Foliations and Lineations in Deformed Rocks

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Foliations and Lineations in Deformed Rocks

• Chapter 11 Twiss and Moores

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Morphological classification scheme for foliations
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Compositional foliationsA. Diffuse
compositional foliation in a dunite.
Concentrations of pyroxene, in this dominantly
olivine rock, defines the foliation.B. Banded
compositional foliation in a high-grade
metamorphic gneiss.
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Sketches showing characteristics of the various
types of disjunctive foliation. C marks
cleavage domains M marks the microlithons.
Note the change in the scale of the spacing
from centimeters in A and B to millimeters in C
through E.
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Preferred orientation within a microlithon (M)
bounded on either side by cleavage domains (C).
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Stylolitic foliation.A. Stylolitic foliation
in limestone layers.B. Stylolite truncating a
pentacrinoid fossil in a limestone.
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Anastomosing foliation in a limestone. Bedding
is parallel to the ruler.
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Rough foliation (S1) in a deformed wacke. Dark
seams are the cleavage domains composed of
insoluble residue.At C, remnant detrital sand
grains are truncated against cleavage
domains.At T, thin plate-like quartz grains
result from solution of the grains along cleavage
domains.B marks mean mica beard overgrowths on
detrital grains. Scale bar is 1 mm
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Smooth foliation in a slate
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Zonal crenulation foliationsThe laminations and
the preferred orientation of the platy minerals
vary continuously from microlithon to cleavage
domain.Within the cleavage domains, the
laminations and platy minerals are not strictly
parallel to the new cleavage domains.
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Asymmetric crenulations in a quartz-rich schist.
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Discrete crenulation foliation in a calcareous
slate.The orientation of the platy minerals in
the cleavage domain is parallel to the domain
boundary.
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Scanning electron micrographs of continuous fine
foliations. A. Micro-domainal continuous fine
foliation in a slate with an asymmetric
micro-crenulation structureB. A
micro-disjunctive continuous fine foliation in a
slate.
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Coarse continuous foliation showing a strictly
continuous structure. A. Photomicrograph of a
schist with the folation defined by mica. B.
Grain-shape foliation parallel to the the pencil
in a very coarse grained marble layer.
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Convergent and divergent fans on folds and the
refraction of foliation across lithologic
contacts.Foliation orientation is refracted at
the contacts of a folded sequence ofI.
sandstoneII. shale or slateIII. siltstone
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A sandstone (I), shale (II), siltstone (III)
sequence showing the refraction of the foliation
at the contacts.
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The use of bedding-foliation relationships to
deduce the location of fold closures and axial
surfaces. Folded layer with an axial foliation.
Hollow arrows point along the foliation. Solid
arrows point along bedding planes. The sense of
rotation of the hollow arrow through the acute
angle from the foliation toward the bedding
(solid arrow) changes across an axial surface.
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Bedding-folaition relationship in interbedded
sandstone and shale. The foliaion is obvious in
the shale layer. The fold closes upward to the
right and downward to the left.
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Use of bedding-foliation (bxc) relationships to
deduce the stratigraphic-up direction in simply
folded layers. The boxes are parts that are
supposed to be observable in the field.The
arrows in the folded layer indicate the
stratigraphic-up direction. A. Bedding (b) is
upright if bedding and foliation (s) dip in
opposite directionsB. Bedding is upright if
bedding and foliation dip in the same direction,
and bedding has a shallower dip.C. Bedding is
overturned if b and s dip in the same direction
and b has the steeper dipD. For two or more
generations of folding, the bxc relationships do
not give reliable results.
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Foliation in an S-C tectonite. Type I S-C
tectonite in a ductile shear zone in a
granodiorite. The S-foliation is a continuous
coarse mica foliation that curves toward an
orientation parallel to the spaced
C-foliation.The sense of curvature is the same
as the general shear sense, and indicates a
sinistral shear
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Type II S-C tectonite in a quartz-rich mylonite.
The S-foliation is defined by the grain-shape
foliation of the quartz and the preferred
orientation of large mica porphyroclasts
(mica-fish).The C-foliation is the shear
plane defined by the trails of fine micas
commonly connected to the tips of the
porphyroclasts. The sense of curvature from the
mica porphyroclasts tips to the mica trails is
the same as the sense of shear on C and indicates
a sinistral shear.
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Possible sequence in the development of a
transposition foliation. S1 is the developing
surface to the folds in bedding. S2 is the
transposition foliation
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Morphological Classification scheme for lineations
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Discrete lineationsA. A stretched pebble
conglomerate showing quartzite pebbles flattened
parallel to the foliation and elongated to define
a lineation.Alteration spots in a slate. The
view is parallel to the folaition (XY).
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True and apparent lineations associaed with
ellipsoidal structures. The true lineation
orientation is shown on any plane containing the
a axiss (longest axis) of the ellipsoid. All
others are elliptical sections of the ellipsoid,
and do not give the true orientation of the
lieation.
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Intersection lineationA. Intersection of
bedding and foliation (bxc). The trace of b on c
and c on b are parallel
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Pencil cleavage in argillite. The intersection
of two foliations (one of which may be bedding)
produces elongate prisms, or pencils, of rock,
which defines the lineation
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Boudins
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MullionsA. Fold mullions in a sandstone at the
contact with a shale (now eroded). The mullion
is restricted to the bedding surface. It is not
a closed structure in cross section.B.
Irregular mullions showing the irregular cross
section and the strongly cylindrical structure of
the lineation. The mullion surface may be coated
with a thin film of mica
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Quartz rod lineationRods are generally parallel
to local fold hinges and they may be isolated
fold hinges or boudinaged fold limbs. So, in
some cases they can be classified as structural
lineation as well.
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Mineral cluster lineation in a quartz-feldspar
biotite schist defined by elongate concentrations
of quartz and feldspar and of biotite.
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Mineral slickenlines on the slickenside of a
fault surface
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Mineral fiber lineations.A. Curvilinear
serpentine slickenfibers on a fault surfaceB.
Quartz fiber overgrowths on a pyrite grain in
phyllite.