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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Chapter 1<br />
Structural <strong>geology</strong> <strong>and</strong> tectonic interpretation <strong>of</strong> compaction b<strong>and</strong>s<br />
in the Aztec s<strong>and</strong>stone <strong>of</strong> southeastern Nevada<br />
1. Abstract<br />
The Aztec s<strong>and</strong>stone in the Valley <strong>of</strong> Fire <strong>of</strong> southeastern Nevada comprises an<br />
exhumed analog for active æolian aquifers <strong>and</strong> reservoirs subjected to tectonic<br />
compression <strong>and</strong> shortening. A planar fabric <strong>of</strong> anastomosing compaction b<strong>and</strong>s (CBs)<br />
pervades the upper 600 m <strong>of</strong> the 1,400-m-thick Aztec. Individual CBs are observed to be<br />
thin, tabular localizations <strong>of</strong> uniaxial porosity-loss compaction that can be considered<br />
kinematically <strong>and</strong> mechanically as anticracks. In outcrop, CBs form sometimes complex<br />
patterns that can, nonetheless, be generally understood as resulting from mechanical<br />
interactions between anticracks. Compaction b<strong>and</strong>s are the oldest structures in the Aztec,<br />
<strong>and</strong> abundant geological evidence indicates that they formed during the earliest period <strong>of</strong><br />
regional compression associated with Cretaceous Sevier tectonism in a weakly cemented<br />
material buried less than a few hundred meters.<br />
Viewed at larger scales (> 100 m), a dominant north-south trend <strong>and</strong> steep east dip are<br />
apparent, as well as localized pockets <strong>of</strong> a coexisting, coeval CB set trending generally<br />
northeast <strong>and</strong> dipping steeply northwest. The mean trend <strong>of</strong> the dominant CB set is<br />
generally orthogonal to the east-vergent direction <strong>of</strong> Sevier thrusting, while the secondary<br />
set is, on average, orthogonal to the dominant set. Based on this geometry, <strong>and</strong> our thin-<br />
section to outcrop-based underst<strong>and</strong>ing <strong>of</strong> CBs as anticracks, we interpret the dominant<br />
set as having formed orthogonal to the maximum compressive paleostress (σ1), with the<br />
secondary set orthogonal to σ2. This completely constrains the orientations <strong>and</strong> relative<br />
magnitudes <strong>of</strong> the principal paleostresses during CB formation—σ1 → 270°/38°, σ2 →<br />
138°/39°, σ3 → 24°/27°—which, however, depart radically from the classic view <strong>of</strong> σ3 as<br />
subvertical in thrust-fault environments. We suggest that a diffuse stress perturbation<br />
caused by as yet unrecognized regional structures—e.g. a detachment beneath the Valley<br />
<strong>of</strong> Fire along which it moved relatively westward—could explain the inferred state <strong>of</strong><br />
paleostress.<br />
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