MYLONITES, S C
MYLONITES, SIZE, STRAIN
Mylonite zones range in scale from the size of thin sections up to zones that are hundreds of kilometers long and perhaps 30 km or so in width. The classical mylonite zone is associated with the outcrop of the Moine Thrust in the northwest highlands of Scotland. This zone is gently dipping and approximately 200 km in length. It ranges in thickness from 10 to 100 meters [Lapworth (1885); McIntyre (1954); Christie (1963); Johnson (1967); McLeish (1971)].
The strain associated with mylonite zones has been a matter of some discussion. A widespread interpretation is that such zones are the product of large shear strains. Where the displacement of the rocks either side of the zone can be establisbed, this is commonly the case [see Ramsay and Graham (1970); Vernon and Ransom (1971); and Hara et al. (1973)]. However, in most very large mylonite zones such displacements cannot be demonstrated and then interpretation is not so clear. Johnson (1967) has proposed that in many mylonite zones and, particularly, the Moine Thrust, the strain is essentially a flattening normal to the foliation and the mylonite layering has an origin much the same as slaty cleavage. Johnson bases this interpretation on the symmetrical patterns of preferred orientation of quartz that are developed in the Moine mylonites [ Christie (1963)]. This may, in fact, be the case for the Moine mylonites but for many other zones sucb patterns of
preferred orientation are strongly asymmetric [see, for instance, Hobbs (l966a); Hara et al. (1973); and Bell and Etheridge (1974)]. It is important to also note that typical mylonite microstructures commonly thought of as the result of shear strain have been produced in axial1y symmetric straining by Tullis et al. (1973). Another problem associated with the strain in mylonite zones is the common paral1elism between lineation and fold axes. Many workers hold that the lineation is an axis of principal extension whereas others claim it to be a direction of shearing within the foliation plane. Generally, however, there is very little evidence to define the strain very clearly, but a problem exists in establishing the relationship of folds and lineation in mylonite zones to the distribution of strain within those zones and in correlating these features with the overall displacement associated with the zone.