Boudinage

{{Short description|Structures in rock caused by extension}}

File:Boudinage.jpg

Image:Samos boudins.JPG vein within blueschist, Samos, Greece.]]

File:Boudinage near Kangerlussuaq.JPG near Kangerlussuaq]]

image:boudin vein.jpg in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia.]]

Image:Skagit-gneiss-Cascades.jpg gneiss with dike of granite orthogneiss; competent banded gneiss is boudinaged by ductile shear.]]

File:Uto Boudinage.jpg, Sweden]]

File:ChocolateTabletBoudinage.jpg

Boudinage is a geological term for structures formed by extension, where a rigid tabular body such as hornfels, is stretched and deformed amidst less competent (resistant) surroundings.{{cite book|last=Fossen|first=H.|title=Structural Geology|url=https://books.google.com/books?id=01PI5jhjiiQC&pg=PA282|year=2010|publisher=Cambridge University Press|isbn=978-0-521-51664-8|page=272}} The competent bed begins to break up, forming sausage-shaped boudins. Boudinage is common and can occur at any scale, from microscopic to lithospheric, and can be found in all terranes.Marques, Fernando O., Pedro D. Fonseca, Sarah Lechmann, Jean-Pierre Burg, Ana S. Marques, Alexandre J.M. Andrade, and Carlos Alves (2012). [http://www.sciencedirect.com/science/article/pii/S0040195111003350 "Boudinage in Nature and Experiment."] Tectonophysics 526-529, 88-96. In lithospheric-scale tectonics, boudinage of strong layers can signify large-scale creep transfer of rock matter. The study of boudinage can also help provide insight into the forces involved in tectonic deformation of rocks and their strength.

Boudinage can develop in two ways: planar fracturing into rectangular fragments or by necking or tapering into elongate depressions and swells.Arslan, Arzu, Cees W. Passchier, and Daniel Koehn (2008). [https://www.researchgate.net/publication/233794378_Arslan_etal_2008_JSG/file/79e4150b8e9698fbbb.pdf "Foliation Boudinage."] Journal of Structural Geology, 30 (3), 291-309. Boudins are typical features of sheared veins and shear zones where, due to stretching along the shear foliation and shortening perpendicular to this, rigid bodies break up. This causes the resulting boudin to take a characteristic sausage or barrel shape. They can also form rectangular structures. Ductile deformation conditions also encourage boudinage rather than imbricate fracturing. Boudins can become separated by fractures or vein material; such zones of separation are known as boudin necks.

In three dimensions, the boudinage may take the form of ribbon-like boudins or chocolate-tablet boudins, depending on the axis and isotropy of extension. They range in size from about 20 m thick to about 1 cm."Boudinage." Encyclopædia Britannica (2010). Encyclopædia Britannica Online. 06 Oct. 2010 .