On the Internal Geometry of Mylonite Zones

Abstract

Mylonite zones are generally characterized by abrupt and very large strain transitions, which commonly result in excessively anastomosing schistosities on a wide range of scales when compared with non-mylonitic foliations. This geometry is very susceptible to remodification during progressive mylonitization, resulting in unusual and complex fold, lineation, and foliation geometries and interrelationships. Open folds of the mylonitic foliation with axes parallel to the stretching lineation in the surrounding mylonite cannot have formed by the rotation of fold axes through a large angle within their axial planes, as has been usually proposed for isoclinal and sheath folds in mylonitic zones. Open folds initiate with axes parallel or close to the stretching lineation due to the geometric effects of folding a mylonitic foliation, which anastomoses around an ellipsoidal pod of less deformed material. This initial geometry also allows the generation of fold axes curved within their axial plane through 180° about the stretching lineation at the time of nucleation. Successive mylonitic foliations develop during this folding and refolding process with boundaries that truncate and isolate earlier fold hinges and portions of fold limbs. As a consequence, stretching and intersection lineations can vary from plane to plane through the mylonite zone, although careful examination often reveals a weak overprinting stretching lineation parallel to the bulk movement direction for the whole zone. Fold asymmetry in mylonite zones is a potential indicator of shear sense across a zone, if the fold axes lie at an angle to the bulk stretching lineation direction. In such circumstances, however, a single asymmetry projected onto a plane perpendicular to the mylonitic foliation and containing the bulk stretching lineation can indicate either sense-of-shear depending on a variety of factors. These include whether the foliation folded is primary or mylonitic, and in the latter case whether the mylonite zone formed with a steep-dip and horizontal stretching lineation or in some other orientation. The most satisfactory sense-of-shear indicator is the asymmetry of S and C planes.