mp 202 l2 pressure and stress

8/4/2019 MP 202 L2 Pressure and Stress

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Metamorphic Petrology 202Igneous and Metamorphic Petrology 242

Ian Fitzsimons

COMMONWEALTH OF AUSTRALIA

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Pressure and stress are the SAME


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Both are the force applied on amaterial per unit area

Measured in N/m2 (= Pascals or Pa) Geologists also use kilobars (kbar)

where 1 bar is atmospheric pressure

(105 N/m2)


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(105 N/m2)

Pressure (or stress) on geologicalmaterials can be divided into two

types:


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(105 N/m2)

Pressure (or stress) on geologicalmaterials can be divided into two

types: NON-DEVIATORIC DEVIATORIC


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Pressure is the same in all directions(1 = 2 = 3)

Also known as LITHOSTATIC orCONFINING pressure where:

P = 1/3 (1 + 2 + 3)P = 1 = 2 = 3

Depends on depth of burial: P = zg

where (density) 2800 kg/m3

and g (gravity) = 9.81 m/s2


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Lithostatic pressure increases as rocks are BURIED anddecreases as they are EXHUMED

Pressure increases by 1 kbar for each 3.5 km of burial Pressure at base of 35 km thick crust is 10 kbar

(or 1,000 MPa, or 1 GPa)


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EFFECT ON MINERALOGY?


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EFFECT ON MINERALOGY? As lithostatic pressure increases,

minerals stable at low pressure are

replaced by minerals stable at high

pressure (e.g. kyanite replaces

andalusite, diamond replaces

graphite)


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EFFECT ON FABRIC?


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EFFECT ON FABRIC?

Because pressure is the same inALL directions, burial will not

cause minerals to align, or rocks to

develop a foliation or lineation


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EFFECT ON FABRIC?

Because pressure is the same inALL directions, burial will not

cause minerals to align, or rocks to

develop a foliation or lineation

NO EFFECT ON FABRIC!


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Pressure is NOT the same in alldirections (123)

Also known as DIRECTED orDIFFERENTIAL pressure

Considered as three PIRINCIPALSTRESSES: 1 = maximum compressive stress 3 = minimum compressive stress

Caused by tectonic forces within thecrust (extension, compression etc.)


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Has values of zero under lithostaticconditions (no tectonic forces)

Magnitude of (1-3) can neverexceed tensile strength of rock

Widely believed that rocks cannotsustain (1-3) values more thanabout 0.1 kbar

Except at the Earths surface, the magnitude of deviatoric

stress is SMALL compared to lithostatic stress


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Changes in deviatoric stress atfixed depth will have little or no

effect on mineralogy as total

pressure is hardly changed

P = 1/3 (1+2+3)


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Changes in deviatoric stress atfixed depth will have little or no

effect on mineralogy as total

pressure is hardly changed

P = 1/3 (1+2+3)

But some geologists arguedeviatoric stress can be as large

as 1 kbar in some situations

(CONTROVERSIAL)


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EFFECT ON FABRIC?


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EFFECT ON FABRIC?

Causes rocks to deform andminerals to align

Responsible for metamorphicfoliations, lineations, and folds

Deviatoric stress is the mostimportant factor controlling rock

fabric


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EFFECT ON FABRIC?

Foliations form perpendicular to 1 Lineations form parallel to 3


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EFFECT ON FABRIC?

Foliations form perpendicular to 1 Lineations form parallel to 3

1

3


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EFFECT ON FABRIC?

Foliations form perpendicular to 1 Lineations form parallel to 3Rocks with a foliation ONLY:

1 > 2 = 3 (S tectonite)Rocks with a lineation ONLY:

1 = 2 > 3 (L tectonite)Rocks with foliation and lineation:

1 > 2 > 3 (LS tectonite)(SIMPLISTIC - ignores simple shear)

1

3


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The terms STRUCTURE, FABRIC, and TEXTURE are all

used to describe features in metamorphic rocks


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STRUCTURE is the COMPLETE SPATIAL andGEOMETRICAL CONFIGURATION of all those

components that make up a rock


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FABRIC is the PENETRATIVE SPATIAL andGEOMETRICAL CONFIGURATION of all those



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FABRIC is the PENETRATIVE SPATIAL andGEOMETRICAL CONFIGURATION of all those


TEXTURE refers to the SIZE, SHAPE andARRANGEMENT of mineral grains


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Some features are structures but not fabrics or textures:


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Some features are structures but not fabrics or textures: Faults, shear zones, veins


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Some features are structures & fabrics but not textures:


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Some features are structures & fabrics but not textures: Foliations, lineations


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Some features form part of the structure, fabric ANDtexture of a rock:


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Grain size, grain shape


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Grain size, grain shapeALL ROCKS HAVE FABRIC, TEXTURE & STRUCTURE


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SCALE OF FABRICS

REGIONAL or MEGA fabrics occur on scale of 10s to100s of km

MESO fabrics occur of the scale of an outcrop or handspecimen

MICRO fabrics are only observable under themicroscope

Many fabrics are SCALE INDEPENDENT the smallscale fabrics mimic large scale fabrics


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0.5 mm


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Satellite image of a pluton

in a shear zone

Sketch of a garnet

porphyroblast

0.5 mm


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Why study metamorphic fabrics?


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Record likely stress field at timethe rock was metamorphosed


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1

1


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Record whether minerals are arein equilibrium or not


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Not in equilibrium:calcite and wollastonite

never in contact

In equilibrium: calcite, quartzand wollastonite all havemutual boundaries


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Provide a relative time frameworkfor metamorphic mineral growth


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Provide a relative time frameworkfor metamorphic mineral growth

History of rock1. Biotite growth2. Chlorite growth3. Fault


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DESCRIPTIVE TERMINOLOGY


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S is used to label foliations


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S is used to label foliations S0 = bedding or igneous foln


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S is used to label foliations S0 = bedding or igneous foln S1 = first metamorphic foln

S1


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S is used to label foliations S0 = bedding or igneous foln S1 = first metamorphic foln S2 = second metamorphic folnS1

S2


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S is used to label foliations S0 = bedding or igneous foln S1 = first metamorphic foln S2 = second metamorphic foln

L is used to label lineations L0, L1, L2 etc.

F is used to label folds F0, F1, F2 etc.

S1, L1, F1 refer to foliation, lineationand fold formed at the SAME timeduring deformation event D1

S1

S2


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MULTIPLE CLEAVAGES


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MULTIPLE CLEAVAGES

D1 produces S1 cleavageperpendicular to maximum

compressive stress


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MULTIPLE CLEAVAGES


compressive stress

D2 folds the S1 cleavage to formF2 folds (CRENULATIONS) and

a second S2 cleavage

(CRENULATION CLEAVAGE)


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MULTIPLE CLEAVAGES


compressive stress

D2 folds the S1 cleavage to formF2 folds (CRENULATIONS) and

a second S2 cleavage

(CRENULATION CLEAVAGE)Crenulation

Cleavage Crenulation


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Changes in lithostatic pressure result from burialand exhumation, and cause changes inmineralogy but no changes in fabric

Changes in directed pressure result fromtectonic forces, and cause changes in fabric but

no changes in mineralogy

Metamorphic fabrics provide a record of ancientstress fields, evidence for equilibrium, and

relative time frameworks for describingmetamorphic histories

mp 202 l2 pressure and stress

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