Everything about Foliation Geology totally explained
Foliation is any penetrative planar
fabric present in
rocks. Foliation is common to rocks affected by regional
metamorphic compression typical of
orogenic belts. Rocks exhibiting foliation include the typical metamorphic rock sequence of
slate,
phyllite,
schist and
gneiss. The
slatey cleavage typical of slate is due to the parallel growth of microscopic
mica crystals. In
gneiss the foliation is more typically represented by compositional banding due to segregation of mineral phases. Foliated rock is also known as
S-tectonite in
sheared rock masses.
Formation Mechanisms
Foliation is usually formed by the preferred orientation of
minerals within a rock.
Typically this is a result of some physical force, and its effect upon the growth of minerals. The planar fabric of a foliation typically forms at
right angles to the principal stress direction. In
sheared zones, however, planar fabric within a rock may not be directly perpendicular to the principal stress direction due to
rotation, mass transport and
shortening.
Foliation may be formed by realignment of micas and
clays via physical rotation of the minerals within the rock. Often this foliation is associated with
diagenetic metamorphism and low-grade burial metamorphism. Foliation may parallel original sedimentary bedding, but more often is oriented at some angle to it.
The growth of
platey minerals, typically of the mica group, as a result of
prograde metamorphic reactions during deformation. Often,
retrograde metamorphism won't form a foliation because unroofing of a metamorphic belt isn't accompanied by significant compressive stress. Thermal metamorphism in the
aureole of a
granite is also unlikely to result in growth of mica in a foliation, although growth of new minerals may overprint existing foliation(s).
Alignment of
tabular minerals in
metamorphic rocks,
igneous rocks and
intrusive rocks may form a foliation. Typical examples of metamorphic rocks include
porphyroblastic schists where large,
oblate minerals form an alignment either due to growth or rotation in the groundmass.
Igneous rocks can become foliated by alignment of
cumulate crystals during
convection in large
magma chambers, especially
ultramafic intrusions, and typically
plagioclase laths. Granite may form foliation due to frictional drag on
viscous magma by the wall rocks. Lavas may preserve a flow foliation, or even compressed
eutaxitic texture, typically in highly viscous
felsic agglomerate,
welded tuff and
pyroclastic surge deposits.
Metamorphic differentiation, typical of
gneisses, is caused by chemical and compositional
banding within the metamorphic rock mass. Usually this represents the
protolith chemistry, which forms distinct mineral assemblages. However, compositional banding can be the result of
nucleation processes which cause chemical and mineralogical differentiation into bands. This typically follows the same principle as mica growth, perpendicular to the principal stress.
Metamorphic differentiation can be present at angles to protolith compositional banding.
Crenulation and intersection lineations are particular types of foliation.
Interpretation
Foliation, as it forms generally perpendicular to the direction of principal stress, records the direction of shortening. This is related to the axis of folds, which generally form an
axial-planar foliation within their axial regions.
Measurement of the intersection between a fold's axial plane and a surface on the fold will provide the
fold plunge. If a foliation doesn't match the observed plunge of a fold, it's likely associated with a different deformation event.
Foliation in areas of shearing, and within the plane of
thrust faults, can provide information on the transport direction or sense of movement on the thrust or shear. Generally, the acute intersection angle shows the direction of transport. Foliations typically bend or curve into a shear, which provides the same information, if it's of a scale which can be observed.
Foliations, in a regional sense, will tend to curve around rigid, incompressible bodies such as granite. Thus, they're not always 'planar' in the strictest sense and may violate the rule of being perpendicular to the regional stress field, due to local influences. This is a megascopic version of what may occur around porphyroblasts. Often, fine observation of foliations on outcrop, hand specimen and on the microscopic scale complements observations on a map or regional scale.
Description
When describing a foliation it's useful to note
- the mineralogy of the folia; this can provide information on the conditions of formation
- the mineralogy in intrafolial areas
- foliation spacing
- any porphyroblasts or minerals associated with the foliation and whether they overprint it or are cut by it
- whether it's planar, undulose, vague or well developed
- its orientation in space, as strike and dip, or dip and dip direction
- its relationship to other foliations, to bedding and any folding
- measure intersection lineations
Following such a methodology allows eventual correlations in style, metamorphic grade, and intensity throughout a region, relationship to
faults,
shears, structures and mineral assemblages.
Further Information
Get more info on 'Foliation Geology'.
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