Pressure shadow
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.
Pressure shadows regions (indicated by dashed lines) around porphyroclasts in a deformed granodiorite.
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.[2]
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References
- Passchier and Trouw (2005). Microtectonics (2nd ed.). Springer. ISBN 9783540293590.
- Blenkinsop, T.G. (2007). Deformation Microstructures and Mechanisms in Minerals and Rocks. Kluwer Academic Publishers. p. 150. ISBN 030647543X.
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