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Strain and Strain Analysis |
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Nature does only occasionally provide us with real ellipsoids to characterise strain so structural geologists have invented ways to visualize ellipsoids where there is none. What follows describe a number of techniques structural geologists use to characterize strain.
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| The Fry Method |
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This is one of the most elegant and easier method to determine strain in rocks made of particles initialy randomly distributed in a matrix. The particle can be anything: quartz grains and quartz aggregates in a rhyolites, pre-strain metamorphic minerals such as garnet in metamorphic rocks, pebbles in matrix supported pebbly sandstones, etc. The main assumption is that before deformation the spacing of these particles should be statistically isotropic i.e. the distance between two particles does not depend on the direction in which the distance is measured. One way to demonstrate this is to plot on a tracing paper the position of particles (in red on the sketch below on the left) neighbouring a choosen particle (crossed white spot) and to repeat this operation for as many particles as possible. The result is shown on the sketch below on the right. This sketch shows that particles are disbributed around a circle of radius the average distance between particles. There is no strain. |
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