Fragmentation and recrystallization of rocks in narrow zones such as faults or thrusts where strong deformation has occurred. Rocks are ground to a fine powder in the zone of deformation and, because of their fine grain size, recrystallize efficiently under the extreme directional stress and release of frictional heat during deformation. The fine-ground powder recrystallizes to a flinty rock which often surrounds fragments of uncrushed country rock in the deformation zone to form a mylonite. During extreme deformation all the fragments in the deformation zone are ground down to powder and recrystallize to form a fine-grained, banded rock known as an ‘ultramylonite’, and rocks can melt to form pseudotachylyte.
Subjects: Earth Sciences and Geography.