ductile shear zones!
DESCRIPTION
Ductile Shear Zones!. ductile shear zone “ zone ”: area with higher strain than surrounding rock This is heterogeneous strain. “ shear ”: simple shear dominates. “ ductile ” deformation mechanisms. similar to faults in that displacement occurs, but no fracture forms. - PowerPoint PPT PresentationTRANSCRIPT
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Ductile Shear Zones!
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ductile shear zone “zone”: area with higher strain than surrounding rock
This is heterogeneous strain.
“shear”: simple shear dominates
“ductile” deformation mechanisms
similar to faults in that displacement occurs, but no fracture forms
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Strain distribution in a Shear zone
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potentially can determine:• sense of displacement• amount of displacement• amount of strain
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shear zones: offset markers
marker shows gradual deflection
marker shows discrete offset
from: Davis and Reynolds, 1996
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deflection and offset of markers across shear zones--sense of shear
similar terminologyto faults
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relationship of shear zonesat depth to
faults near surface
thrust displacement
normal displacement
from: Davis and Reynolds, 1996
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100°
300°
500°
10 km
20 km
depthgouge
cataclasite
mylonite
gree
nsch
ist
amph
ibol
ite
consider a fault from surface to depth…brittle (frictional) shallow; ductile (plastic) deep
brittle-plastic transition
quartzplasticity
feldsparplasticity
deformationmode
faultrock
T°C
relative crustalstrength curve
d
frictional
plastic
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Mylonites
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Mylonites often have lineations
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L-S tectonites have both foliation and lineation
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These feldspars are mostly brittlely deformed
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Feldspars are not as deformed as quartz
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Quartz is black, feldspars are elongated
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from: http://www.rci.rutgers.edu/~geolweb/slides.html
a refresher on mylonite
from: http://www.geolab.unc.edu/Petunia/lgMetAtlas/meta-micro
marble mylonite and quartz mylonite form at lower temperatures• dynamic recrystallization of calcite > 250°C• dynamic recrystallization of quartz > 300°C
feldspar mylonites form at higher temperatures • dynamic recrystallization of feldspar > 450°C
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types of mylonites
protomylonite: matrix is < 50% of rockultramylonite: matrix is 90-100% of rock
rocks with 50-90% matrix simply called mylonites
http://www.geo.umn.edu/teaching/microstructure/images/079.html
myloniteultramylonite
http://uts.cc.utexas.edu/~rmr/images
protomylonite
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main goal is to identify sense of shear: need shear-sense indicatorswhere to look? optimal surfaces are those perpendicular to
foliation or shear zone boundaries
shear zone and foliation
from van der Pluijm and Marshak, 1997
1) determine orientation of shear zone
2) find perpendicular (profile) plane
3) identify line of transport…direction along which relative displacement
occurred… (in perpendicular plane)
perpendicular plane is sense-of-shear plane (SOS)
SOS plane
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what are they? • offset markers• foliations• S-C fabrics and shear bands• grain-tail complexes• disrupted grains (mica fish)• folds
…now we know to look in SOS plane for indicators…
offset markers usually obvious …make sure similar features
on both sides are same
from: http://www.leeds.ac.uk/learnstructure/index.htm
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foliations
from: Davis and Reynolds, 1996
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what does foliation subparallel to boundary in center imply? --either: • coaxial strain (normal to zone) • noncoaxial strain with very high shear • foliation represents very thin shear zones …this leads to S-C fabrics…
from: Davis and Reynolds, 1996
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S-C fabricsmost shear zones have one foliation at angle < 45° to boundary; this foliation is s-foliation (schistosité from French);
…crystal-plastic processes elongate crystals to extension
s points in shear direction; displacement on c’ is same as shear zone
from van der Pluijm and Marshak, 1997
another foliation parallels shear zone boundaries;this foliation is c-foliation (cisaillement from French);…shear direction is within c plane
a third foliation may be oriented oblique to boundary;this foliation is c’-foliation and crenulates mylonitic foliation;…shear bands…
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S-C pattern is similar to that for foliation in shear zone as a whole
from: Davis and Reynolds, 1996
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s-c fabrics
• s points in direction of shear• c parallel to shear direction• c’ displacement same as that
of shear zone
from: http://www.earth.monash.edu.au/Teaching/mscourse
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pressure shadowsform on flanks of rigid inclusions in shear zones
…rigid inclusion shields matrix on flanks from strain……crystallization of quartz, calcite, chlorite, etc.
most pressure shadows are microscopic--see in thin-section
growth accompanies each increment of extension
orientation of fibers depends on coaxialversus
noncoaxial (rotational)strain
from: Davis and Reynolds, 1996
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different types of pressure shadows:
pyrite: material mineralogically same as matrixbut different from inclusion
…fibers grow in crystallographic continuity with matrixcrinoid: material similar to inclusion not matrix
…fibers grow in crystallographic continuity with inclusioncomposite: aspects of both
from: Davis and Reynolds, 1996
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Impressive evidence for rotation of cleavage during its formationcan sometimes be read from fibrous mineral growth in the strain
shadows of resistant minerals such as pyrite (From Passchier and Trouw, 1996)
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grain-tail complexes (inclusions; porphyroclasts; porphyroblasts)
grains in matrix may have tails that form during deformation…tails are distinguishable from matrix
…tails may be.. ..attenuated, preexisting minerals..dynamic recrystallization at grain rim
..synkinematic metamorphic reactions
grains are rigid bodies that rotate during deformation …tails give sense of displacement…
to use grain-tail complexes to indicate shear-sense, need reference frame…relative to shear zone foliation..
…two “winged” types of tails: -type and -type
grain
tail
tail
…grains may be…inclusions…porphyroclasts (relics from protolith)…porphyroblasts (grow during deformation)
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two asymmetric types: -type and -type
wedge-shaped tails that do not cross reference plane when tracing tail away from grain; looks like
tails wrap around grain so they cross-cut reference plane when tracing tail away from grain; looks like
right-lateral
right-lateral
reference plane is shear zone foliation
right-lateral (dextral) shear: clockwise rotationleft-lateral (sinistral) shear: counter-clockwise rotation
from van der Pluijm and Marshak, 1997
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Sense of Shear Indicators
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two-types related: relationship between rate of crystallization and rotation of grain
…formation fast relative to rotation: -type…rotation fast relative to formation: -type
(tail dragged and wrapped around grain)
presence of both types indicates different:• rates of tail growth• initial grain shape• times of tail formation
development of from from van der Pluijm and Marshak, 1997http://www.geo.umn.edu/teaching/microstructure/images/079.html
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Shear Sense Indicators
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other minerals, such as phyllosilicates, display useful geometryphyllosilicate grains (micas) connected by mylonitic foliation …basal planes oriented at oblique angle to foliation… …point in direction of instantaneous elongation
grains have stair-step geometry in direction of shear
when large enough to see in hand specimen,
…look like scales on a fish (“mica fish”)
from van der Pluijm and Marshak, 1997
QuickTime™ and a
Photo CD Decompressor
are needed to use this picture
from: Simpson, Microstructures CD-ROM
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can determine asymmetry of mica fish by observing reflections in sunlight… …fish flash…
• mark north arrow on sample• put back to sun and sample in front of you• view parallel to lineation• tilt sample• note if flashy or dull
QuickTime™ and a
Photo CD Decompressor
are needed to use this picture
from: Davis and Reynolds, 1996
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veins…veins commonly associated with shear zones… …form perpendicular to instantaneous extension…
…initially form at 45° to shear zone… …subsequently rotate to steeper angle
while new part of vein forms at 45°
from: http://www.rci.rutgers.edu/~geolweb/slides.html
from: http://www.science.ubc.ca/~eosweb/slidesets/keck
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Evidence for Rotation during non-coaxial deformation
Garnet in Qtz-mica schist