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
Slide 3
Strain distribution in a Shear zone
Slide 4
Slide 5
Slide 6
Slide 7
potentially can determine: sense of displacement amount of
displacement amount of strain
Slide 8
shear zones: offset markers marker shows gradual deflection
marker shows discrete offset from: Davis and Reynolds, 1996
Slide 9
deflection and offset of markers across shear zones--sense of
shear similar terminology to faults
Slide 10
relationship of shear zones at depth to faults near surface
thrust displacement normal displacement from: Davis and Reynolds,
1996
Slide 11
100 300 500 10 km 20 km depth gouge cataclasite mylonite
greenschist amphibolite consider a fault from surface to depth
brittle (frictional) shallow; ductile (plastic) deep
brittle-plastic transition quartz plasticity feldspar plasticity
deformation mode fault rock TC relative crustal strength curve dd
frictional plastic
Slide 12
Mylonites
Slide 13
Mylonites often have lineations
Slide 14
L-S tectonites have both foliation and lineation
Slide 15
These feldspars are mostly brittlely deformed
Slide 16
Feldspars are not as deformed as quartz
Slide 17
Quartz is black, feldspars are elongated
Slide 18
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 > 250C dynamic recrystallization of
quartz > 300C feldspar mylonites form at higher temperatures
dynamic recrystallization of feldspar > 450C
Slide 19
types of mylonites protomylonite: matrix is < 50% of rock
ultramylonite: 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
Slide 20
main goal is to identify sense of shear: need shear-sense
indicators where 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
Slide 21
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
Slide 22
foliations from: Davis and Reynolds, 1996
Slide 23
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
Slide 24
S-C fabrics most shear zones have one foliation at angle <
45 to boundary; this foliation is s-foliation (schistosit 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
Slide 25
S-C pattern is similar to that for foliation in shear zone as a
whole from: Davis and Reynolds, 1996
Slide 26
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
Slide 27
pressure shadows form 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
coaxial versus noncoaxial (rotational) strain from: Davis and
Reynolds, 1996
Slide 28
different types of pressure shadows: pyrite: material
mineralogically same as matrix but different from inclusion fibers
grow in crystallographic continuity with matrix crinoid: material
similar to inclusion not matrix fibers grow in crystallographic
continuity with inclusion composite: aspects of both from: Davis
and Reynolds, 1996
Slide 29
Impressive evidence for rotation of cleavage during its
formation can sometimes be read from fibrous mineral growth in the
strain shadows of resistant minerals such as pyrite (From Passchier
and Trouw, 1996)
Slide 30
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 framerelative to shear zone foliation.. two winged
types of tails: -type and -type grain tail grains may be inclusions
porphyroclasts (relics from protolith) porphyroblasts (grow during
deformation)
Slide 31
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
reference plane is shear zone foliation right-lateral (dextral)
shear: clockwise rotation left-lateral (sinistral) shear:
counter-clockwise rotation from van der Pluijm and Marshak,
1997
Slide 32
Sense of Shear Indicators
Slide 33
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, 1997
http://www.geo.umn.edu/teaching/microstructure/images/079.html
Slide 34
Shear Sense Indicators
Slide 35
other minerals, such as phyllosilicates, display useful
geometry phyllosilicate 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 from: Simpson, Microstructures CD-ROM
Slide 36
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 from: Davis and Reynolds, 1996
Slide 37
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
Slide 38
Slide 39
Evidence for Rotation during non-coaxial deformation Garnet in
Qtz-mica schist