Reversible Deformation, Permanent Fabric Development
Boulder, Colo., USA: Earth is a stressed planet. As plates move, magma
rises, and glaciers melt—just to mention a few scenarios—rocks are subject
to varying pressure and compressional and extensional forces. The effect of
these stresses on rock mineralogy and texture is of great interest to the
tectono-metamorphic community. Yet the link between process and outcome
remains elusive.
There are two possible states of stress: either all principal stresses are
equal (lithostatic or hydrostatic pressure), or one prevails (differential
stress). Both scenarios are ubiquitous in nature.
The conventional knowledge is that lithostatic pressure alone (without
differential stress) is sufficient to affect thermodynamic equilibrium and
thus prompt metamorphic reactions on its own. The energy stored in the
system leads to a change in phase stability, and reactions occur to
accommodate that according to bulk composition.
Metamorphic fabrics, such as lineation and foliation, require differential
stress to develop instead. But how much differential stress is truly
needed? Beyond elastic loading, there is no question—we know that mineral
orientation and stresses are related. Whether reversible, elastic
deformation is sufficient to prompt fabric development remains to be
ascertained. This is a fundamental question because it is at the core of
the role of stress in metamorphism.
Dr. James Gilgannon, a structural geologist currently conducting research
as a postdoctoral scholar at the University of Edinburgh, set up a series
of experiments to assess any potential effect of elastic differential
stress on the development of mineral fabrics. His team’s work fits into a
larger picture emerging collectively from cutting-edge synchrotron
experiments on the role of differential stress and elasticity in
metamorphism and tectonics. They dehydrated gypsum samples under different
elastic stress states, and their experiments took place in a synchrotron
line, so that sample changes could be documented in 4D. “The advantage of
this technique is that you can see everything that is happening, whereas in
nature rocks have very complex, hidden histories,” explains Gilgannon.
What they saw is the early-onset development of a fabric orthogonal to the
largest principal stress in a sample experiencing an elastic differential
stress, long before accumulation of irreversible strain. And if the elastic
term, usually neglected, is producing a fabric, this will condition
whatever comes next. He states, “Elasticity could be considered boring, but
it is really exciting. It could be enough to produce an anisotropic
texture, which would then control fluid flow.”
There are still many questions to be answered. For one, it is still unclear
how to differentiate elastic and permanent stress fabrics in nature. “I
expect that many metamorphic rocks have patterns related to this small
elastic stress—not the big permanent one,” says Gilgannon. “But it might be
the case that because the elastic stress is always present, it will be hard
to deconvolve its contribution.” The next set of planned experiments will
be at an even higher spatial resolution and will focus on what happens to
sample fabric once the elastic stress is relieved.
FEATURED ARTICLE
Elastic stresses can form metamorphic fabrics
J. Gilgannon et al.
Contact: James Gilgannon, The University of Edinburgh,
jamesgilgannon@hotmail.com
https://doi.org/10.1130/G51612.1
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