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

when the ambient stress state is nearly isotropic and/or bands are spaced less than a meter
apart.
One other factor mentioned in the section on mechanical theory as a potential
influence on the propagation paths of interacting CBs is the effective drag or “inertia”
imposed by their pre-existing length. As a test of the general validity of this notion, we
undertook a series of three simulations identical in every respect except for the initial
lengths of the model bands: two 25-m bands (as above and in Figure 4.21), one 25 m
band and one 5 m band, and two 5 m bands. In order to ensure adequate space and
propagation reactivity to render variations in path distinguishable on a single plot, a
spacing of 2 m and an isotropic remote stress state were chosen (Figure 4.22). When both
bands are 25 m long, their mutual influence is substantially similar to that when spacing
is 2.5 m. When one band is 5 m long, its impact on the continued propagation of the
longer band is perceptibly diminished, both in terms of the initial repulsion and the
subsequent attraction. The effect of the longer band on the shorter one, however, is even
more noticeably enhanced. When both bands are 5 m long, their responses again mirror
each other, but are completely different from either of the first two cases. In particular,
they initially repel and attract each other somewhat less robustly than the pair of 25-m-
long bands, but, at the point when parallel propagation resumes in both earlier examples,
attraction between the shorter bands re-intensifies and they propagate toward intersection
at a relatively high angle of approach (> 30°). The final path has a slight hitch in it that
produces a mild, s-shaped curvature. This simple example demonstrates both that band
length can influence the strength of tip interactions, and that the most general case
involving different length bands interacting is likely to produce noticeably asymmetric
patterns of hooking.
Finally, we note that, although the simulations presented in Figures 4.21 and 4.22
generally predict the resumption of parallel propagation paths following initial overlap,
the normalized magnitude of σθθ max rapidly drops toward one. This suggests that, for any
reasonable critical threshold value, propagation would cease. Outcrop observations,
however, demonstrate that long, sub-parallel overlaps between bands spaced even
millimeters apart are common.
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