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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