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Structural > Week 2 > Flashcards

Week 2 Flashcards

1
Q

Forms of deformation

A

Translation

Rotation

Distortion

Dilation

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

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2
Q

Is ACW +ve or -ve?

A

+ve

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3
Q

Is compression +ve/-ve

A

+ve

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4
Q

What is the finite strain ellipsoid?

A

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

X>=Y>=Z

XY, XZ, YZ

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5
Q

What is strain in 1 dimension?

A

Extension

= (ld-lo)/lo

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

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6
Q

Magnitude of strain =

A

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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7
Q

Equation of strain ellipsoid

A

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

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8
Q

How do you determine the orientation of the strain ellipsoid?

A

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

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

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9
Q

Types of strain (+how many dimensions?)

A

PROLATE (3D)

OBLATE (3D)

PLANE (2D)

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10
Q

PROLATE STRAIN

A

X > Y = Z

Lineation

L-tectonite

Linear fabric

‘rods’

// to x-axis of FSE

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11
Q

OBLATE STRAIN

A

X = Y > Z

Foliation/cleavage

S-tectonite

‘sheets’

Planar fabric

// to XY plane of FSE

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12
Q

PLANE STRAIN

A

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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13
Q

What is a Flinn plot as a graph?

A

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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14
Q

k =

A

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

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15
Q

Importance of y values in the Flinn plot

A

Prolate Y < 1 ; k=infinity

Oblate Y > 1; k=0

Plane Y = 1; k=1

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16
Q

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

A

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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17
Q

Volumetric strain equation

A

/\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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18
Q

/\V>0

A

Volume gain

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

19
Q

/\V<0

A

Volume loss

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

20
Q

/\V=0

A

Compensation

21
Q

Strain;

A

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

22
Q

Kinematics;

A

how geometry of rock body and strain evolved during deformation

23
Q

Dynamics;

A

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

24
Q

Why is structural geology important?

A

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
25
Q

Stress =

A

forces that cause the strain

Stress = force/area

26
Q

Confining pressure;

A

force equal in all direction

27
Q

Differential stress;

A

force not equal in all directions

28
Q

Elastic deformation =

A

rocks return to original state when stress removed

29
Q

Permanent deformation =

A

preserved after stress relaxation

30
Q

Brittle vs ductile deformation

A

Brittle = cracks/fracture/cold-shallow

Ductile = flow/bend/hot-deep

31
Q

Primary structures =

A

related to original deposition and formation of rock

32
Q

Secondary structures =

A

formed by deformation (modify primary)

e.g. faults/folds/foliations

33
Q

Kinematic predictions from geometric and evolutionary models

A

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

34
Q

Kinematic and stress predictions from mechanical models

A

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

35
Q

Fundamental challenges in structural geology

A

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

36
Q

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

A

X axis

37
Q

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

A

XY plane

38
Q

Rxz =

A

Rxy x Ryz

39
Q

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

A

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

40
Q

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

A

/\V>0

Plot along k=infinity line

41
Q

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

A

/\V<0

Plot along k=0 line

42
Q

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

A

/\V = 0

Plot along k=1 line

43
Q

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

A

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

Structural (17 decks)

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