Gardner et al., 1999, 2001). The Pajarito fault, a steeply east-dipping normal fault, dominates the fault zone, with maximum surface displacement of over 120 m on the central portion of the fault (Fig. 2) (e.g., Lewis et al., 2009). The fault zone consists of a series of mechanically linked segments in_ _For clarity and consistency with a basis in previous local work (Wilson et al., 2003, 2006), we apply two explicit definitions. The term ‘fracture’ in this study refers to discrete surfaces across which the rock has broken, lost cohesion, and has a measurable aperture. In contrast, deformation bands in the Bandelier Tuff are zones where reductions in clast size, pore size, and porosity accommodated cm-scale displacements. They are interpreted to be compactive shear deformation bands sensu Aydin et al. (2006), Fossen et al. (2007), and Schultz and Fossen (2008), but are hereafter referred to as ‘deformation bands’ for simplicity. Glassy nonwelded units contain deformation bands (Fig. 5a). Crystallized nonwelded units and an iron-oxidized portion of a glassy nonwelded unit have both deformation bands and_ Ключевые слова: nonwelded ignimbrite, previous study, grande rift, bandelier tuff, brittle faulting, map, evans, broxton, faulkner, pajarito fault, implication, geological, engineering, faults, band, reneau, welded ignimbrite, shallow crust, uncorrected data, mechanical stratigraphy, welded ignimbrites, factor, journal, kelley, frijoles canyon, stress, sanderson, plot, riley, ignimbrite, otowi member, member, journal geophysical, weighting factor, bandelier, strength, welded, study, welding, sawyer, sandstone journal, e-striking joint, plumose structure, rio grande rift, fossen, laboratory, transects, bland canyon, protolith, wall, mexico, spatial variation, greater, scale, journal structural, hanging wall, geological society, pavement, eds basins, damage, frijoles canyons, geology, alamos, frijoles, map-scale, documented, los alamos, orientation distribution, formation, strike, crystallized, internal structure, cooling joint, schultz, qbo, pole, trace length, mexico implication, elsevier, table, pattern, fault-zone, structural, data collected, high angle, fracture, slip sense, wilson, location, zone width, fault core, fault zone, joint, vaniman, asymmetry, orientation, bailey, shipton, rio, main fault, rio grande, nonwelded, keller, slip surface, local trace, eds faults, rock strength, journal structural geology, footwall, trace, variation, monograph, damage zone, structural geology, fault-zone structure, fracture density, distance, wohletz, terzaghi, normal fault, rift, slip, ignimbrites, fault, transect data, map-scale fault, porosity, rock, major fault, glassy nonwelded, glassy, alamos national, pajarito plateau, sawyer bland, goodwin, smaller, bulletin, segall, throw, uid, wider range, parallel, tuff, cather, cooling unit, pollard, deformation band, lithologic control, hanging, fracture sub-parallel, post-depositional crystallization, ha, transect, oblique, geophysical, deformation, ?ow, structure, unit, water, national laboratory, correction, olson, density, normal, data, national, m-scale weakening, lewis, inversely proportional, exhibit, applied, increase, cooling, control, zone, canyon, los, observation, surface, gardner, sawyer canyon, fracture parallel, pajarito, welded unit, smith, tshirege member, riley journal, angle, olsen, society, sandstone, displacement, grande, increasing distance, bland, correction factor, rst-order control, width, mechanical, aydin