boudinage

thumb|Small-scale boudinage thumb|Boudinaged quartz vein within [[blueschist, Samos, Greece.]] thumb|Boudinage in Greenland near [[Kangerlussuaq]] thumb|upright|Boudinaged Vein (geology)|quartz vein in shear foliation, Starlight Pit, Fortnum Gold Mine, [[Western Australia.]] thumb|upright|Foliation (geology)|Banded [[gneiss with dike of granite orthogneiss; competent banded gneiss is boudinaged by ductile shear.]] thumb|Boudin on the Island of Uto, Stockholm Archipelago, [[Sweden]] thumb|Chocolate-tablet boudinage structures in a low grade metasedimentary rock outcropping in Deception Pass, Washington 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. 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. 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. 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.