Geoscience Reference
In-Depth Information
Table 8.3
Some common tectonic fabrics.
Fabric
Typical setting
Formed by
Clues in the fi eld
Pressure solution
cleavage
Upper crust, outer zones
of mountain belts
Dissolution of soluble
grains due to directed
stress
Dark/pale colour striping;
partially dissolved fossils,
clasts; stylolitic surfaces
Slaty cleavage
Upper crust, outer zones
of mountain belts;
fi ne-grained rocks
Alignment of platy grains
by rotation, dissolution and
recrystallization during
applied stress
Fine fabric that rock cleaves
along; typically associated
with folds
8
Fracture cleavage
Upper crust, outer zones
of mountain belts;
competent rocks
Tensional failure under
high fl uid pressure* in
competent rock types
Spaced cracks in competent
rock type
Mylonitic foliation
High strain faults and
shear zones at all but
shallowest depths
Extreme fl attening and
stretching in narrow,
high-strain zones of
shearing
Strongly planar fabric; other
high-strain features (see text
for examples)
Schistosity
Middle crust, inner
zones of mountain belts;
metamorphosed rocks
Mineral alignment under
applied stress, during
metamorphic crystallization
Visible mineral grains;
millimetre- to centimetre-scale
folia, rougher than slaty
cleavage
Crenulation
cleavage
Middle crust, inner
zones of mountain belts;
metamorphosed rocks
Microfolding of a pre-
existing planar fabric
(tectonic or sedimentary)
Microfold hinges, crenulation
lineation (Section 8.3.5)
*Many of the mechanisms for cleavage formation (especially fracture cleavage) are a matter of debate.
Figure 8.15
Examples of tectonic foliations. (a) Mylonitic foliation,
northwest Himalaya. High strain is indicated by the strong planar fabric
and tightly wrapped porphyroclasts with tails streaked out into the fabric.
(b) Slaty cleavage, visible as fi ne lines running from top left to lower right
(two separate cleavage planes are arrowed). The cleavage cuts obliquely
across bedding (dark/pale subhorizontal layers) in these fi ne-grained
mudstones and siltstones from Cumbria, UK. (c) Spaced fractures
(arrowed) cut across subvertical bedding in a limestone, southwest Wales,
UK. This fabric is sometimes referred to as 'fracture cleavage'. (d) Pressure
solution cleavage (thin, dark lines) in siltstones, west Wales, UK. Note also
the cleavage refraction, where cleavage orientation changes abruptly
across some bedding planes (arrowed), refl ecting grain-size changes.
There is also a very fi ne (barely visible) slaty cleavage parallel to the
solution cleavage. (e) Subhorizontal crenulation cleavage in schist, NW
Himalaya, showing clear microfold hinges. (f) Close-up view looking down
on schistosity planes showing visible mineral grains, including mica. The
surface of the sample cuts through numerous, irregular, millimetre-scale
foliation planes. View is 4 cm across. (a-f: Tom W. Argles, The Open
University, UK.)
