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GLG310 Structural Geology
GLG310 Structural Geology
Cleavage, Foliation, Lineation
• Rock fabric: Total sum of grain shape, grain
size, and grain configuration in a rock.
• Foliation is a planar fabric
– Cleavage is a common kind of foliation
• Lineation is a linear fabric
Note that most of the graphics in this lecture
come from the 3rd edition of our textbook.
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GLG310 Structural Geology
Cleavage
• Rock cleavage: Broadly refers to spaced,
aligned, planar to curviplanar surfaces
that tend to be associated with folds and
oriented parallel to or fan-shaped relative
to the axial surfaces. Commonly
penetrative and form without apparent
loss of cohesion.
GLG310 Structural Geology
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GLG310 Structural Geology
16 November 2004 GLG310 Structural Geology
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GLG310 Structural Geology
GLG310 Structural Geology
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GLG310 Structural Geology
GLG310 Structural Geology
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GLG310 Structural Geology
Foliation
• “…mesoscopically penetrative parallel
alignment of planar fabric elements in a
rock, usually in a metamorphic rock”
– Can be primary (igneous or sedimentary)
GLG310 Structural Geology
Primary foliation in a volcanic rock
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GLG310 Structural Geology
GLG310 Structural Geology
Lineation
• “…subparallel to parallel alignment of
elongate, linear fabric elements in a rock
body, commonly penetrative at the
outcrop and/or hand sample scales of
observation”
– Can be primary (igneous or sedimentary)
– Includes slickenlines
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GLG310 Structural Geology
Lineation vs. foliation
16 November 2004 GLG310 Structural Geology
Lineation
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GLG310 Structural Geology
Mullions
16 November 2004 GLG310 Structural Geology
Boudins—French for Sausage;
another form of lineation
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16 November 2004
More
Bou
din
s
GLG310 Structural Geology
Why do we care?
• Tectonites
• Sense of shear
Structures can relate small scale deformation to
large scale context
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GLG310 Structural Geology
Predominantly
flattening
--single foliation
Predominantly
constriction
--single lineation
Mixed
--lineation
contained in the
foliation
16 November 2004 GLG310 Structural Geology
Plane strain—all deformation only in
the S1-S3 plane, S2 = 1
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16 November 2004 GLG310 Structural Geology
Sense of Shear
• Identify sense
of shear
plane:
perpendicular
to the
foliation and
containing
the lineation
GLG310 Structural Geology
Sense of shear
S-C fabric: S surface = schistosity,
C surface = “cisaillement” (high shear strain) shear bands
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16 November 2004 GLG310 Structural Geology
Sense of
shear