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

concentration of quartz plasticity within the bands and the spatial uniformity of the
plastic strain thereby accommodated; and the apparent distribution of perturbed
compressive stress that suggests grain damage could only occur within a few cm of the
tip.
Ultimately, we anticipate that more refined microscopic examinations will reveal the
presence of a near tip CB process zone defined by quartz grain damage and incipient
plasticity, and that the diameter of this softened damage zone may correlate to a
maximum attainable band thickness independent of trace length. Certainly, most of the
uniform inelastic compaction accommodated within a CB occurs well behind the tip-line
in an environment of reduced compressive stress that would seem to preclude thickening
of the band by lateral propagation into undamaged sandstone. We suggest that the
characteristically elliptical profile of an isolated band could be due primarily to relatively
rapid mechanical propagation of the tip-line, compared to relatively slow processes of
plastic relaxation and collapse that lead to progressive thickening within the rind of
damaged grains left in its wake.
In terms of the bulk material and stress-strain conditions conducive to compaction
localization in sandstone, as well as the mechanical characteristics of the phenomenon,
this analysis of natural CBs in the Aztec sandstone presents a scenario seemingly at odds
with results and interpretations reported in the recent experimental and theoretical rock
mechanics literature. Firstly, regarding material and load conditions, we understand the
phenomenon to have occurred in well consolidated, saturated, but essentially uncemented
sandstone at moderate mean compressive stresses consistent with burial of less than 2.5
km in a thrust-faulting tectonic regime. Laboratory efforts to induce compaction
localization in triaxial experiments have tended to focus on moderately to well-cemented
sandstones (e.g. Berea, Bentheim and Castlegate) generally subjected to much greater
confining pressures reaching as high as 300 MPa (e.g. Wong et al., 2001), which is
equivalent to more than 12 km of overburden and enough to induce brittle-ductile
transition behavior. The grain-scale textures of compaction resulting from such
experiments also tend to involve intense grain crushing and comminution far in excess of
that observed in natural CBs.
72