Geology Reference
In-Depth Information
(1) The fracturing of the granules consists largely of diametral splitting in
response to compressive loading at contacts with granules of similar size and
so is strongly influenced by the size distribution of neighbouring granules.
(2) The size distribution that develops is self-similar with respect to change in
magnification, constituting a fractal distribution with a fractal dimension D of
about 2.6. This observation suggests that the fragmentation process is scale
invariant (Turcotte
1986
).
(3) Assuming that each fracture event is accompanied by a local relative shearing
displacement b (b = actual displacement divided by granule diameter), and
using the averaging procedure of Molnar (
1983
) the mean macroscopic shear
strain in the gouge is deduced to be about 6b in the case of D = 2.6. Applying
this relationship gives quite large values of b, of the order of 10 or more,
indicating that the amount of sliding and/or rolling that accompanies each
fracture event must be quite large. Also the latter contribution possibly
increases at larger strains relative to that of the local displacements associated
with the fracturing itself (Biegel and Sammis
1988
). These considereations
give some idea of the local kinematics that may be involved in the mecha-
nisms of cataclastic granular flow.
The second aspect of such flow on which one might seek guidance from studies
on gouge is the dynamics. The dynamics of cataclastic granular flow can be
expected to be dominated by some combination of friction and fracture effects.
Since the friction itself will tend to reflect fracturing of asperities, the overall or
macroscopic flow behaviour can thus probably be expected to follow a Coulomb-
type law, analogous to that for macroscopic shear fracturing (Paterson and Wong
2005
,
Sect. 3.3
) but with perhaps somewhat lower values for the coefficient of
internal friction if rolling of granules is a significant factor. However, from the
existing experimental literature, it is difficult to draw general conclusions and to
develop a clear physically-based theory of cataclastic granular flow. The obser-
vations on gouge layers often refer to sliding at the gouge-slider interface (for
example, Shimamoto and Logan
1981b
). Even if the main body of the gouge is
involved, as it tends to be at very large relative displacements (Blanpied et al.
1988
), the deformation is often concentrated in Riedel shears (for example, Logan
et al.
1981
). Therefore, the stability of the deformation is likely to figure as a major
aspect in any comprehensive theory. Complementary experimental observations
relevant to cataclastic granular flow are also offered by studies on porous rocks,
such as sandstones, with porosities of the order of 0.1 or higher (Borg et al.
1960
;
Hadizadeh and Rutter
1982
,
1983
; Handin and Hager
1957
,
1958
; Handin et al.
1963
; Hirth and Tullis
1988
). In such experiments, there tends to be a preliminary
phase of deformation during which cataclasis associated with pore collapse con-
tributes a component to the deformation. As noted in
Sect. 7.1.2
, large strains are
needed for complete neighbour exchanges in granular flow and therefore steady-
state conditions can generally only be expected at very large strains; transient
behaviour, involving either strain hardening or strain softening, tends to be
characteristic for the normal range of strains explored in axisymmetric tests. Such
