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

almost certainly involves movement along regional bounding faults, such that tractions
applied to a tectonic block containing the Aztec sandstone resulted in the orientation of
paleostress inferred from the CBs. Such a configuration is difficult to visualize in 3-D.
A highly schematized 2-D treatment that ignores the secondary CB set, however, serves
to illustrate the essential concept (Figure 1.11b).
For this simple model, assuming homogeneous, isotropic, linear elasticity and using
the finite element method software Abaqus®, we consider a vertical, east-west cross
section through the Mesozoic units of the study area as a block sliced orthogonally to the
mean orientation of the dominant CB set and sitting atop a horizontal detachment at the
top of the Hermit shale. If this 3.6-km-thick block is moving stably westward along the
detachment (i.e. being pushed quasi-statically), the trajectories of σ1 induced a few km in
from its eastern edge are as shown by the tick marks in Figure 1.11b. Within the zone
indicated, 200 to 800 m deep and 5 to 9 km from the eastern edge of the block, which
coincides with a lateral section of the CB-rich upper Aztec, these trajectories roughly
coincide with our inferred orientation of σ1 that produced east dipping CBs. This
rudimentary treatment is not well constrained, but it does demonstrate that the inferred
orientation of σ1 can be produced over a broad, km-scale region in a geologically realistic
way. In fact, any variety of geologically reasonable configurations of mechanical
stratigraphy, and detachment and lateral bounding faults can be expected to produce the
inferred paleostress orientations. Nonetheless, even the simple 2-D model presented here
provides two relatively robust predictions testable by observation. Does the dip of the
dominant band set systematically decrease with increasing depth and increase from west
to east? And is there any independent evidence for westward motion of the study area
along a relatively shallow detachment?
On this last point, we note the interpretation of Carpenter and Carpenter (1994) that
west-vergent reverse faulting along what they called the North Buffington back-thrust
system comprised the earliest expression of Sevier tectonism in southeastern Nevada,
predating the formation of the dominant east-vergent thrusts (Muddy Mountain, Willow
Tank, etc.) now exposed in the study area.
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