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

ands are also fairly common. Generally up to hundreds of meters in lateral extent,
although some may be larger, we speculate that these holes in an otherwise pervasive CB
fabric represent combinations of loading and mechanical properties not conducive to
band formation. Whether they are due primarily to small differences in porosity,
cementation and cohesion, disadvantageous bedding orientations, pore-pressure
variations or location within larger thrust structures remains to be deciphered. However,
abundant evidence that sedimentary architecture can strongly influence CB propagation
(Hill, 1989; Sternlof et al., 2004; Flodin and Aydin, 2004) suggests that mechanical
anisotropy related to depositional bedding, and perhaps also relative movement between
adjacent cross-bed packages that alters the local stress state, might play decisive roles.
(a)
(b)
σ 3
σ 1
x 3
x 1
x 1
Figure 1.5. Schematic of anticrack-inclusion conceptual model. (a) Axisymmetric
geometry of eccentric ellipsoidal band aligned with the principal remote stresses (σ1 > σ2
> σ3, compression positive). (b) Cross-sectional area of the band (solid ellipse) relative to
the pre-compacted area originally occupied by the same detrital grains (dashed ellipse).
Net inward movement of the boundary (u1) during uniaxial compaction corresponds to a
closing-mode (anti-mode I) sense of displacement. Their extreme eccentricity (~ 1)
allows compaction bands to be modeled as anticracks (adapted from Sternlof et al., 2005).
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u 1
x 2
x 2
σ 2