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Brian Evans
Abstract: Constraints from the Laboratory
Generalized
models of the rheology of the Earth's lithosphere often rely on simplifying
assumptions of petrology, deformation geometry, and thermodynamic conditions,
assuming for example, that the Earth is a layered half-space, deforming with
uniform strain rate throughout. Not surprisingly, when the actual geology or
tectonic loading is more complicated, such general schemes fail to describe
the deformation accurately. How well do we know the strength of the Earth's
crust and mantle, and what factors are most important in determining it? Among
the confounding factors to be prescribed are the following: the mechanical and
chemical influence of water and its variation with depth; the exact evolution
of the structure and properties of the rock mass with time or strain, and
precise formulations of the constitutive equations for the mechanisms of
deformation. Comparisons of the strength differences caused by variations in
water fugacity and melting provide an interesting example of the complex
behavior. The strength of water-saturated peridotite is predicted to be 2
orders of magnitude less than that of dry peridotite at the same temperature
and pressure. A similar decrease in strength of peridotite occurs only if
greater than 5-10% partial melting occurs. Trace amounts of water have an even
larger effect on the strength of anorthite aggregates, causing a decrease in
strength by 3-4 orders of magnitude. Introducing a silica-rich melt at the
grain boundaries in plagioclase rocks caused a decrease of 0.5-1 order of
magnitude. Thus, large differences in strength could result if water fugacity
is incorrectly predicted.
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