Mylonite is a fine-grained, foliated rock that forms in fault zones due to intense shearing and deformation. The study of mylonites is important in understanding the mechanics of faulting, the conditions of deformation in the Earth’s crust, and the evolution of tectonic boundaries.
Source Passchier, C. W., & Trouw, R. A. J. (2005). “Microtectonics.” Springer.