Structural Geology lecture - Deformational
fabrics
Lecture Index: Definitions
for and description of fabrics. / Mechanisms
of foliation formation. / Axial planar
cleavages. / Linear structures and
lineations. /
Reading:
Chapt. 11, Fabrics: Foliations and Lineations
in van der Pluijm and Marshak text.
Key
Terms:
- various manifestations of cleavage:
- preferred orientation of inequant minerals,
crystallographic axes, and of deformed grains.
- distributed solution and fracture surfaces.
- domainal structure (compositional layering).
- formation mechanisms:
- dislocation glide and climb.
- solution.
- mechanical rotation.
- dewatering.
- growth in a stress field.
- axial planar cleavage.
- bedding cleavage patterns.
- cleavage as a flattening plane, versus a
slip surface.
- bedding-cleavage relationships and pattern
recognition.
- cleavage fans.
- cleavage refraction.
- interplay of pure shear and folding.
- boudinage as an associated feature.
- primary lineations.
- intersection lineations .
- elongation lineations.
- rodding lineations.
USGS photo from Appalachians of a well developed
cleavage (the 'layering' running from lower left to upper right)
as it intersects folded bedding, which is close to horizontal
to the right, but bends to a subvertical position as one traces
it to the left, as part of an asymmetric synform. Image source:
http://pubs.usgs.gov/of/2002/of02-437/gallery.htm
.
Definitions for and description of fabrics
foliation: any
planar, relatively penetrative, fabric in a rock that
is not primary, i.e. due to the interaction of deformational
processes and metamorphic processes.
Rocks with fabrics are anisotropic with respect
to strength and seismic properties
How are foliations manifest in rocks?
- preferred orientations of:
- inequant minerals: especially micas, but
also amphiboles.
- deformed mineral grains: can be especially
evident if an outline of the original clastic texture is preserved.
- of crystallographic axes: commonly of quartz
and calcite, can produce strength and seismic anisotropy.
- folia: concentrations
of certain minerals, usually micas along with opaques, represents
a compositional segregation at thin section to hand specimen
scale.
- microlithons:
are the areas between the folia which retain more of their original
character.
- compositional layering (e.g. gneisses)
- fracture and discrete solution planes.
spaced vs. penetrative fabrics: This distinction is a matter of scale, but the default
perspective is hand specimen scale.
- flow cleavage: penetrative
- crenulation cleavage: spaced and usually
associated with microfolding.
- fracture cleavage.
View of migmatitic gneiss pavement.
The gneissic layering here has been formed by either melt segregations
or by intimate injection of igneous material. Deformation (flattening
and shearing) then enhanced the layering. Note also the crenulations
that deform the gneissic layering, and how the grain size and
layer thickness is decreased in the short limbs of the crenulation.
This can be considered a second, spaced fabric that developed.
The deformation history of this rock is very complex.
Mechanisms of foliation formation
Foliations are polygenetic. Even in one specimen
different mechanisms can contribute to the foliation character.
- intracrystalline strain (dislocation glide
and climb - will learn about next week).
- solution (grain boundary diffusion) and local
reprecipitation (or not).
- mechanical rotation (usually minor, inefficient
in producing a fabric).
- dewatering.
- growth of minerals in a stress field in a
preferred orientation.
- foliations as flattening planes, as shear
planes.
Axial planar cleavages.
A structural association
and pattern between the fold and cleavage geometry exists that
can be very useful in the field. We can develop it for two
different cases:
- for an upright fold.
- for overturned folds.
What information that can be garnered at
one outcrop from bedding cleavage relationships?
- direction of fold axes.
- position on fold form, and direction to the
antiform or synform.
- minimum tightness of fold.
- vergence direction.
What are deviations from a simple axial
planar pattern?
- cleavage fans.
- cleavage refraction.
- sigmoidal cleavage.
- cleavage overprints.
- simple shear modification.
axial planar cleavage and the history of
fold development:
- if flexural slip or buckling occurs during
or after cleavage development occurs what should happen to the
cleavage?
- evolution of strain mechanisms in folded-cleaved
rocks: flexural slip to pure shear.
We will consider fabrics associated with ductile
shears zones separately.
These are Carboniferous sandstones
and shales on Svalbard that are involved in an upright fold with
a subvertical, semi penetrative cleavage that is preferentially
developed in the shalier strata and can be seen as the planar
vertical fractures in the fold hinge zone. If you carefully try
to trace out layers small scale slip surfaces throughout this
outcrop become apparent, some of them along the cleavage planes.
Looking at a recumbent tight fold
in Triassic shales of Midterhuken, Spitsbergen. Note the hinge
with a spaced cleavage just meters to the left and above the person
for scale (OK - my brother).
Close up of fold hinge showing a
fanning axial planar cleavage. Also note how the cleavage is not
really planar, but anastomosing, even at this scale. The platey
talus debris is not from breakage along bedding. but along the
cleavage.
Linear structures and lineations
primary linear structures:
- parting or current lineations.
- current ripples.
- sole markings.
- long axes of pebbles (till fabrics)
- oriented fossils
- flow lineation in igneous rocks.
intersection lineations: the classic example is bedding cleavage intersection,
but can be between any two surfaces.
mineral lineations:
again, preferred growth position in a stress field. Easier to
grow ends if in elongation direction. Common with amphiboles.
crinkle lineations:
micro-folding, common in phyllites.
elongation lineations:
due to a cigar shaped strain ellipse, nicely developed in metaconglomerates.
View of granitic dike in the Pelona schist. Note how
the dike layer this and thickens, and how the schistosity bends
into the area where the dike has thinned. This is known as a pinch
and swell structure, and is due to layer-parallel extension, that
is causing necking as the more competent granitic dike is stretched.
Think of pulling taffy apart. If the deformation where to have
continued, the dike segments would actually separate, producing
boudins. Photo source: http://scamp.wr.usgs.gov/scamp/html/scg_sgm_vincent.html
.
boudins:
- related to competency contrasts.
- information in different boudin profiles.
- long axis parallel to fold axes, extension
perpendicular to boudin axis.
- symmetric vs.
asymmetric.
- chocolate tablet boudinage.
mullions and quartz rods (viscosity contrasts in a stress field.
A few references for future
follow-up:
Early thought on foliation development:
- Sedgewick, A., 1835, Remarks on the structures
of large mineral masses ...; Trans. Geol. Soc. London, vol. 2,
p. 461.
- Darwin, C., 1846, Geologic Observations in
South America; Smith and Elder & Co., London.
- Sorby, 1856, On the theory of the origin
of slaty cleavage; Philosophical Magazine, #4, 12, p. 27.
Some more recent thought:
- Wood, D. S., 1974, Current views of the development
of slaty cleavage; Annual Review of Earth and Planetary Sciences,
vol. 2, p. 369-401.
- Williams, P. F., 1977, Foliation, a review;
Tectonophysics, vol. 39, p. 305-328.
- Wilson, G., 1982, Introduction to small scale
geologic structures; George Allen and Unwin, London, 128 p..
- Gray, D. R. & Durney, D. W., 1979, Crenulation
cleavage differentiation: implication of solution-deposition
processe; Journal of Structural Geology, vol. 1, 73-80.
Copyright Harmon D. Maher Jr., This may be
used for non-profit educational purposes as long as proper attribution
is given. Otherwise, please contact me. Thank you.