Foliations Lineations

7/30/2019 Foliations Lineations

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Announcements

http://www.geo.arizona.edu/geo3xx/geo304/Foliations_Lineations.ppt

Field trip to Tanque Verde this Saturday!

8:00 AM departure; loading dock
http://www.geo.arizona.edu/geo3xx/geo304/Foliations_Lineations.ppthttp://www.geo.arizona.edu/geo3xx/geo304/Foliations_Lineations.ppthttp://www.geo.arizona.edu/geo3xx/geo304/Foliations_Lineations.ppthttp://www.geo.arizona.edu/geo3xx/geo304/Foliations_Lineations.ppt


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In brittle regime:

joints, tensile fractures,

shear fractures(faults!), pressure

solution (cleavage

development)-

deformation

mechanisms depend

on pressure!

What about

deformation in the

deeper crust?


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Today: Foliations, stretching lineations,

and tectonites- deformation in the deeper

crust

(D&R, pp. 456-479; 485-492)


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"General" definition:

Foliation: Penetrative (at outcrop and microscopicscale) and parallel planarfabric elements in a rock.

"Structural geologist's" definition:

Planar fabric is secondary and due to mineral

recrystallization and/or plastic behavior during

deformation at elevated temperatures


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phyllitic

structure is a

type of

foliation


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Schistosity:

coarser grainedfabric- also a

foliation


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Gneissic structure:

Compositional banding

produced duringdeformation.


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Migmatite: Compositional banding due to in-

situ partial melting. Swirly appearance


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In a conglomerate, flattened pebbles may

define a foliation- "flattening fabric"


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Flattening of strong layers surrounded by

weak layers may cause strong layers to "neck"

and form boudins.


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more boudins


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Boudins in 3-D


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Mylonitic foliation: Forms due to grain-size

reduction by a mix of brittle and plastic deformation

in shear zones

brittle deformation of feldspar porphyroclasts

plastic deformation of quartz "ribbons" and mica


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Coarse-grained mylonitic augen gneiss. The large

porphyroclasts are called augen ("eyes")


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A strongly mylonitized rock- note the extremely fine

grain size due to "pulverization" during shearing


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Lineation: penetrative linear fabric. We will focus

on those that are related to deformation.

How does it differ from other linear structures we

have talked about, like slickenlines on a fault

surface?

Types of lineations:

1) Intersection2) Crenulation

3) Mineral

4) Stretching


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Intersection lineation: Intersection of two planar

features- an "apparent" lineation in that there is no

fabric that is linear.

e.g., intersection between cleavage and planar surface


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Crenulation lineation: Intersection between fold

hinges and foliation


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Mineral lineation: preferred alignment of minerals

due to deformation and/or recrystallization duringdeformation

St t hi li ti l ti f i l d


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Stretching lineation: elongation of minerals due

to "stretching" deformation


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Stretched

calcite


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Lineation defined by stretched pebbles in a

conglomerate


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Tectonites: Rocks that are

pervaded by foliation and/or

lineation- flowed in solid state

S: Schistosity (foliation) only

due to flattening- no lineation

L: Lineation only, due to

unidirectional stretching/

constriction

LS: Foliation and Lineation,

related to noncoaxial strain-

shearing


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Strain ellipse and tectonites

S-tect = S1= S2 > S3 (coaxial)

L-tect = S1 > S2 = S3 (coaxial)

LS-tect = S1 > S2 > S3 (non-coaxial)

L t t itWh t ki d f t t it i thi ?


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What is it?

L-tectoniteWhat kind of tectonite is this?

Coaxial (pure shear) or non-

coaxial strain?Coaxial

Wh t ki d f t t it i thi ? L S t t it


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What kind of tectonite is this?

Coaxial vs. non-coaxial?

L-S tectonite

non-coaxial


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S-C fabrics- occur in L-S tectonites and serve as

excellent sense-of-shear indicators

S-Surfaces- planes of schistosity/foliation (flattening)

C-Surfaces- planes of maximum shear "shear bands"

C comes from cisaillement, French for shear

When studying S C fabrics must look


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When studying S-C fabrics- must look

perpendicular to lineation!


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S-C fabrics and the strain ellipse

First step find shear bands (C surfaces)


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First step- find shear bands (C-surfaces)

Second step- find flattening planes (S-surfaces)

Third step- sense of shear from strain ellipse!left-lateral sense of shear

Practice!


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Practice!

CS

top to right sense of shear

For fine grained mylonites S C fabrics can


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For fine-grained mylonites- S-C fabrics can

be studied using a microscope

Saturday's field


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Saturday s field

trip to Tanque

Verde Wash

(RedingtonPass area)

A look at

deformation in ashear zone

related to the

Catalina

detachment


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Hypothesis for

evolution of

metamorphic

core complexes

including the

Catalina-Rinconcore complex

and detachment

system.


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Next Lecture: Shear zones and shear sense

indicators

Please read (D&R, pp. 493-551)

I t t t i l / t


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Important terminology/concepts

foliation

gneissic structuremigmatite

boudins

mylonites

augen gneiss

lineation (intersection, crenulation, mineral, stretching)

tectonites (L, S, L-S)

tectonites and strain ellipsoidS-C fabrics and sense-of-shear

Structural evolution of metamorphic core complexes

Documents

Foliations Lineations