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Pressure shadow

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Pressure shadow
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A pressure shadow (also called strain shadow) is a term used in metamorphic geology to describe a microstructure in deformed rocks that occurs adjacent to a relatively large, undeformed particle, such as a porphyroclast. Pressure shadows often appear in thin sections as pairs of roughly triangular regions that are elongated parallel to the foliation around a clast of a different mineral.[1] Pressure shadows that contain fibrous mineral textures are also termed pressure fringes or strain fringes.[2]

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Pressure shadows regions (indicated by dashed lines) around porphyroclasts in a deformed granodiorite.
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Formation

During deformation, minerals can migrate by plastic flow or may grow by diffusive mass transport into the lower-stress regions created by a rigid porphyroclast or porphyroblast.[3]

References

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