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

interact is inversely proportional to the magnitude of the remote differential stress acting
upon them. This model observation suggests that the tendency toward anastomosis along
the trend of a subparallel CB array—and thus the extent to which fluid flow in that
direction may be impeded—can be predicted from knowledge of the remote stress state in
which it formed. Conversely, the orientations and relative magnitudes of all three
principal paleo stresses can be estimated from directional variations in the degree of
anastomosis revealed by a well-exposed (or imaged) CB array.
3. Field evidence and interpretation
Extensive arrays of generally sub-parallel, NNW-trending, steeply E-dipping CBs
pervade the upper 600 meters of the 1,400-m-thick Aztec sandstone as exposed in and
around the Valley of Fire State Park of southeastern Nevada (Sternlof et al., 2005)
(Figures 4.1 and 4.2). The Aztec is an æolian, subarkosic, chronostratigraphic equivalent
of the early Jurassic Navajo and Nugget sandstones (Marzolf, 1983). It remains weakly
lithified and is typified by large-scale tabular and trough cross-bedding, average porosity
of 20-25%, and a mean grain diameter of 0.25 mm within a range of 0.1 mm to 0.5 mm
(Flodin et al., 2005). The CBs, which comprise the oldest structural fabric present, are
relatively thick at a centimeter or more, typically extend for tens of meters in outcrop
trace length, and represent highly localized tabular inclusions of compacted detrital grains
and secondary clay accumulation. With no obvious association to pre-existing structures,
the bands appear to represent a pervasive compressional tectonic fabric—in essence the
kinematic and structural opposite of a regional joint set (Sternlof et al., 2005). Noting
their relative age and dominant orientation, Hill (1989) first suggested that the CBs
formed as a result of and perpendicular (symmetric) to ENE-directed regional
compression associated with tectonic shortening during the Cretaceous Sevier orogeny.
The CB fabric was subsequently crosscut and offset by relatively low-angle shear bands
related to final, proximal emplacement of the overlying Sevier thrust sheets, while
overprinting by joints and joint-based strike-slip faults associated with Basin and Range
extension followed in mid Tertiary time (Eichhubl et al., 2004; Flodin and Aydin, 2004;
Hill, 1989; Myers and Aydin, 2004; Sternlof et al., 2005; Taylor et al., 1999).
Sternlof et al. (2005) present a coherent conceptual and mechanical model for CBs as
anticrack inclusions that is grounded in detailed outcrop and petrographic observations
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