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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

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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

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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