LECTURE 6 DEFORMATION
MICROSTRUCTURES
MECHANISMS
AND
LECTURE PLAN
1) INTRODUCTION
2) DIFFUSIVE MASS TRANSFER
3) CRYSTAL PLASTICITY
4) FRICTIONAL SLIDING, FRACTURE PROCESSES AND
CATACLASIS
Recoverable ~ no microstructures
Peak stress and strain
Ascending stress/strain
Strain
1) INTRODUCTION
In this Lecture, the mechanisms which allow rocks to change
shape during deformation are described.
There are 2 fundamental categories of deformation:-
Failure or yield stress,
beyond which
does not occur
Stress
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Non-recoverable ~ microstructures formed
Peak and final stress
and strain
a) Recoverable- elastic deformation and thermal expansion.
Atomic bonds are elastic and are not broken during
recoverable deformation. When the source of stress or heat
are removed, the body resumes its original pre-deformational
shape.
b) Permanent- If the stress exceeds a critical value termed the
yield stress, then permanent deformation will occur. In this
lecture, we will deal only with permanent deformation, involving
diffusive mass transfer, crystal plasticity and fracture
processes.
Descending stress/strain
Ascending stress/strain
Strain
Failure or yield stress,
beyond which the material
is permanently deformed
through the development
of microstructures
Stress
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The activation of specific deformation mechanisms is reliant on
the prevailing temperature, stress magnitudes, fluid pressures,
strain rate, chemical conditions and deformation history of the
material (see below).
Brittle deformation (generally low temperature) is defined as
strongly-pressure dependent deformation involving an increase
in volume (dilatancy). Brittle strengths increase with increasing
pressure because frictional strength increases and normal
stresses fight against dilatancy. Gouges generally form.
BRITTLE CATACLASTIC FLOW (high confining pressure)
High confining pressures do not allow grains to slide past eachother. Cataclastic flow occurs within
zones of fault gouge composed of a multitude of small fragments of the original grains, formed by
grain size reduction during frictional grain boundary sliding.
(2)
(1)
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PLASTIC FLOW
Original
BRITTLE CATACLASTIC FLOW (low confining pressure)
(1) Starting material composed
of spherical particles. Low
confining pressures allow grains
to slide past eachother.
(2) Brittle deformation where frictional grain boundary
sliding has re-arranged the grains.Further slip
between grains causes rotations. Note senses of
rotation and shear between grains.
(3) Volume changes (dilatancy) occur as the
grains move past eachother. The volume
changes mean the deformation is sensitive
to the confining pressure
Deformed
Plastic deformation through internal shape
changes in the grains/crystals. Volume
changes are unimportant so the deformation
is pressure insensitive
(4) Sliding and grain-rotation alternate
Either (low temperature)
Or (high temperature)
Ductile deformation
with no visble breaks
or discontinuities in the
deformation (at the
scale of viewing)
Plastic deformation (generally high temperature) is defined as
strongly temperature and time dependent deformation which is
a constant-volume mechanisms. Plastic strengths are
insensitive to pressure, but usually decrease exponentially with
temperature. Mylonites with internally deformed grains/crystals
generally form.
So:
- the terms plastic and brittle describe separate deformation
mechanisms.
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Gneiss
- the term ductile describe the geometry of the deformation
where no breaks or discontinuities can be seen in the
deformation at a particular scale of viewing (e.g. macroscopic
flow).
Therefore ductile deformation by microscopic fracturing is
termed cataclastic flow, whilst ductile deformation by crystal
plasticity is plastic flow.
Mylonite
Close-up
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Close-up view of a low-temperature thrust zone cutting Cretaceous chalk
with thin chert (flint) beds. Material within the thrust zone has been
fractured and grain-size reduced during frictional sliding involving
fracture. The resulting fault rock is a gouge. Note the smaller-scale R
and P shears within the fault zone. The black spots in the gouge are
grain-size reduced chert. The deformation is brittle.
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Close-up view of a high temperature mylonitic thrust zone emplacing
amphibolite facies gneisses of the Pre-Cambrian Lewisian complex onto
Cambrian quartzites (not show), Ben Arnaboll, N.W. Scotland. The pink,
orthoclase-rich pegmatites and gneisses have been stretched into thin
layers within the myloniti c foliation. The deformation appears ductile
and plastic.
Start of this Lecture
Formation of cements in pore spaces
2) DIFFUSIVE MASS TRANSFER
D = dissolution
Diffusive mass transfer (DMT) induces deformation by the
transfer of material away from zones of relatively high
intergranular normal stress to interfaces with low normal
stresses.
D
C = cementation
C
Material dissolved from the stylolitic
contact between grain may be reprecipitated as cements in pore spaces.
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Be
dd
av
Cle
The removal of material can lead to volume losses and strain
accommodation by chemical compaction.
ing
eg Flattened quartz grains with beards of quartz & chlorite
growing in the spaces between the grains.
age
The driving force for DMT depends on the variation in chemical
potential in the rock aggregate induced by stress variations
within the aggregate, fluid pressure gradients or variations in
the internal strain energy of grains.
Spaced pressure dissolution cleavage in
Cretaceous pelagic limestones (Scaglia
Rosata), Umbria Marche Thrust belt,
central Italy. The cleavage has formed at
angle to bedding due to c. NE-directed
overthrusting. Material has been
dissolved along the cleavage, and may
have caused volume losses of a few tens
of percent. The material is transported in
solution and then re-precipitated
elsewhere within the thrust belt.
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DMT is most likely to dominate the deformation in fine-grained
material where the diffusion path length is low.
Can be considered a 3 stage process:-
Peloids and small
ooids with an
anhydrite cement
Ooids with
an anhydrite
cement
Mudstone with
anhydrite crystals
a) Source mechanisms- These occur along stylolites and
dissolution seams. How the material enters a diffusion path.
Include the processes which control the activation of diffusion,
corrosion of existing material and reaction processes.
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Stylolite in Zechstein (Permian) carbonates within an oil/gas field in the Netherlands.
Material has been dissolved along the cleavage, and may have caused volume losses of a
few tens of percent. The material is transported in solution and then re-precipitated
elsewhere within the region. Bitumen has accumulated along the stylolite, either as an
insoluble residue or due to later fluid flow along the stylolite. Injected blue glue shows pores.
Start of this Lecture
One of the source mechanisms is pressure dissolution. Areas
of high internal stress in rocks such as point contacts between
grains have high internal elastic strain. The strain energy
makes the stressed solid more soluble in the pore fluid than
the un-strained material.
Formation of pitted pebbles
greatest
stress and
elastic strain
dissolution
Microstructures include stylolites, pitted pebbles and generally
areas of dissolution.
Microstructures form such as
stylolites, cleavage and pitted pebbles
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b) Migration or diffusion mechanisms- Includes:Diffusion along:i) the discontinuities within the crystal structure.
ii) thin fluid film along grain boundaries
iii) transport in a bulk fluid which is undergoing flow.
Microstructures may be difficult to pinpoint.
PPL
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c) Sink processes- where material is precipitated in the sites of
crystal growth.
CL
Two views of a calcite vein from the Vercors, thrust belt, French Alps. In plane-polarised light
almost-clear calcite has been precipitated from pore-waters circulating through a fracture.
Under cathodoluminescence (electic current passed across the thin-section in a vacuum and no
light), the growth zones in the calcite produce different luminescence colours, revealing subtle
variations in the pore water iron/manganese chemistry and the growth faces of the crystals. The
crystals have grown into a fluid-filled cavity.
Microstructures include cement overgrowths, pressure
shadows and veins.
Start of this Lecture
Click to enlarge
Click to enlarge
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Red stained stylolites that have allowed pressure dissolution of fossiliferous
limestones (see on a polished table top made of “marble”). Low strain rates
and low temperatures force deformation to occur via dissolution in the presence of water within pore spaces. Insoluble residues (haematite in this case)
accumulate along the stylolite.
3) CRYSTAL PLASTICITY
Crystal plasticity through movement of dislocations
Crystal plasticity involves the accumulation of strain by
intracrystalline processes such as the movement of
dislocations and twinning. Crystals commonly contain defects
such as missing atoms or impurities which are orders of
magnitude weaker than the crystal structure. Crystal plasticity
involves the motion of these defects through the crystals in
response to stress .
At low temperatures deformation occurs by dislocation glide
where dislocation motion is confined to slip planes (low
temperature plasticity).
Deformation may become become easier to accomplish if one
of the atomic bonds is broken at a defect or dislocation, where
there are less atomic bonds to break along a plane in order for
it to slip.
Leads to dislocation tangles and strain-hardening
(characterised by an increasing resistance to straining during
deformation).
straining
grain
strain in
adjacent
grain
adjacent
grain
dislocation
Time 1
Time 2
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Strain Hardening
Ascending stress/strain
Strain
Stress
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Twin gliding occurs in bands where the crystal structure is
sheared into a mirror image of its neighbouring material.
Start of this Lecture
Onset of strain hardening
where strain accumulates
with more stress required
per unit strain.
At higher temperatures, thermally activated recovery
processes such as dislocation climb (movement of the
dislocation out of their slip planes) reduce the effect of the
work hardening process. The deformation mechanism is
termed dislocation creep.
Microstructures which indicate the action of crystal plasticity
include:-
Strain Softening
Ascending stress/strain
Strain
Stress
- Twins
- preferred crystallographic orientations
Onset of strain softening,
where strain accumulates
with less stress required
per unit strain.
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- undulose extinction indicating bent or twisted crystal structure
- sub-grain structures within grains
- new grains developed during dynamic recrystallisation of
grain boundaries during grain-boundary migration or sub-grain
rotation.
- high dislocation densities within the crystals
Strain-softening of shear zones may result as easy glide
horizons become aligned.
Start of this Lecture
Sample of a mylonite from the Moine Thrust Zone, N.W. Scotland. Note the intense foliation defined by crystals that have become aligned during crystal plastic deformation. The
green color is due to the presence of chlorite beteen the quartz crystals. The lighter bands
have less chlorite because they were worm burrows before the deformation when the
rock was a sedimentary silt/sandstone.
Views onto the top and end of the Moine mylonite sample. A stretching lineation is seen on the top, paralllel to the pen, defined by quartz crystals aligned by
crystal plastic deformation. The ellipse shapes on the end of the sample are flattened worm burrows.
Stretching direction
m
u
b
r
s
r
o
w
s
r
o
w
r
u
b
m
r
o
W
r
o
W
Worm burrows
Worm burrows
Elliptical worm burrows
flattened during the shearing
The original sedimentary rock with worm burrows is progressively sheared
at temperatures high enough to allows crystal plasticity. This results in a
foliated rock with ellipses seen when viewed parallel to the direction of
maximum elongation (known as the x direction).
4) FRICTIONAL SLIDING, FRACTURE PROCESSES AND
CATACLASIS
Frictional grain boundary sliding without fracture
a) Frictional grain-boundary sliding without fracture
Deformation by frictional grain-boundary sliding involves the
sliding of grains past eachother. Individual grains are
essentially undeformed and behave as rigid bodies. This
deformation mechanism is termed independent particulate
flow.
Also known as independent particulate flow
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Sliding starts when the cohesion and friction between grains is
overcome.
Therefore, this is a pressure sensitive mode of deformation
which is promoted by low confining pressures and high fluid
pressures.
The initiation of sliding is critically dependent on the amount
and strength of cement bridges holding the grains together.
Fault gouge on a normal fault near Delphi,
central Greece. Although the grains have
been formed by sliding involving fracture
and grain size reduction, it is likely that at
least some strain will have been
accommodated by frictional grainboundary sliding without fracture (like ballbearings rolling past eachother). The
microstructure produced by sliding without
fracture will simply be grain re-packing. As
you can see, it will be difficult to recognise!
Complex volume changes accompany this style of deformation
as grains move apart and compact closer together to
accomplish displacements.
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Deformation occurs through the following sequence of events:dilation + fluid influx disaggregation and displacement
>collapse and grain alignment
Microstructures may be difficult to recognise as grain sizes and
shapes are not disturbed.
Start of this Lecture
A sample of carbonate fault gouge that has been stained to highlight the ferroan
nature of the calcite within the sample. Sample from a thrust in the French Alps.
Late fractures that
formed during
erosion and uplift
Individual clast
within the gouge
Matrix between
clasts is actua;y
made of extremely
fine grained clasts.
Fault gouge with relatively
coarse clast sizes
Gouges such as these form during
high strain rate slip events
(earthquakes) through cataclasis.
Cataclasis involves intense clastsize reduction due to the action of
friction causing crushing of clasts.
Fault gouge with
extremely fine clast size
Scratches on the surface of the specimen: ignore
b) Frictional grain boundary sliding involving fracture
Fracture processes involve the nucleation, propagation and
displacement along new surfaces created during the
deformation.
If the pieces fit together after fracture then it is called brittle
fracture. e.g. dropping a plate.
If the fracture occurs after distortion of a material, and the
pieces no longer fit together, then this is termed ductile
fracture. Ductile fracture is accompanied by strain surrounding
the fracture accommodated by another deformation
mechanism (plasticity). e.g. bending a piece of metal until
fatigues and fractures.
Thrust fault cutting steeply dipping beds of chalk and siliceous chert (the thin
black layer). Frictional sliding involving fracture and grain-size reduction has
broken both the chalk and the chert into fragments. The arrows indicate the
contact between the gouge (where fragments are completely surrounded by a
fine-grained rock flour) and brecciated chalk (where fractures surround clasts
but the clasts are essentially in the place they originated). Note the irregular
nature of the gouge-to-breccia contact. The movement sense is in
and
out
of the page.
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The fragmentation of material, together with the rotation and
associated grain-boundary sliding and dilation, constitute
cataclasis which dominates faulting at shallow crustal levels
and produces gouges and fault breccias.
In the absence of water, frictional heating associated with rapid
seismic slip may melt the fault rocks to produces glassy
pseudotachylites.
Start of this Lecture
Fracture mechanisms are usually associated with fast crack
propagation termed brittle failure.
Propagation at lower velocities is termed sub-critical crack
growth (e.g. ductile fracture).
Mechanisms include:- Elastic strain accumulation- Fractures follow weaknesses
such as crystallographic cleavages and may be transgranular
and exploit grain boundaries.
- Crystal plastic processes- can cause fracture when dense
dislocation tangles or intense twinning induce work hardening
and failure. Also, voids can open at the grain boundaries
between minerals whose ease of crystal plastic deformation is
different during the same deformation.
- Diffusion processes- The opening of voids during dissolution
can lead to failure as the voids are linked by a propagating
fracture.
- Phase transformations- Volume changes associated with
recrystallisation can open voids which can nucleate a fracture.
- Fluid processes- High fluid pressures can cause fracturing by
reducing the effective normal stress across a fault or fracture
so that the frictional resistance to sliding is decreased. Also,
corrosion of minerals at crack tips due to the presence of the
fluid can induce sub-critical crack growth.
Start of this Lecture
FURTHER READING AVAILABLE
FROM THE ELECTRONIC LIBRARY
John P. Craddock, Kimberly J. Nielson and
David H. Malone, 2000. Calcite twinning
strain constraints on the emplacement rate
and kinematic pattern of the upper plate of
the Heart Mountain Detachment, Journal of
Structural Geology, 22, 983-991
Peter Vrolijk and Ben A. van der Pluijm, 1999.
Clay gouge, Journal of Structural Geology, 21,
1039-1048
Trenton T. Cladouhos, 1999. Shape preferred
orientations of survivor grains in fault
gouge, Journal of Structural Geology, 21, 419436
J. F. HipperttF. D. Hongn, 1998. Deformation
mechanisms in the mylonite/ultramylonite
transition, Journal of Structural Geology, 20,
1435-1448
B. Bos, C. J. Peach and C. J. Spiers, 2000,
Frictional-viscous flow of simulated fault
gouge caused by the combined effects of
phyllosilicates and pressure solution,
Tectonophysics, 327, 173-194