Page 124 - Microtectonics

5.2 · Brittle Fault Rocks 113
Fig. 5.1.
Optical (a) and cathodolumines-
cence (b) image of a dilatational
brittle fault zone in a quartzite
host rock of the Muth Formation
(India; sample of Erich Draga-
nits, Vienna). Sense of shear is
dextral (see rotated quartz chips
that flake off from the larger
grains). Created voids are ce-
mented with quartz that is dark
in the cathodoluminescence
image. Thus fluids have to be
involved in the cementation of
the brittle fault. The brittle par-
ticles acted as precipitation nu-
clei. Notice microcracks and
transgranular fractures in the
quartz grains outside the fault
zone. Width of view 4 mm.
(Photographs courtesy of
Erich Draganits and Michel
Bestmann)

5.2.3 Deformation bands are brittle fault zones that develop very
Deformation Bands close to the Earth’s surface in poorly or even unconsolidated
porous sediment (Fig. 5.1; Underhill and Woodcock 1987;
Deformation bands are mm-wide planar brittle shear Antonellini et al. 1994; Cashman and Cashman 2000). They
zones in undeformed, porous, quartz-rich, clay-poor sedi- are normally planar and up to tens of metres long, with
mentary rocks (Aydin 1978; Aydin and Johnson 1978, 1983; occasional ramp or branching structures. They typically
Antonellini et al. 1994; Davis 1999; Mair et al. 2000, 2002; have a small to very small displacement homogeneously
Davatzes et al. 2003; Main et al. 2001). Notice, however, distributed over the band, without localisation as on faults.
that the term deformation band is also used for a special Most deformation bands have significantly lower poros-
type of undulose extinction (Sect. 3.6). ity than the undeformed wall rock associated with cataclasis.