chapter 7 - deformation mechanisms

8/21/2019 CHAPTER 7 - Deformation Mechanisms

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Goals of this chapter

0 To be able to analyzed and understand how rock

deform on a microscopic to submicroscopic scale .

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To learn how microscopic scale deformation is relatedto larger structures.

0 To be to identify and understand the deformation

mechanism that cause rock deformation.


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Inroduction

0 If we can understand the conditions that produces the

dislocations and deformation mechanisms, we will

able to better trace the conditions that affected an

ancient rock mass that was formed at great depth

before erosion exposed it for us to study at the

present surface.


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Lattice Defects and

Dislocations0 Crystalline solid- is a regular geometric arrangement

of atoms, ions or groups in a compound.

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Crystalline lattice- is a periodic and systematicarrangement of atoms that are found in crystals with

exception to amorphous solid and gases.


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0 Simple models like this can affect crystal lattices on atomicscale to larger scale structure we observe in minerals androck.

0 Perfect crystal-a crystal lattice where all the sites arefilled with the right atoms, ions or groups and whichcontains no interstitial atoms between lattices.

0 Ideally perfect crystals exist only at O Kelvin(-273 degreeCelsius) where no thermal or other disturbances affect the

lattice0 This crystals are rare or nonexistent in nature. In nature

most crystal contains imperfections or defects.


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0 Vaccancies- unoccupied sites in crystal lattices

0 it change the stacking order of layers, or otherwise

distorting the lattice.0 Knowledge of lattice defects helps us to understand

the varied ways rocks deform on microscopic and

larger scales.


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Point Defects

0 Point Defects- where an atom is missing or is in

irregular in the lattice structure.

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Vaccancies in a lattice may also be produced duringductile deformation, by irradition by high energy

particles, and by sudden cooling of a crsytal from high

temperature.


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Planar defectsStacking

faults0 Consist of irregularities in the repetition of order in a

series of layers in a closed packed lattice.

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Example when a sequence of ABAB there is nopossibilities of planar defects but in a sequence of

ABCABC there is a possibility.


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Line Defects--Dislocations

0 Incision in a hypothetical crystal lattice, partly separating 2layers, and then inserting an extra partial layer producesan edge of dislocation.

0 Half Plane- inserted plane

0 In real crystals it is attained by subjecting a lattice tosimple shear, leading to distortion of the lattice so that partof a layer of atoms is isolated halfway between 2 normallayers

0 Edge dislocationis derived from location from location of

the edge of the extra layer separating the dislocation.0 Screw dislocation- involves hellical rotation within the

dislocation plane


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Deformation Mechanisms


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Cataclasis

0 Brittle deformation is concentrated in a rock mass

along surfaces of movement and no cohesion surfaces

that separate undeformed regions producing the rock

Cataclasite

0 Consists of brittle granulation of rock at low temp and

low to moderate confining pressure, but may be

accelerated by high fluid pressure.

0 Where strain rate is high, failure occurs under brittle

conditions by exceeding the elastic limit of the

material.


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0 Total volume of the rock may be reduced by closer packing ofgrains and by fracturing and granulation

0 Fracturing has also been observed in rocks deformed atmoderate to high temperature where strain rate is very high.

0 CATACLASTIC FLOW- flow of grained material produced bybrittle defromation

0 The process of cataclastic flow may also describe large scalechanges in shape due to small scale brittle deformation

0 Triboplastic behavior- apparent megascopic ductile behavior

shown to be brittle in microscopic scale

0 It occurs in shallow low depth where rocks are readily ground tofine grain size during motion along a fault


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Pressure solution

0 is a deformation mechanismthat involves

the dissolutionof minerals at grain-to-grain contacts

into an aqueousporefluid in areas of relatively

high stressand either deposition in regions of

relatively low stress within the same rock or their

complete removal from the rock within the fluid. It is

an example of diffusive mass transfer.
http://en.wikipedia.org/wiki/Deformation_mechanismhttp://en.wikipedia.org/wiki/Solvationhttp://en.wikipedia.org/wiki/Aqueous_solutionhttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Aqueous_solutionhttp://en.wikipedia.org/wiki/Solvationhttp://en.wikipedia.org/wiki/Deformation_mechanism


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0 Parts of quartz, calcite or other mineral grain may be

dissolved at points of greatest stress difference,

frequently in points of high stress where on crystal

touches another.

0 These points of high stress are the most soluble and

so dissolution starts there.

0 The remaining surfaces are able to withstand the

applied pressure, and therefore the rate of pressure

decreases with distance away from points or zone of

high stress.


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0 Minerals that dissolved are sometimes precipitated inzones of lower pressure as overgrowths, deposited in veinsor remain as a solution and are completely removed in therock.

0 Pressure solution occurs during diagenesis, particularlyduring compaction and cementation of carbonatesediments whereStylolitesirregular surfaces coated byinsoluble mineralsmay form parallel to beddingindicating that maximum stress occur was vertical.

0

Pressure solution may proceed grain by grain if the rockmass water is saturated and is sufficiently porous .

0 Most porosity in a rock mass tends to concentrate alongbedding and fracture planes.


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Translation and Twin Gliding

0 Deformation may occur in a crystal lattice by slipalong an existing crystallographic plane calledtranslation gliding

0 Slip systems- is the specific planes which slip occurs.0 Deformation of crystal lattices may occur by slip

segment of the lattice in the crystallographic planes,producing strain-induced twinning of the lattice,another form of translation called twin gliding.

0 This two type of mechanisms are importantdeformation mechanism in rock at low temperatureand pressure.


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Creep Processes

0 Creep processes is rate dependent, but strain is not limitedby deformation rate.

0 Creep mechanisms may involve mass transport or diffusion

of atoms or ions at grain boundaries, glide, climb ofdislocations within a lattice, and diffusion of point defectsthrough the lattices

0 Each is a separate mechanism that is most efficient over aparticular range of temperature and pressure.

0 These processes also overlap and compete with each otherand with pressure solution under the right conditions sothat more than one mechanism may occur simultaneously,although one usually dominates


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0 Deformation map- is a way of representing the dominant deformationmechanism in a material loaded under a given set of conditions andthereby its likely failure mode.

0 Grain boundary diffusion creep- involves mass transport throughdiffusion at grain boundaries. As temperature increases, diffusionprocesses become more efficient and pressure solution no longer is thedominant is the dominant mass transport mechanism.

0 Dislocation creep- dislocations are produced by strain and are removedthrough combination of glide and climb motion of the dislocationsthrough the crystal lattices, occuring at moderate to high temperature.

0 Dislocation are constantly produced and migrate through the lattice as

the rock is strained, causing each crystal to change shape.0 At low temperature end of the process, dislocations may pinned at grain

boundaries and other discontinuities within the rock. As a result, therock mass becomes strain-hardened and greater strain is required toincrease the amount of strain further.

0 At higher temperature, the amount of energy permits dislocation bypass obstacles by climbing or slipping from one lattice to another.


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0 Volume diffusion or Nabarro Herring creep-it

involves diffusion of point defects through crystals

and occurs at high temperature and low stress. It

would overlap and compete at lower temperature

with grain boundary diffusion, but it is a very slow

process even at high temperature and cannot be

considered an important deformation mechanism in

crustal rocks.


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Grain Boundary Sliding

0 The slip of grains past one another, as sand grains or

ball bearings move when a stick is pushed into them.

0 It occurs in association with coble creep at moderate

to high temperature.


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Superclastic flow

0 Occur by combination of grain boundary sliding andflow.

0 A single grain being deformed by a creep mechanism

will assume the shape of the entire mass of grains.0 If the mechanism is grain boundary sliding, a single

grain will maintain its initial shape even though theshape of the aggregates changes significantly.


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Unrecovered strain,Recovery and

recrystallization0 Subgrains- small parts of grains with lattice orientations

differing in adjacent parts of the same grain.

0 Low angle boundaries- boundaries between subgrains andold strained grains which indicates only a slight latticemisorientation across the boundary.

0 In a crystal being deformed by dislocation glide and climb,steady-state flow may be accomodated by one or twomechanisms: dynamic recovery and dynamicrecrystallization


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0 Recovery- is a term used to describe processes that reducedislocation density and dislocation interaction andincrease the glide and climb in a lattice being deformed by

dislocation creep.0 The main difference between the two is that dynamic

recrystallization involves strain softening and dynamicrecovery does not.

0 Recovery occurs at higher strain rates; Recrystallization

occurs at lower strain rates and higher temepratures.

chapter 7 - deformation mechanisms

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