Microstructures

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Kink Bands in Biotite

Click image to enlarge

65. Kink-band formation in biotite - Biotite in very weakly deformed granodiorite from Borrego Springs, southern California, has deformed by brittle fracture, cleavage plane slip (analogous to bedding plane slip on a larger scale) and kink band formation. Deformation was at low to middle greenschist facies. Adjacent plagioclase grains contain a few fractures, but are otherwise undeformed. The biotite crystal does not bend smoothly around the kink bands, but shows patchy extinction, indicative of microfaulting.

FOV 3.2 mm, Nicols Crossed.

66. Kink-band formation in biotite - Biotite in protomylonitic granodiorite from Borrego Springs, has deformed predominantly by kink band formation. Deformation was at middle to upper greenschist facies. Kink bands are mostly rather narrow and lenticular (trending top left to lower right in image), but some are more open (lower right part of biotite grain). The titanium-rich biotite crystal has exsolved tiny ilmenite grains along the narrow kink band boundaries (see image #67), but not along the more open kink bands.

FOV 1.5 mm, Plane Light.

67. Kink-bands in biotite - A detail of a single, narrow kink band tip from the sample in image #66 shows that (001) cleavage planes have undergone rotation inside the kink band. Ilmenite (very fine, black grains) decorates the kink band boundaries.

FOV 0.8 mm, Plane Light.

Kink Bands in Biotite	Click image to enlarge
65. Kink-band formation in biotite - Biotite in very weakly deformed granodiorite from Borrego Springs, southern California, has deformed by brittle fracture, cleavage plane slip (analogous to bedding plane slip on a larger scale) and kink band formation. Deformation was at low to middle greenschist facies. Adjacent plagioclase grains contain a few fractures, but are otherwise undeformed. The biotite crystal does not bend smoothly around the kink bands, but shows patchy extinction, indicative of microfaulting. FOV 3.2 mm, Nicols Crossed.
66. Kink-band formation in biotite - Biotite in protomylonitic granodiorite from Borrego Springs, has deformed predominantly by kink band formation. Deformation was at middle to upper greenschist facies. Kink bands are mostly rather narrow and lenticular (trending top left to lower right in image), but some are more open (lower right part of biotite grain). The titanium-rich biotite crystal has exsolved tiny ilmenite grains along the narrow kink band boundaries (see image #67), but not along the more open kink bands. FOV 1.5 mm, Plane Light.
67. Kink-bands in biotite - A detail of a single, narrow kink band tip from the sample in image #66 shows that (001) cleavage planes have undergone rotation inside the kink band. Ilmenite (very fine, black grains) decorates the kink band boundaries. FOV 0.8 mm, Plane Light.