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

the hooking patterns commonly observed in outcrop. We conclude that the anticrack
model provides an adequate approximation to the outcrop-scale mechanics of compaction
bands and suggest specifically that:
1. CBs propagate as a consequence of self-generated compressive stress
concentrations along paths that attempt to maintain incremental symmetry
with (orthogonality to) the maximum circumferential compressive stress
immediately ahead of their tips;
2. In the absence of external influences, CBs form symmetric with (orthogonal
to) the maximum remote principal stress and will continue to propagate in
plane;
3. Small-scale directional perturbations of the tip of a well-developed band
caused by material heterogeneities are self-correcting, regardless of the
magnitude of the remote differential stress;
4. Large-scale heterogeneity and anisotropy in the surrounding material aside,
the primary cause of curving (out-of-plane) CB propagation is mechanical
interaction between adjacent bands, particularly between tips;
5. The degree of these interactions is acutely sensitive to the magnitude of the
remote differential stress—nearly isotropic stress conditions allow significant
interaction, while realistically high values of differential stress virtually
eliminate interaction;
6. The degree of interaction is also sensitive to the material properties inside the
propagating CBs, as well as their length (absolute in-plane dimensions)—
lower internal resistance to shear enhances interaction as does shorter band
lengths.
Despite the general success of the current anticrack numerical model in producing
realistic propagation behavior, considerable room for improvement exists. Among the
physical phenomena that cannot as yet adequately be addressed are long overlaps
between bands spaced sometimes mere millimeters apart, the checkerboard patterns
comprised of two distinct CB sets crossing each other at high angle, and the regular zig-
zag behavior exhibited by both individual bands and clusters of bands. As the scope of
observation shrinks to focus on these issues at the grain scale, thereby challenging the
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