Geoscience Reference
In-Depth Information
rocks of variable grain size may change orientation, gradually
within a single layer, and abruptly across bedding planes
separating beds with contrasting grain size. This phenomenon
is known as cleavage refraction (Figure 8.15d, p. 178): in
coarse-grained rocks, the cleavage tends to form at a high angle
to bedding, but as the grain size becomes fi ner, cleavage planes
may curve round until they are almost parallel with bedding.
Slaty and disjunctive fabrics (pressure solution and fracture
cleavage) are common in the outer zones of mountain belts,
and their relationship with systematic, regional folding (Section
8.3.5) can be critical to understanding regional deformation. In
the cores of mountain belts, the orientations of ductile
foliations largely refl ect regional tectonic stresses.
8
Some rocks that are affected by more than one deformation
event may develop multiple fabrics. For instance, the
crenulation cleavage in Figure 8.15e may retain evidence for
the earlier, microfolded foliation, and in some cases the
original bedding is also visible. The orientations of these earlier
fabrics and associated lineations, and their relationships with
later fabrics, can reveal detailed information on how the rocks
were progressively deformed. In some cases, the latest
deformation is so strong that any earlier fabrics are largely
rotated into parallelism with the crenulation axial planes and
cannot be traced, a process known as transposition.
(a)
Many coarse-grained, high-grade rocks have a planar tectonic
fabric defi ned by aligned and/or fl attened mineral grains, which
has been termed 'gneissosity'. However, the same term has also
been applied to centimetre-scale compositional banding in
gneisses (Figure 8.17), and so is best avoided. Gneissic banding
may refl ect processes other than deformation (e.g. partial
melting, metamorphic segregation, transposition of original
lithological layering), whereas gneissic foliation is simply a
coarser form of schistosity.
(b)
Figure 8.16
High-strain features
typical of mylonites. (a) Boudinage
of a competent layer, northwest
India. (b) Intrafolial folds, northwest
India (view about 1.5 m across).
(Tom W. Argles, The Open
University, UK.)
8.3.2 Direction of shear/stretching:
Stretching lineations
When rocks deform, stretching tends to dominate in one
direction, resulting in a preferred orientation of linear
elements (e.g. stretched mineral grains, pebbles and fossils;
or aligned elongate crystals). Lineations are most easily
measured on foliation planes (if they exist: see Figure 8.18).
Large areas of consistent lineations are likely to refl ect regional
stress orientations, whereas lineation trends in narrow zones
generally indicate the direction of motion on those major
shear zones. You should always record what defi nes a
lineation (e.g. stretched quartz grains, aligned belemnites) at
the same time as recording its orientation, as this may provide
information on the metamorphic history of a deformed rock.
Look closely in the hinge
areas of mesoscale folds,
and in 'strain shadows'
where the main foliation
wraps round rigid objects
such as boudins and veins,
for earlier fabric relationships
that are commonly preserved
there.
