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

1973; Mardon, 1988; Peck et al., 1985). Given that our observations come from the tips
of CBs that did naturally cease to propagate, however, raises the possibility that damage
halos were originally present, but were consumed by subsequent compaction. On the
other hand, small pods of grains involved in what appears to be incipient collapse can be
distinguished outside of established CBs and interpreted as Griffith-type flaws or proto-
bands (Sternlof et al., 2005).
3.3 Anticrack-inclusion interpretation
The elliptical tip-to-tip profiles and uniform uniaxial internal compaction observed
for straight (planar) CBs corresponds to a pure closing-mode sense of relative boundary
displacement, albeit one distributed across a thin, but finite band thickness rather than the
knife-edge surface of displacement discontinuity that defines solution surfaces (Fletcher
and Pollard, 1981; Mardon, 1988). This elliptical distribution of closing-mode (anti-mode
I) displacement (Figure 4.9) defines the bands kinematically and mechanically as
anticracks (Mollema and Antonellini, 1996; Sternlof et al., 2005). In fact, Sternlof et al.
(2005) show that the local state of stress induced in a linear elastic material by the plastic
compaction of a perfectly planar CB oriented symmetric to the maximum remote
compressive stress is, for all intensive purposes, independent of the evolving material
properties inside the band.
Corroborating the anticrack interpretation is the fact that many of the characteristic
CB patterns observed in Aztec outcrop—particularly the hook and eye configurations—
are distinctly reminiscent of those described for opening-mode cracks (joints), many of
which can be explained as resulting from mechanical interactions between propagating
tips within the framework of linear elastic fracture mechanics (e.g. Cotterell and Rice,
1980; Cruikshank et al., 1991; Fleck, 1991; Olson and Pollard, 1989; Pollard and Aydin,
1988; Segall and Pollard, 1983; Sempere and Macdonald, 1986; Sumi et al., 1985; Swain
and Hagan, 1978; Thomas and Pollard, 1993). This suggests by analogy that CB patterns
may also largely express the mechanical interactions of brittle cracks or, in this case,
anticracks.
Complicating this virtual anticrack approximation of CBs, however, is their physical
reality as highly eccentric, oblate ellipsoidal inclusions of compacted grains, whose
internal material properties must exert nonzero tractions along their boundaries with the
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