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]

Pressure shadows regions (indicated by dashed lines) around porphyroclasts in a deformed granodiorite. Aar02 pressureshadows.tif
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]

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<span class="mw-page-title-main">Porphyroclast</span>

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<span class="mw-page-title-main">Cleavage (geology)</span> Planar fabric in rock

Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the kind of cleavage feature that develops. Generally, these structures are formed in fine grained rocks composed of minerals affected by pressure solution.

<span class="mw-page-title-main">Pressure solution</span> Rock deformation mechanism involving minerals dissolution under mechanical stress

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In geology and materials science, a deformation mechanism is a process occurring at a microscopic scale that is responsible for deformation: changes in a material's internal structure, shape and volume. The process involves planar discontinuity and/or displacement of atoms from their original position within a crystal lattice structure. These small changes are preserved in various microstructures of materials such as rocks, metals and plastics, and can be studied in depth using optical or digital microscopy.

<span class="mw-page-title-main">Fabric (geology)</span> Spatial and geometric configuration of all the elements that make up a rock

In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks, the fabric developed depends on the depositional environment and can provide information on current directions at the time of deposition. In structural geology, fabrics may provide information on both the orientation and magnitude of the strains that have affected a particular piece of deformed rock.

The Thiviers-Payzac Unit is a metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the southern Limousin in France. The unit geologically forms part of the Variscan basement of the northwestern Massif Central.

In geology oblique foliation, steady state foliation or oblique fabric is a special type of a tectonically produced foliation or fabric, most commonly in quartz-rich layers. The microtectonic structure can be used to determine the shear sense in shear zones and their associated rocks, usually mylonites.

<span class="mw-page-title-main">Dynamic quartz recrystallization</span> Process of crystal regrowth under conditions of stress and high temperature

Quartz is the most abundant single mineral in the Earth's crust, and as such is present in a very large proportion of rocks both as primary crystals and as detrital grains in sedimentary and metamorphic rocks. Dynamic recrystallization is a process of crystal regrowth under conditions of stress and elevated temperature, commonly applied in the fields of metallurgy and materials science. Dynamic quartz recrystallization happens in a relatively predictable way with relation to temperature, and given its abundance quartz recrystallization can be used to easily determine relative temperature profiles, for example in orogenic belts or near intrusions.

In structural geology, strain partitioning is the distribution of the total strain experienced on a rock, area, or region, in terms of different strain intensity and strain type. This process is observed on a range of scales spanning from the grain – crystal scale to the plate – lithospheric scale, and occurs in both the brittle and plastic deformation regimes. The manner and intensity by which strain is distributed are controlled by a number of factors listed below.

References

  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. Bibcode:2000Sci...288.2195M. doi: 10.1126/science.288.5474.2195 . PMID   10864865.
  3. Blenkinsop, T.G. (2007). Deformation Microstructures and Mechanisms in Minerals and Rocks. Kluwer Academic Publishers. p. 150. ISBN   978-0-306-47543-6.