Announcements
Field trip to Tanque Verde this Saturday!
8:00 AM departure; loading dock
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?
Today: Foliations, stretching lineations,
and tectonites- deformation in the deeper
crust
(D&R, pp. 456-479; 485-492)
"General" definition:
Foliation: Penetrative (at outcrop and microscopic
scale) and parallel planar fabric 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
phyllitic
structure is a
type of
foliation
Schistosity:
coarser grained
fabric- also a
foliation
Gneissic structure:
Compositional banding
produced during
deformation.
Migmatite: Compositional banding due to insitu partial melting. Swirly appearance
In a conglomerate, flattened pebbles may
define a foliation- "flattening fabric"
Flattening of strong layers surrounded by
weak layers may cause strong layers to "neck"
and form boudins.
more boudins
Boudins in 3-D
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
Coarse-grained mylonitic augen gneiss. The large
porphyroclasts are called augen ("eyes")
A strongly mylonitized rock- note the extremely fine
grain size due to "pulverization" during shearing
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) Intersection
2) Crenulation
3) Mineral
4) Stretching
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
Crenulation lineation: Intersection between fold
hinges and foliation
Mineral lineation: preferred alignment of minerals
due to deformation and/or recrystallization during
deformation
Stretching lineation: elongation of minerals due
to "stretching" deformation
Stretched
calcite
Lineation defined by stretched pebbles in a
conglomerate
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 strainshearing
Strain ellipse and tectonites
S-tect = S1= S2 > S3 (coaxial)
L-tect = S1 > S2 = S3 (coaxial)
LS-tect = S1 > S2 > S3 (non-coaxial)
What kind of tectonite is this?
Coaxial (pure shear) or noncoaxial strain?
What is it?
L-tectonite
Coaxial
What kind of tectonite is this?
Coaxial vs. non-coaxial?
L-S tectonite
non-coaxial
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
perpendicular to lineation!
S-C fabrics and the strain ellipse
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!
top to right sense of shear
S
C
For fine-grained mylonites- S-C fabrics can
be studied using a microscope
Saturday's field
trip to Tanque
Verde Wash
(Redington
Pass area)
A look at
deformation in a
shear zone
related to the
Catalina
detachment
Hypothesis for
evolution of
metamorphic
core complexes
including the
Catalina-Rincon
core complex
and detachment
system.
Next Lecture: Shear zones and shear sense
indicators
Please read (D&R, pp. 493-551)
Important terminology/concepts
foliation
gneissic structure
migmatite
boudins
mylonites
augen gneiss
lineation (intersection, crenulation, mineral, stretching)
tectonites (L, S, L-S)
tectonites and strain ellipsoid
S-C fabrics and sense-of-shear
Structural evolution of metamorphic core complexes