1. High Strain Rocks GLY 4310 - Spring, 2019
Petrology Lecture 10
High Strain Rocks
GLY Spring, 2019
Rock materials formed under high-strain conditions may be either cohesive or non-cohesive.
We have already seen the non-cohesive products called fault breccia and fault gouge. Cohesive rocks can be divided into foliated and non-foliated rocks.
In addition, we can use the percent of fine matrix versus large clasts.
Table 22-1 shows a classification.

2. High-Strain Terminology
Non-foliated cohesive rocks include microbreccias, if they have less than 70% clasts, or cataclasites, if they have more than 70% clasts. Foliated cohesive rocks are called mylonites, or phyllonite if they are mica rich. Protomylonites are around 50% clasts, mylonites are near 70%, and ultramylonites are near 90% clasts. Grain size reduction is very important in all fault zone rocks. If significant recrystallization, probably during a subsequent event, has occurred, a cohesive rock may be called blastolmylonitic.
Under extreme conditions, frictional heating will produce partial melting, creating glass. Rocks with glassy seams are called pseuodotachylyte. (Tachylyte is a volcanic glass that may be black, green, or brown, because of abundant crystallites. It is formed from basaltic magma, and is commonly found as chilled margins of dikes, sills, or flows.)
The distribution of fault zone rocks with depth is shown in Figure 22-2.

3. Shear Zone Cross-Section
Non-cohesive rock fragments are found near the surface, in narrow fault zones. Lack of pressure means the rocks will behave as brittle solids. At greater depths, confining pressure squeezes the fault zone walls together, further deforming and crushing the rock fragments. At greater depths the rocks grade into cataclasites and microbreccias, as they become cohesive. At greater depths, recrystallization begins, and the rocks become foliated. At this depth, the width of the shear zone increases. Ductile flow allows shearing to be more evenly distributed throughout. At great depths, shear is widely distributed, and not very intense. Gneisses, indistinguishable from regional metamorphic gneisses, develop.
Much of the terminology for fault zone rocks dates back more than a century. Recent research has revealed a great deal of new information, making it necessary to reformulate terminology. Cataclasis referred to mechanical crushing and grinding, with no recrystallization. It was thought to be the dominant process in fault and shear zones. Today, cataclasis is known to be limited to the zone of brittle deformation very near the surface. Mylonite was originally used to describe a fine-grained, laminated rock from the Moine Thrust, Scotland. It came from a Greek word which means to grind. The work of Bell and Etheridge (1973) showed that mylonization was a ductile process, with rapid recovery from strain, and recrystallization, and was not a crushing process.
One attempt to straighten out the linguistic problems was that of Wise et al. (1984) shown in Figure 22-3.
Figure Schematic cross section through a shear zone, showing the vertical distribution of fault-related rock types, ranging from non-cohesive gouge and breccia near the surface through progressively more cohesive and foliated rocks. Note that the width of the shear zone increases with depth as the shear is distributed over a larger area and becomes more ductile. Circles on the right represent microscopic views or textures. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin.

4. Shear Zone Terminology
This scheme depends on the rate of strain, and the rate of recovery. Both of these factors are important. But how can they be determined from a hand specimen sample? Thus figure 22-3 is basically used to gain understanding of processes occurring, but is also useful for actually naming a rock.
Students should look at Figure in the text (page 487) for photos of progressive mylonitization in hand specimen and thin section. This figure is not available from the publisher or it would be included here.
Figure Terminology for high-strain shear-zone related rocks proposed by Wise et al. (1984) Fault-related rocks: Suggestions for terminology. Geology, 12,