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

5. Compaction band orientations
Orientations were collected for 484 CBs intersected along 28 orthogonal scan lines
totaling 359 m in length at 23 locations around the Valley of Fire. One additional set of
data (33 orientations) was collected as a control from outcrops of Aztec sandstone located
~15 km southwest of the Valley of Fire in Buffington Pockets, a window through
Paleozoics of the overriding Muddy Mountain thrust plate (Figure 1.3). All orientation
data were collected as dip azimuth/dip angle, and only deformation bands identifiable as
CBs were measured (Figure 1.6). A band was judged to be a CB if it was relatively thick
(> 5 mm toward the middle), planar and continuous, and did not show discernable shear
offset. These simple criteria were applied to avoid counting low-angle shear bands, as
well as subsidiary bands (e.g. linking structures) judged to be expressions of more
localized stress regimes.
As a point of interest, we note that bands clearly belonging to the dominant CB set do
occasionally show right-lateral offsets on the order of centimeters. These bands are
generally encountered in close proximity to much younger, but similarly north-northwest
trending dextral-slip faults (Myers and Aydin, 2004; Flodin and Aydin, 2004) and display
an unusually high degree of induration. We suggest that these bands represent Cretaceous
CBs reactivated in shear during Miocene-Pliocene faulting, possibly due both to their
favorable orientation and their potential for low shear strength as a result of the grain
damage, comminution and preferential clay accumulation accommodated. Alternatively,
one could argue that decreased porosity within these bands resulted in increased shear
modulus, causing them to concentrate subsequent fault-related stresses and, ultimately, to
fail in synthetic shear themselves. In either case, the greater induration of these bands
today is likely due to frictional heating and healing of the shattered quartz grains,
although this has yet to be confirmed with photomicroscopy. During our collection of
orientation data, these bands were included as CBs.
Figure 1.7 displays all the CB orientation data as planar poles in equal area, lower
hemispheric projection stereograms for each location. These data clearly demonstrate the
pervasive dominance of the steeply east-dipping CB set, although two other distinct
orientation groups are apparent—steeply northwest dipping and very steeply southwest
dipping. Figure 1.8 shows the stereograms for the five locations where multiple CB sets
24