Abstract
The study of controls, processes, and deformation
behaviors of large fault zones, especially along active
plate margins, has been the focus of much recent lab, field,
and theoretical work. These shear zones are often
characterized by broad zones of deformation, anastamosing
fault strands, damage zones, and thick accummulations of
gouge. My recent research interests are on the gouge, which
separates the fault blocks and softens the fault zones.
Gouge derives from the disaggregation and alteration of wall
rocks, and appears to evolve through time. Distinctive
textures and fabrics. such as grain size and particle size
distribution (PSD, which defines the relative abundance of
large and small particles) in the gouge preserve a record of
this evolution, and probably have strong influence on the
strength of fault zone, the deformation behavior, and the
seismogenic potential. Despite numberous lab and field
studies focusing on correlating gouge properties and
structure with strength and behavior, we still have very
little understanding of the controls on fault behavior.
Numerical simulations provide us the opportunity to look
inside the deforming system. Due to the discrete nature of
fault gouge, fault zone properties tend to be very
heterogeneous, and marked by distinct discontinuities, e.g.,
zones of localized slip. Such complexity is difficult to
capture using typical continuum theories or numerical
techniques. The distinct element method (DEM) offers a
unique numerical technique to study granular shear zones,
preserving the discrete character of the material and
enabling direct correlations among granular micromechanics
and shear zone behavior. Over the last few years, our group
has carried out DEM simulations of granular shear zones,
exploring controls on deformational fabrics, shear zone
strength, and frictional stability.
We have conducted two-dimensional (2D) simulations of
shear on compact granular assemblages to explore
correlations among strength, dilation, and deformational
response. Second order effects introduced by variations in
particle size and PSD were examined as well, in order to
explore the evolution of cataclastically deforming gouge.
Scaled shear zones about 1 cm thick were filled with
power-law distributed particles (radii of 500, 250, 125, and
62.5 µm), and sheared to 200% strain to reach residual
strength conditions. The 2D power law exponent, D,
controlling PSD was varied from 0.81 to 2.60; normal
stresses ranged from 40 to 140 MPa. The numerical
experiments reveal a direct correspondence between shear
strength and dilation rate (i.e., changes in volume with
strain) which correlates well with deformational behavior.
Episodes of peak strength correlate to high rates of
dilation, and are accompanied by interlocking of the shear
zone and distributed shear. Abrupt drops in strength
correspond to decreases in dilation rate followed by
eventual contraction, as localized zones of slip form and
propagate through the shear zone. These observations are
consistent with predictions of others (e.g., Marone et al.,
1990, J. Geophys. Res., 95, 7007; Beeler et al., 1996, J.
Geophys. Res., 101, 8697), based on energy considerations.
Variations in numerically determined shear strength also
suggest second order dependencies on net dilation and PSD.
The former results from changes in porosity, interparticle
contact abundance, and magnitude and orientation of contact
forces, all of which influence particle mobility. The PSD
effects appear to arise from the partitioning of deformation
between interparticle sliding and rolling. Enhanced
interparticle rolling occurs in high D assemblages, leading
to the self-organization of rolling particles into localized
slip bands and a reduction in shear zone strength.
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