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

possible, and use these to better constrain the azimuth and plunge of the tilting axis, as
well as the amount of tilting. It would also be reasonable to avoid the southeastern limb
of the Overton syncline as adding an unnecessary complication and source of potential
error.
These refinements, however, are unlikely to alter substantially the existing paleostress
assessment. Varying the tilting axis azimuth and plunge by ± 15° around the 315°/0°
orientation used produces a mild range of variation, while the amount of the tilt (~25°) is
reasonably well constrained by field measurements. In any event, no geologically
reasonable restoration can bring both sets of CBs to paleo vertical, as this would also
result in near vertical Cretaceous strata during their deposition.
Could the underlying interpretation of CBs as anticracks be flawed? The specific
mechanics of CB propagation remains an area of active research—observationally,
theoretically and experimentally—and it is possible that the bands will turn out to
propagate obliquely to the principal stresses. This could be the case so long as anti-mode
I deformation dominates such that shear displacements are on the order of the mean grain
size (0.25 mm) and thus nearly impossible to distinguish. Such a result would, of course,
alter our principal paleostress interpretation based on the CB orientations. In this regard,
it is interesting to note that, in individual outcrops containing both CB sets, the mean
dihedral angle is ~80°, and can be less (Figure 1.8). This departure from the more nearly
orthogonal result when looking at the mean band orientations could be a clue that some
degree of resolved shear is involved in propagation at the micromechanical scale. On the
other hand, 80° is closer to 90° than to 60° (the typical dihedral angle between conjugate
shear planes) and the preponderance of available data points strongly to the dominance of
uniaxial compaction (Sternlof et al., 2005; Chapter 4, this thesis). We suggest therefore
that a significantly different interpretation of paleostress direction relative to CB
orientation—such as σ1 bisecting the dihedral angle between the primary and secondary
band sets, and σ2 corresponding to the axis of intersection between them—is highly
unlikely. In any case, no plausible reinterpretation would yield principal paleostress
directions in agreement with Andersonian expectations.
This leaves the third possible interpretation—that the current analysis is substantially
correct and points to an as yet unrecognized tectonic explanation. Any such explanation
34