Plagioclase twins mechanically on the albite law. Twinning is most likely to occur when the resolved shear stress on the twin plane, (010), and in the glide direction, perpendicular to [100] in (010), is at a maximum. Only those crystals oriented in the stress field to approximately meet this condition will twin. Thus, in a rock containing mechanically twinned grains, the principal stress directions affecting the rock may be approximated by measuring the twin elements of these grains. The plagioclase fabric must be taken into account if the results are to be significant. Study of samples of anatectic granodiorite of the Chewack River Gneiss in the northeastern Cascade Range, Washington, where the unit was deformed adjacent to the Chewack-Paysaten fault, demonstrate the use of the method.

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