structural geology, propagation mechanics and - Stanford School of ...

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