Week 2 Flashcards by Elle Kinsman

Forms of deformation

Translation

Rotation

Distortion

Dilation

(Latter two = strain, previous two can occur without strain)

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Is ACW +ve or -ve?

+ve

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Is compression +ve/-ve

+ve

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What is the finite strain ellipsoid?

3 mutually perpendicular axes (3D) and 3 principle planes

X>=Y>=Z

XY, XZ, YZ

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What is strain in 1 dimension?

Extension

= (ld-lo)/lo

e(x), e(y), e(z)
= principal longitudinal strain // to x/y/z

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Magnitude of strain =

axial ratio (ellipticity, R) of cross section containing X and Z (longest and shortest) axes

R(xz) = X/Z = (1+e(x))/(1+e(z))

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Equation of strain ellipsoid

(x/1+e(x))^2 + (y/1+e(y))^2 + (z/1+e(z))^2 = 1

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How do you determine the orientation of the strain ellipsoid?

Plunge/azimuth of principle axes
(X = mineral stretching lineation)

Strike/dip/dip direction of principle planes
(XY = foliation/cleavage planes)

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Types of strain (+how many dimensions?)

PROLATE (3D)

OBLATE (3D)

PLANE (2D)

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PROLATE STRAIN

X > Y = Z

Lineation

L-tectonite

Linear fabric

‘rods’

// to x-axis of FSE

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OBLATE STRAIN

X = Y > Z

Foliation/cleavage

S-tectonite

‘sheets’

Planar fabric

// to XY plane of FSE

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PLANE STRAIN

X > Y > Z, Y = 1

Foliation and lineation (along foliation planes)

LS-tectonite

Planar and linear fabrics

‘Sheets with lineations on surface’

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What is a Flinn plot as a graph?

y-axis = R(xy) = X/Y

x-axis = R(yz) = Y/Z

Goes through the point (1,1)

k = 1

GREATER MAGNITUDES OF STRAIN PLOT FURTHER AWAY FROM ORIGIN

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k =

(R(xy)-1) / (R(yz)-1)

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Importance of y values in the Flinn plot

Prolate Y < 1 ; k=infinity

Oblate Y > 1; k=0

Plane Y = 1; k=1

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Where do different types of strain plot on the Flinn Plot?

Prolate to left of k=1 line

apparent constriction
L-tectonite

Plane on the k=1 line
- L-S tectonite

Oblate to the right of the k=1 line

apparent flattening
S-tectonite

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Volumetric strain equation

/\V= (V(d)-V(o)) / V(o)

Since volume of ellipsoid = 4/3 x pi x (XYZ)

Volume of sphere = 4/3 x pi

/\V = XYZ - 1

= (1+e(x)) (1+e(y)) (1+e(z)) -1

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/\V>0

Volume gain

e. g. veins/dykes/sills
- can identify X as EXTENDING

/\V<0

Volume loss

e. g. cleavage/stylolites
- can identify z as SHORTENING

/\V=0

Compensation

Strain;

how rock body has changed shape, vol, orientation and position during deformation

Kinematics;

how geometry of rock body and strain evolved during deformation

Dynamics;

orientation, magnitude and origin of forces/stresses that caused deformation

Why is structural geology important?

RECONSTRUCT EVENTS

deformation processes
plate motions, mountain building, basin formation

PETROLEUM

MINING

minerals/metals in veins
distribution of veins related to fractures and local/regional stresses

CONSTRUCTION + ENGINEERING

e.g. landslide risk
house foundations

Stress =

forces that cause the strain Stress = force/area

Confining pressure;

force equal in all direction

Differential stress;

force not equal in all directions

Elastic deformation =

rocks return to original state when stress removed

Permanent deformation =

preserved after stress relaxation

Brittle vs ductile deformation

Brittle = cracks/fracture/cold-shallow Ductile = flow/bend/hot-deep

Primary structures =

related to original deposition and formation of rock

Secondary structures =

formed by deformation (modify primary) e.g. faults/folds/foliations

Kinematic predictions from geometric and evolutionary models

Approximates each limb of folds to stack of rigid layers (equivalent to "beds") Layers are free to slide past each other (Follow evolutionary model as they do this) GEOLOGICAL REALITY CHECK

Kinematic and stress predictions from mechanical models

Approximates each bed to a series of "elements" with stresses acting along their edges Use computer simulation/mathematical solution = predict how shape/vol/position/orientation of each element and therefore over fold shape will change GEOLOGICAL REALITY CHECK

Fundamental challenges in structural geology

Parts of structure missing = difficult to define geometry/quantify strain We don't see intermediate stages = difficult to test evolutionary model/kinematic predictions Do not know state of stress at time of deformation = difficult to verify mechanical models Do not know material properties at time of deformation = above

Which axis of the FSE is mineral stretching lineation parallel to?

X axis

Which plane of the FSE is foliation/cleavage // to?

XY plane

Rxz =

Rxy x Ryz

Assumptions when applying the Flinn plot to find the type of strain

Assume strain recorded by (e.g. deformed quartz pebbles) reflects the bulk finite strain experienced by the surrounding rock i.e. strain was homogeneous - reasonable if pebbles and host rock have same composition Original shape of particle = sphere

Where would a veined rock plot on the Flinn plot? What is /\V?

/\V>0 Plot along k=infinity line

Where would a cleaved rock plot on the Flinn plot? What is /\V?

/\V<0 Plot along k=0 line

Where would a veined and cleaved rock plot on the Flinn plot? What is /\V?

/\V = 0 Plot along k=1 line

What does it mean if, across a shear zone, data plots will progressively lower values of K with greater strain?

RELATIVE lengths of X, Y and Z are not constant across the shear zone