structural geology, propagation mechanics and - Stanford School of ...
structural geology, propagation mechanics and - Stanford School of ...
structural geology, propagation mechanics and - Stanford School of ...
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dihedral angle <strong>of</strong> 80° or more, <strong>and</strong> so appear to have formed contemporaneously (Sternl<strong>of</strong><br />
et al., 2004).<br />
4.2. Anticrack model<br />
The highly eccentric elliptical tip-to-tip pr<strong>of</strong>iles <strong>and</strong> uniform uniaxial internal<br />
compaction observed for individual CBs corresponds to a pure closing-mode sense <strong>of</strong><br />
relative boundary displacement distributed across a very thin b<strong>and</strong>. This geometry <strong>and</strong><br />
anti-mode I sense <strong>of</strong> displacement discontinuity defines CBs kinematically <strong>and</strong><br />
mechanically as anticracks (Mollema <strong>and</strong> Antonellini, 1996; Sternl<strong>of</strong> et al., 2005). As<br />
such, they would form symmetric with (orthogonal to) the direction <strong>of</strong> maximum<br />
compressive stress in an otherwise homogeneous, isotropic material (Figure 1.5), except<br />
when mechanically interacting with each other (Sternl<strong>of</strong> et al., 2005; Chapter 4 this<br />
thesis). The anticrack interpretation suggests that the normal to the dominant planar<br />
orientation <strong>of</strong> CBs in the Valley <strong>of</strong> Fire coincides with the direction <strong>of</strong> maximum<br />
compressive paleostress (σ1) acting regionally when they formed. In locations where a<br />
subsidiary set <strong>of</strong> cross-hatch CBs also formed, we suggest these reflect the orientation <strong>of</strong><br />
the intermediate principal paleostress (σ2), speculating that σ1 ≈ σ2 locally <strong>and</strong> that the<br />
preferred CB orientation flip-flopped between the two (see Chapter 4, this thesis).<br />
4.3. Outcrop to regional scale<br />
As described in the section on deposition, the Aztec s<strong>and</strong>stone is neither truly<br />
homogeneous nor isotropic. Rather, it is a granular material deposited in variable bedding<br />
orientations arranged as packages within a complex æolian sedimentary architecture. We<br />
suggest that this heterogeneous reality, coupled with apparently strong mechanical<br />
interactions between adjacent CBs, leads to the highly variable b<strong>and</strong> patterns commonly<br />
observed at outcrop scales. As the scale <strong>of</strong> observation increases, however, the influence<br />
<strong>of</strong> these local complications drops away. At a scale <strong>of</strong> hundreds <strong>of</strong> meters (Sternl<strong>of</strong> et al.,<br />
2006) to kilometers, the approximately planar fabric defined by north-northwest trending,<br />
steeply east-dipping b<strong>and</strong>s dominates, suggesting distributed formation in response to,<br />
<strong>and</strong> generally orthogonal with a paleo direction <strong>of</strong> regional compression.<br />
Finally, while CBs can be found in upper Aztec s<strong>and</strong>stone outcrops throughout<br />
southeastern Nevada, it is important to note that large exposures essentially devoid <strong>of</strong><br />
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