Oblique foliation

In mylonites, grains can be strained into very long ribbons parallel to the main foliation plane. Elongated, recrystallized grains within the ribbons may develop a grain-shape alignment oblique to the main foliation in the rock. This oblique, grain shape foliation indicates deformation faster than recrystallization, which tends to revert grain shapes to equidimensional. The obliquity is consistent with the bulk sense of shear.

Grain shape preferred orientation (GSPO) in a mylonite. From Jean-Pierre Burg.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Oblique foliation in quartz in a mylonite. XPL image, 2x (Field of view = 7mm)	Oblique foliation in quartz in a mylonite. XPL image, 2x (Field of view = 7mm)	Oblique foliation in quartz in a mylonite. XPL image, 2x (Field of view = 7mm)