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were encountered, giving the dihedral angles between the mean orientations for each
set—labeled P (primary), S (secondary) and T (tertiary), based on relative abundance. P
and S share a dihedral angle of about 80° at all five locations, as do S and T at the two
locations where they coexist. P and T, however share a variably low dihedral angle at
these same two locations, having similar trends, but opposite dip directions.
Figure 1.9 displays the combined data for all locations (n = 484) as poles, density
contours, and Rose diagrams of both strike and dip orientations. Table 1.1 provides a
brief statistical summary of the numbers, including mean and median dihedral angles, and
relative abundance. Two basic conclusions are immediately apparent. Firstly, wherever it
occurs, the secondary CB set forms essentially orthogonal to the primary set. Secondly,
the tertiary CB group is a steeply dipping subset of the primary orientation. Although the
data were not collected to provide a statistically valid measure of relative abundance
between the CB sets, they do also reasonably illustrate the dominance of the primary set
at ~83% of all bands.
6. Tectonic interpretation
Hill (1989) first suggested that high-angle deformation bands in the Aztec sandstone
that do not exhibit macroscopic shear (CBs in current usage) resulted from tectonic
compression related to the Sevier orogeny. His primary arguments were that the bands
trend generally parallel to the encroaching thrust front and orthogonal to the east-vergent
tectonic transport direction, and that they comprise the oldest structures present based on
cross-cutting relationships. Subsequent workers (Taylor et al., 1999; Taylor and Pollard,
2000; Myers and Aydin, 2004; Flodin and Aydin, 2004; Eichhubl et al., 2004; Sternlof et
al., 2004, 2005, 2006) have all come to the same conclusion and we concur, offering a
more in-depth examination of the evidence and implications below.
6.1. Timing, spatial and material constraints
The analysis of Taylor and Pollard (2000) demonstrates that CBs were already
present in the Aztec to influence the initial upward and eastward expulsion of reducing
basinal brines from beneath the advancing Sevier thrust front, which bleached the middle
and upper parts of the sandstone as observed today (Eichhubl et al., 2004) (Figure 1.1).
Thus, CBs formed in the sandstone while it was stained uniformly red with hematite
grain coatings, suggesting that this trace (~1% by volume) cement (Flodin et al., 2003;
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