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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possible, <strong>and</strong> use these to better constrain the azimuth <strong>and</strong> plunge <strong>of</strong> the tilting axis, as<br />
well as the amount <strong>of</strong> tilting. It would also be reasonable to avoid the southeastern limb<br />
<strong>of</strong> the Overton syncline as adding an unnecessary complication <strong>and</strong> source <strong>of</strong> potential<br />
error.<br />
These refinements, however, are unlikely to alter substantially the existing paleostress<br />
assessment. Varying the tilting axis azimuth <strong>and</strong> plunge by ± 15° around the 315°/0°<br />
orientation used produces a mild range <strong>of</strong> variation, while the amount <strong>of</strong> the tilt (~25°) is<br />
reasonably well constrained by field measurements. In any event, no geologically<br />
reasonable restoration can bring both sets <strong>of</strong> CBs to paleo vertical, as this would also<br />
result in near vertical Cretaceous strata during their deposition.<br />
Could the underlying interpretation <strong>of</strong> CBs as anticracks be flawed? The specific<br />
<strong>mechanics</strong> <strong>of</strong> CB <strong>propagation</strong> remains an area <strong>of</strong> active research—observationally,<br />
theoretically <strong>and</strong> experimentally—<strong>and</strong> it is possible that the b<strong>and</strong>s will turn out to<br />
propagate obliquely to the principal stresses. This could be the case so long as anti-mode<br />
I deformation dominates such that shear displacements are on the order <strong>of</strong> the mean grain<br />
size (0.25 mm) <strong>and</strong> thus nearly impossible to distinguish. Such a result would, <strong>of</strong> course,<br />
alter our principal paleostress interpretation based on the CB orientations. In this regard,<br />
it is interesting to note that, in individual outcrops containing both CB sets, the mean<br />
dihedral angle is ~80°, <strong>and</strong> can be less (Figure 1.8). This departure from the more nearly<br />
orthogonal result when looking at the mean b<strong>and</strong> orientations could be a clue that some<br />
degree <strong>of</strong> resolved shear is involved in <strong>propagation</strong> at the micromechanical scale. On the<br />
other h<strong>and</strong>, 80° is closer to 90° than to 60° (the typical dihedral angle between conjugate<br />
shear planes) <strong>and</strong> the preponderance <strong>of</strong> available data points strongly to the dominance <strong>of</strong><br />
uniaxial compaction (Sternl<strong>of</strong> et al., 2005; Chapter 4, this thesis). We suggest therefore<br />
that a significantly different interpretation <strong>of</strong> paleostress direction relative to CB<br />
orientation—such as σ1 bisecting the dihedral angle between the primary <strong>and</strong> secondary<br />
b<strong>and</strong> sets, <strong>and</strong> σ2 corresponding to the axis <strong>of</strong> intersection between them—is highly<br />
unlikely. In any case, no plausible reinterpretation would yield principal paleostress<br />
directions in agreement with Andersonian expectations.<br />
This leaves the third possible interpretation—that the current analysis is substantially<br />
correct <strong>and</strong> points to an as yet unrecognized tectonic explanation. Any such explanation<br />
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