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10
SUPPLEMENTARY NOTE ON ROCK MECHANI CS
PROBLEMS AT ACTIVE GINS
Important deformation mechanisms include fracture,
frictional sliding, cataclastic flow, pressure solution,
diffusion creep, and dislocation creep. For any given rock type,
one deformation process will be dominant under a particular set
of conditions. As either the conditions, the rock type, or the
grain size change, the operative process may change with an
accompanying change in the Theological behavior. Several
examples are instructive for understanding the range of
Reformational behavior that might be expected in continental
margins.
It is commonly believed that the transition from the upper
to the lower crust is characterized by an abrupt increase in
strength because of a change in rock type from a more silicic to
more mafic composition. This occurs because at the temperature
and pressure of the transition, the upper crustal rocks -can flow
by dislocation creep at moderate deviatoric stress, whereas the
stress required for flow to operate in the lower crustal rocks is
much higher. Thus, the lower crustal rocks may deform by brittle
processes of fracture and frictional sliding because the stress
required for this process-is lower than that for dislocation ~
creep e Qualitatively, this idea is very useful, but we lack
sufficient knowledge of the constitutive parameters for likely
rock types to permit useful quantitative calculations. For
example, the constitutive relationship for wet quartzite is often
used to represent the upper crust even though it is unlikely-that
upper crustal rocks are actually wet quartzites. If they were
granitic, for example, the expected stresses would be different
because the rheology of granite under these conditions is not the
same as that of quartzite. -
Local fracture and pervasive cataclastic flow result
reduction in grain size. This lower grain size may allow
processes, such as pressure solution, diffusion creep, or
dislocation creep, to become the dominant Reformational
mechanism. In this way, the relationship between stress and
strain rate change with time from mean stress-dependent
plasticity to linear or nonlinear viscosity.
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Sliding on faults may occur either by earthquakes or by
stable sliding, depending on the constitutive behavior of the
rocks in the fault zone. If the constitutive behavior is such
that an increase in salting velocity results in a reduction in
shear resistance (velocity weakening), then unstable sliding is
possible; if it results in an increase in resistance (velocity
strengthening), sliding should be stable. This appears to
explain the transition from earthquakes at shallow depths on
strike-slip faults to stable sliding at greater depths, since an
increase in temperature can cause a transition from velocity
weakening to velocity strengthening. It might also help to
explain the fact that large subduction earthquakes occur at some
convergent margins and not at others, because there could be
systematic differences in the rock types from one boundary to
another e One possibility is that: the constitutive behavior for
subducted sediments might be very dif ferent than for serpentinite
or for mafic rocks.
To know the rheology of various parts of continental
margins, it is necessary to have two different data sets. One
includes the rheological behavior of important rock types under a
wide range of conditions, and the other includes knowledge of the
rock types and ambient conditions that exist in the continental
margins. The first of these comes largely from laboratory
experiments, and the second, from a variety of sources.
Laboratory Experiments
Experiments must~be done under a wide range of physical
conditions, from the low temperatures and pressures found in
accreti-onary wedges, to the high temperatures and pressures
associated with partial melting deep in subduction zones. In
addition, it is important that experiments be done with
controlled fluid pressures, because fluids man affect the
rheology, both by the purely mechanical fluid pressure effects
embodied in the effective pressure concept and through chemical
effects. It is also important that experimental-data be obtained
at large strains because the strains are often large in
continental margin deformation, and it is important that the
experimental results can be shown to apply to this situation.
The wide ranges in-experimental requirements present
challenges to experimental design. A single design of an
experimental deformation apparatus cannot be used over the entire
range of conditions mentioned above. For any deformation
apparatus, digital data collection and computer control offer
significant advantages and should be included in the design.
Because the study of low permeability clay-rich sediments is
important, such apparatus must be designed for conducting
experiments that; may last weeks or months. It could be useful to
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employ rotary shear so that deformation in shear zones can be
studied to high strain.
Defo Cation of consolidated rocks requires apparatus capable
of attaining higher pressures and temperatures. This means that
for a given sample size the entire apparatus must be larger than
for experiments at lower temperatures and pressures. However, it
is not easy to design an apparatus that can cover the entire
range of interesting conditions. A number of apparatus exist
that have some of the desired capabilities, but design and
construction of new, more sophisticated equipment is necessary.
Only a small number of laboratories exist that are sufficiently
well equipped to conduct the necessary research.
Conditions in Continental Margins
To apply laboratory experimental results to continental
margins, it is necessary to know both the rock types and the
physical conditions found in this setting. Relevant physical .
conditions include temperature, mean stress, deviatoric stress,
and fluid pressure. Deformation processes are highly sensitive
to these factors; so unless the operative process is known, it is
not possible to predict the appropriate constitutive behavior.
Paradox of Stress Magnitudes on Faults
In divergent, convergent, and strike-slip continental
margins, faults commonly appear to slip at shear stress
magnitudes that are lower than one would expect to be possible.
Our expectations are based on the simple concept that frictional
sliding can only occur if the shear stress is equal to the normal
stress times a coefficient of friction. If the normal stress and
the coefficient of friction are known, the shear stress can be
calculated. The existence of low angle normal faults in
extensional margins, the apparently low shear stress on the-San
Andreas fault, and the existence of nearly horizontal detachment
faults in a number of places, such as the Los Angeles basin, are
all phenomena that do not fit this simple mechanical view. What
is wrong?
It might be tempting to say that, for some reason,
laboratory experimental friction coefficents do not apply to
natural faults. However, this is not a satisfactory explanation,
because there are situations in which natural faults behave
exactly as expected based on laboratory measurements. One good
example is the field experiment done at Rangely, Colorado, in
which fault slip was repeatedly started and stopped by changing
the pore fluid pressure through pumping water into and out of
boreholes drilled for Chat purpose. The example of Rangley is
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particularly valuable because it is a location in which the
stresses were known by hydraulic fracturing measurements, the
pore pressures were directly measured, and laboratory friction
measurements were made on rock samples that were known to exist
where the faulting was occurring. In this well-characterized
case, the frictional behavior was exactly what was expected from
Mohr-Coulomb failure analysis using the effective pressure
concept.
Thus, the paradox that some faults seem to move with low
ratios of shear stress to normal stress is not solved by
dismissing the results of laboratory experiments and known
mechanical relations e There may be some reason that these
concepts are not applicable to low stress faults. For instance,
some other deformation mechanism might operate instead of
frictional sliding If so, we need to discover what it is. In
some cases, the rock types might be unusual, with low
coefficients of friction, and if so we need to determine what
these rock types are and conduct appropriate laboratory
experiments on them. The magnitudes and distributions of fluid
pressure may be very different from what we now believe is
possible. The stress magnitudes may be higher than they appear
to be.
If we know the operative processes, the mechanical
properties of the relevant rocks associated with these processes,
and the actual conditions existing on the faults, we should be
able to solve the paradox. Our current knowledge of one or more
of these factors is inadequate, and we do not even know now in
which area the inadequacy lies. Whatever the explanation, we
have the opportunity to resolve the paradox. In so doing, we
will undoubtedly discover that our current view of the mechanics
of these faults is somehow incorrect. Discovering the correct
explanation will be a fundamental advance. The implications of
this new understanding could be far reaching in many areas of
earth science. To solve this paradox, we must combine inputs
from many different geological approaches, all focused on the
fundamental problem of deformation at continental margins.
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Representative terms from entire chapter:
continental margins
