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

Pollard, 2002). The particular utility of this hybrid interpretation lies in combining the
analytical insights of Eshelby inclusion theory (Eshelby, 1957; Eshelby, 1959) with the
power of boundary element method (BEM) simulations (Crouch and Starfield, 1983) to
understand the mechanics of CB evolution. Starting with targeted field and petrographic
observations of CBs from the Aztec sandstone in the Valley of Fire, we build a
mechanical analysis of these structures as Eshelby-type inclusions to establish the
validity and utility of a two-dimensional anticrack treatment of CBs using a displacement
discontinuity BEM.
The basic hypothesis underlying this research is that a fundamental mechanism of
incremental, quasi-static and (anti)crack-like propagation was at work throughout the
Aztec such that individual CBs tended to form orthogonal to the most compressive
remote stress. The diversity of outcrop patterns observed therefore reflects the
interactions of CBs with local material and stress heterogeneities through this
fundamental propagation mechanism. Beyond contributing to the understanding of an
interesting and generally overlooked mode of structural failure in porous, granular earth
materials, this research is motivated by the practical goal of using mechanical
understanding of the phenomenon to forecast its occurrence and associated fluid-flow
effects in active reservoirs and aquifers. Although the research presented here is built on
detailed CB data acquired from a specific outcrop of Aztec sandstone in the Valley of
Fire, we suggest that the intrinsic attributes identified are common to all CBs in the Aztec
and, by extension, to other porous sandstones with similar depositional and tectonic
histories.
3. Methods
We focused on a “type-locality” outcrop of CBs in Aztec sandstone near Silica Dome
in the Valley of Fire State Park (Figure 2.1, inset) where elementary patterns of
approximately parallel bands are well exposed. Using a steel tape and calipers, we
measured tip-to-tip thickness profiles of the two-dimensional band traces in outcrop
(Figure 2.2), focusing on one example as representing the prototypical compaction band.
Eleven cores were collected along the trace of one CB from tip to middle, as well as
from essentially undisturbed sandstone nearby. These were vacuum-impregnated with
optical-grade blue epoxy and used to make 37 standard and eight polished thin sections.
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