Definitions and images to illustrate geological terms, links to images and website articles


typical example of shear boudinage in deformed jasperoid in sheared basalt, Fortnum Gold Mine, Australia. Courtesy of Roland GotthardBoudinage refers to structures deformed by extension in ductile shear zones. Boudinage structures contain a rigid tabular body that has been stretched and deformed where embedded within more deformable (less competent) rocks.

Banded Skagit gneiss with dike of granite orthogneiss; competent banded gneiss is boudinaged by ductile shearCompetent tabular bodies that are susceptible to boudinage include veins and strata such as sandstones. Where conditions favor brittle fracture rather than ductile deformation, imbricate (overlapping) fracturing occurs.

In boudinage, the competent bed break ups into sausage-shaped boudins – forming structures such as ribbon-like boudins or chocolate-tablet boudins (depending upon the axis and isotropy of extension).

boudinaged quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, Western Australia. Courtesy of Roland Gotthard.
[links: images: Amphibolite boudins in gneisses; formations: Zoroaster Veining, boudins composed of quartz and plagioclase, boudin of metagabbro (HP mafic granulite) in tonalitic gneiss]

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