A fine-grained, typically banded rock produced
by the grain size reduction and recrystallization produced
during ductile deformation of other rocks by shearing and
flattening. Mylonites usually form in shear zones that may be
a few millimeters to many kilometers wide and may be linked
to other structures that accommodate regional tectonic deformations.
They form at depths that are at or below the brittle
ductile transition at 10–15 kilometers, at medium to high
metamorphic grades.
Protomylonites represent the early stages of formation of
mylonitic rocks and are characterized by flattened and stretched
mineral grains, showing signs of internal strain such as undulose
extinction, deformation twins, and kink bands. Some
types of minerals may be more deformed than other types,
depending on their relative strength and the temperature and
pressure conditions of deformation. As deformation becomes
more intense, the grain size is reduced and a strongly foliated
matrix develops, with some isolated larger grains known as
porphyroclasts. The matrix foliation may flow around these
porphyroclasts and become more closely spaced on sides of the
grains that face the principal flattening direction, with pressure
shadow zones developed on the opposing two sides. This
asymmetric structure is one of the ways to determine the kinematics,
or sense of shear within the mylonite zone.
With continued deformation, the mylonite may become
extremely fine-grained and very strongly foliated, forming an
ultramylonite. If the temperature and pressure are high
enough, new metamorphic grains may form, known as porphyroblasts,
and these may grow over and rotate with the
mylonite fabric forming distinctive kinematic indicators useful
for determining the sense of shear of the mylonite zone.
See also DEFORMATION OF ROCKS; STRUCTURAL GEOLOGY.
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