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Geologists
examine an outcrop of fault gouge along the Badwater
detachment fault, Death Valley California. Deformation is
distributed asymmetrically across the fault, grading upward
from fractured footwall, to coarse breccia, and finally into
fine-grained fault gouge with a mesoscopically ductile flow
fabric. The fault zone is bounded above by a nearly planar
slip surface thought to have accommodated the greatest
displacements.
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Abstract
The objective of this study is to understand the controls
on the kinematics of fault gouges and breccias: what kinds
of strains and deformation mechanisms are responsible for
the structures and fabrics preserved in outcrop exposures.
We have focused our study in the Death Valley area, where
low-angle detachment faults reveal extensive exposures of
fault rocks. The rocks range from coarse crush breccias
with very angular rock fragments to gouges with
well-rounded, dispersed clasts floating in a fine-grained
granular or clay-rich matrix. The latter rock types display
extraordinary flow structures and color banding, and lack
discrete throughgoing shears; this suggests that these
gouges have experienced extreme shear strains largely
through distributed granular flow. The group's work has
involved detailed mapping of outcrops in the field and from
photos, optical and microprobe examination, grain size and
shape determination, measurement of phyllosilicate preferred
orientation, and compositional analysis.
Particle size distributions (PSD - relative abundance of
particle sizes) in these fault rocks are of particular
interest, as PSD appears to evolve with strain of the rock,
and influence mechanical strength as it does. Early studies
of cataclastically deformed fault rocks yielded fractal PSDs
with characteristic 3-D fractal dimensions (D) of about 2.6;
this result is attributed to the kinematics of grain
fragmentation during shear. We measured PSDs in 2D and in
3D, and correlated the results with mesoscopic fabric.
Values of D range from 2.74 to 3.31. All estimates for D
are significantly higher than 2.6 predicted from earlier
studies. In part, this results from our revised methodology
which eliminates biases due to scaling and binning of
particle abundances. Values of D also appear to vary with
independent characteristics of the fault rocks, including
grain size, matrix fraction, clay abundance, and particle
rounding; we suspect that particle comminution leading to
fine grain size may cause particles to accumulate at the
"grinding limit" of the mineral, causing an increase in D;
the increase in fines may also reduce interactions among
large particles, causing deformation to be accommodated by
interparticle sliding and abrasion, rather than by
fragmentation; in addition, the decrease in grain size may
enhance low-temperature alteration of minerals to clays,
weakening the matrix and easing interparticle sliding.
Mesoscale fabrics within the fault zones suggest that the
highest shear strains occurred within fine grained rocks
with high D values; their deformation may have been
accommodated by distributed creeping flow, rather than by
stick-slip failure along discrete surfaces suggested for
less mature, cataclastic gouges.
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