Mylonite

This process, sometimes referred to as subgrain rotation recrystallization,[3] acts to reduce the mean grain size.

Mylonites generally develop in ductile shear zones where high rates of strain are focused.

The finite strain axes are initially parallel to the incremental axis, but rotate away during progressive deformation.

Because of the constraints imposed by simple shear, displacement is assumed to occur in the foliation plane in a direction parallel to the mineral stretching lineation.

Therefore, a plane parallel to the lineation and perpendicular to the foliation is viewed to determine the shear sense.

All of these indicators have a monoclinic symmetry which is directly related to the orientations of the finite strain axes.

An amphibolitic mylonite showing a number of (rotated) porphyroclasts : a clear red garnet left in the picture while smaller white feldspar porphyroclasts can be found all over. Location: the tectonic contact between the ( autochthonous ) Western Gneiss Region and rocks of the ( allochthonous ) Blåhø nappe on Otrøy , Caledonides , Central Norway .
A mylonite (through a petrographic microscope ) showing rotated so-called δ-clasts. The clasts show that the shear was dextral in this particular cut. Strona -Cenery zone, Southern Alps , Italy .
Mylonite, Owl Mountains , Poland
Peridotitic mylonite in a petrographic microscope