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

Strain distribution in a Shear zone

potentially can determine:• sense of displacement• amount of displacement• amount of strain

shear zones: offset markers

marker shows gradual deflection

marker shows discrete offset

from: Davis and Reynolds, 1996

deflection and offset of markers across shear zones--sense of shear

similar terminologyto faults

relationship of shear zonesat depth to

faults near surface

thrust displacement

normal displacement

from: Davis and Reynolds, 1996

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

Mylonites

Mylonites often have lineations

L-S tectonites have both foliation and lineation

These feldspars are mostly brittlely deformed

Feldspars are not as deformed as quartz

Quartz is black, feldspars are elongated

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

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

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

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

foliations

from: Davis and Reynolds, 1996

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

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…

S-C pattern is similar to that for foliation in shear zone as a whole

from: Davis and Reynolds, 1996

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

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

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

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)

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)

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

Sense of Shear Indicators

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

Shear Sense Indicators

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

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

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

Evidence for Rotation during non-coaxial deformation

Garnet in Qtz-mica schist