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