Page 125 - Microtectonics

114 5 · Shear Zones

Fig. 5.2. Distribution of the main types of fault rocks with depth in the crust. a Schematic cross-section through a transcurrent shear zone.
The zone may widen, and changes in geometry and dominant type of fault rock occur with increasing depth and metamorphic grade.
b Schematic representation of four typical fault rocks (out of scale) and the local geometry of the shear zone in a 1-m-wide block, such as
would develop from a phenocryst granite. Inclined (normal or reverse) shear zones show a similar distribution of fault rocks and shear zone
geometry with depth. No vertical scale is given since the depth of the transition between dominant ductile deformation and brittle fractur-
ing depends on rock composition, geothermal gradient, bulk strain rate and other factors (Sect. 3.14). MF: Main fault vein
Deformation bands form in high porosity rocks or at high Deformation bands that are associated with a change
differential stress, high mean stress and high strain (criti- in porosity are of great economic importance since they
cal state theory: Schofield and Wroth 1968). In such bands, influence rock permeability and the shape of water and
cataclasis is associated with collapse of the pore space, grain hydrocarbon reservoirs in rocks (e.g. Aydin 2000; Fisher
rotation and cataclastic flow (Menéndez et al. 1996), espe- and Knipe 2001; Ogilvie and Glover 2001).
cially in rocks with high porosity and good sorting. Good
sorting implies high stress concentration points where 5.2.4
grains touch, while poor sorting effects more equal stress Cohesive Fault Rocks
distribution over grains. In such cases, polycrystalline grains
and feldspar grains are fragmented while monocrystalline Cohesive fault rocks can be subdivided into cohesive breccia
quartz grains are more resistant. Deformation bands with (Fig. 5.3), cohesive cataclasite and pseudotachylyte. The dis-
decrease in porosity are also known as compaction bands tinction between breccia and cataclasite is as discussed for
(Mollema and Antonellini 1996). Some deformation bands incohesive fault rocks. The cohesive nature of the rock is
show no significant change in porosity, but are defined by due to precipitation crystallisation of minerals such as
preferred orientation of grains in the band. There are also quartz, calcite, epidote, chlorite or K-feldspar from a fluid.
deformation bands with an increase in porosity. These form K-feldspar only precipitates if the solution is highly alka-
mainly at low strain in low porosity rocks without much line, which could occur if fluid infiltration into a freshly
cataclasis, and probably at very low mean stress close to the crushed rock is limited (Wintsch et al. 1995; Wintsch 1998).
Earth surface (Antonellini et al. 1994). Such bands are also Cohesive breccia and cataclasite are less easily identi-
known as dilatation bands (Du Bernard et al. 2002). Clay fiable in outcrop than incohesive fault rocks; for example,
content of the rock also influences deformation behaviour. incohesive cataclasite in quartzite is obvious because of
Deformation bands can occur single or, more com- weathering contrasts, but cohesive cataclasite may differ
monly, in bundles or zones of subparallel bands which from undeformed host rock only by a darker colour. Cohe-
taken together can lead to significant displacement (Main sive breccia and cataclasite can be formed in any rock type.
et al. 2001). These zones probably develop due to strain Usually, fragments of all sizes occur hampering a clear dis-
hardening in individual deformation bands, leading to tinction between matrix material and fragments (Figs. 3.5,
development of new ones. Cataclasis and hardening in a 5.3). The contact between the fault rock and the intact wall
deformation band may eventually be followed by locali- rock is usually a gradual transition of decreasing brittle
sation of motion on a brittle slip plane, and softening. deformation intensity. Cohesive cataclasite and breccia