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

1. Abstract
Chapter 4
Propagation of compaction bands in sandstone as anticracks:
Field evidence, mechanical theory and numerical simulation
Outcrop and petrographic observations of compaction bands exposed in the Aztec
sandstone of southeastern Nevada suggest that the mechanics of their propagation and
interaction can be approximated using anticrack theory. Our numerical simulations of
anticrack band propagation using the boundary element method confirm this, while
suggesting refinements to better account for the compacted-inclusion character of the
bands. Using realistic, field-based ranges for all physical parameters, we find that, as with
opening-mode cracks, the degree to which adjacent anticrack bands interact is inversely
proportional to the magnitude of the remote differential stress acting upon them. This
model observation suggests that the tendency toward anastomosis along the trend of a
subparallel compaction band array—and thus the extent to which it would impede fluid
flow in that direction—can be predicted from knowledge of the remote stress state in
which it formed. Conversely, the orientations and relative magnitudes of all three
principal paleo stresses can be estimated from directional variations in the degree of
anastomosis revealed by a well-exposed (or imaged) compaction band array.
2. Introduction
Compaction bands (CBs) are a phenomenon of localized compressive failure
commonly observed in outcrops of porous sandstone. They represent one kinematic end-
member of a family of structures known collectively as deformation bands (DBs), which
also includes shear and dilation bands, as well as mixed-mode combinations (Antonellini
et al., 1994; Aydin, 1978; DuBernard et al., 2002; Mollema and Antonellini, 1996;
Rudnicki and Sternlof, 2005; Sternlof et al., 2005). As thin, tabular features of porosity-
loss compaction and order-of-magnitude permeability reduction that are millimeters to
centimeters thick and tens of meters or more in planar extent, compactive DBs act as
baffles to subsurface fluid flow under saturated conditions (Pittman, 1981; Freeman,
1990; Antonellini and Aydin, 1994; Crawford, 1998; Gibson, 1998; Taylor and Pollard,
2000; Sigda and Wilson, 2003; Sternlof et al., 2004). Where present as pervasive arrays,
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