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.[2]

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.[3]

References

Passchier and Trouw (2005). Microtectonics (2nd ed.). Springer. ISBN 978-3-540-29359-0.
Müller, W.; Aerden, D.; Halliday, A. N. (2000). "Isotopic dating of strain fringe increments: duration and rates of deformation in shear zones". Science. 288 (5474): 2195–2198. doi:10.1126/science.288.5474.2195.
Blenkinsop, T.G. (2007). Deformation Microstructures and Mechanisms in Minerals and Rocks. Kluwer Academic Publishers. p. 150. ISBN 0-306-47543-X.

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[1] Passchier and Trouw (2005). Microtectonics (2nd ed.). Springer. ISBN 978-3-540-29359-0.

[2] Müller, W.; Aerden, D.; Halliday, A. N. (2000). "Isotopic dating of strain fringe increments: duration and rates of deformation in shear zones". Science. 288 (5474): 2195–2198. doi:10.1126/science.288.5474.2195.

[3] Blenkinsop, T.G. (2007). Deformation Microstructures and Mechanisms in Minerals and Rocks. Kluwer Academic Publishers. p. 150. ISBN 0-306-47543-X.