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1B ESA > Maps and Structures (L1-8) > Flashcards

Maps and Structures (L1-8) Flashcards

1
Q

Define drift lithologies

A

Quaternary deposits that obscure the bedrock geology

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2
Q

Define solid lithologies

A

Pre-Quaternary units

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3
Q

In the generalised vertical section of a map, what do wavy lines indicate?

A

An unconformity

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4
Q

In the generalised vertical section of a map, what is the meaning of a wedge shape unit?

A

The unit is only present over part of the map area

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5
Q

What are the kinds of stratigraphy? (6)

A 
Litho
Bio
Chrono
Magneto
Chemo
Cyclo
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6
Q

Which structures are looked for in ‘structural geology’? (6)

A 
Faults
Folds
Bedding orientations
Foliations
Lineations
Lines of intersection between other structures
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7
Q

How is planar data represented? (2)

A

Strike/dip
OR
dip direction/dip

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8
Q

How is linear data represented? (1)

A

Plunge/trend

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9
Q

How can the profile shape of folds be estimated from bedding pole dispersion on stereonets? (2)

A

Poles in two clusters = angular fold

Poles spread along a great circle = rounded fold

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10
Q

How can the interlimb angle of a fold be estimated from bedding pole dispersion on stereonets? (2)

A

ILA = 180 - x

Where x is the largest angle between two bedding poles on the great circle

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11
Q

How can the fold axial plane be estimated on a stereonet? (1)
When may this be very wrong? (1)

A

A great circle containing the fold axis and the ILA bisector

Wrong for asymmetric folds and overturned limbs

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12
Q

What is stress?

A

Stress = force/area

Unit: Pa

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13
Q

What happens to stress when a force acts obliquely to the plane? (2)

A

Normal component: maximum at 0, minimum at 90

Shear component: minimum at 0 and 90, maximum at 45

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14
Q

When the three principal stress axes are equal, what is the state of stress?
What kind of change can/can’t be brought about by this stress state?

A

Hydrostatic (shear stress = 0)

Causes volume change, not shape change

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15
Q

In rocks at depth, where does the stress come from? (2)

A

Weight of overlying rocks

Stress = ρgh

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16
Q

What is a system with unequal principal stresses broken down into? (2)

A

Mean stress P = (σ1 + σ2 + σ3)/3 = hydrostatic component stress field
Remainder = deviatoric stress

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17
Q

What does deviatoric stress control?

A

Shape change

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18
Q

Define strain

A

Change in size and shape of a body resulting from an applied stress field

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19
Q

Define homogeneous strain
What is the result?
What is the converse?

A

Strain in all parts of a body is equal
Straight lines remain so, parallel lines remain so, identically shaped and oriented objects remain so
Heterogeneous strain

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20
Q

Define deformation

A

The transformation from an initial to a final geometry by means of a rigid body translation, rigid body rotation and strain

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21
Q

What is extensional strain (extension)?

How is extension related to stretch?

A 
e = (l - l(0))/l(0)
s = 1 + e
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22
Q

What is shear strain (γ)? (2)

A

γ = tan(ψ)

ψ is the deflection of an original right angle

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23
Q

What does the strain ellipsoid represent? (2)

A

Homogeneous deformation in 3D

Deformed shape of an imaginary sphere

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24
Q

Give an example of a strain marker

A

Deformed reduction spots in slate

Assuming they start life as a sphere

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25
Q

What is pure shear?

A

The orientation of the principal strain axes don’t change during deformation

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26
Q

What is simple shear?

A

Lines parallel to the principal strain axes rotate away

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27
Q

Define rheology (2)

A

Study of flow

Linking the response of a material to the forces acting upon it

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28
Q

Outline elastic rheology (4)

A

Linear stress-strain relationship
Gradient = E = Young’s modulus
Instant response to stress
Non-permanent strain

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29
Q

Outline viscous rheology (4)

A

Linear stress-strain rate
Gradient = η = viscosity
Time-dependent response to stress: more time = more strain
Permanent strain

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30
Q

In viscoelastic rheology, how do elastic and viscous components compare as a function of time? (4)

A

Elastic dominates on short time scales
Viscous dominates on long time scales
Importance is given by Maxwell time = 2η/E
When t > tM, material mainly deforms in a viscous manner

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31
Q

What does power law creep match?
What is power law creep?
What is the significance of n? (2)

A

Ductile deformation behaviour of rocks
ė = A σ^n exp[-Q/RT]
n = 3 in lithospheric mantle
n = 1 in asthenospheric mantle

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32
Q

What can folds be viewed as?

A

A stacked series of deformed surfaces

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33
Q

In a 2D cut of a fold, what are the key points on each surface?
What happens to these points in 3D?

A

The hinge points (maximum curvature) and inflexion points (curvature changes sense)
Points join to form hinge lines and inflexion lines

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34
Q

What is maintained in a cylindrical fold?

A

The same shape in successive profile planes perpendicular to the fold axis

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35
Q

What is fold size specified by?

A

Amplitude and wavelength of the fold train

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36
Q

What is fold attitude specified by? (2)

A

Dip and strike of the axial surface

Plunge and trend of the fold hinge

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37
Q

Define closure direction

Define facing direction

A

The direction in which the limbs converge towards the hinge

The direction in which the strata get younger

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38
Q

How do an anticline and a syncline differ?

A

Anticline has the oldest rocks in the centre, a syncline has the youngest rocks in the centre

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39
Q

How do an antiform and a synform differ?

A

Antiforms close upwards, synforms close downwards

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40
Q

Outline the quantitative descriptors of interlimb angle (4)

A

Isoclinal: 0-30
Tight: 30-70
Open: 70-120
Gentle: 120-180

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41
Q

Outline the quantitative descriptors of fold attitude (3)

A

Based on dip of axial surface and plunge of fold axis
Plunge variation: horizontal, plunging, vertical
Dip variation: Upright, inclined, recumbent

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42
Q

Define reclined folds

A

Fold hinges plunging down dip in the axial plane

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43
Q

Outline the rigorous scheme of fold classification (5)

A

Drawing dip isogons: lines joining points of equal dip on adjacent folded surfaces
Class 1 folds have convergent isogons
Class 2 folds have parallel isogons
Class 3 folds have divergent isogons
Can also be looked at by layer thickness variations: parallel folds (Class 1B) show constant thickness, whereas similar folds thicken at the hinge zone

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44
Q

Alternating Class 1B and Class 3 folds can be seen as what?

A

Class 2 overall (which is stackable)

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45
Q

What are parasitic folds? (2)

Outline their relationships (3)

A

Z-, M- and S- folds
Minor folds within a larger fold structure
Z- and S- folds are asymmetric
Z- and S- folds verge towards the hinge zone
Symmetric M-folds are located in the hinge zone

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46
Q

How are parasitic folds formed? (3)

A

Less competent layer above is previously folded by buckling
Competent layer below is folded by buckling
Layer above presents as parasitic folds

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47
Q

What are the three mechanisms of fold formation? (3)

A

Buckling
Bending
Passive folding

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48
Q

When does buckling occur?
What does buckling require?
Buckled folds of competent layers are usually which Class?

A

When a competent layer in a less competent matrix is shortened parallel to the length of the layer
Asperities on which folds can nucleate
Class 1B

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49
Q

How does layer thickness effect buckling?

A

Thicker layers fold with longer wavelength

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50
Q

When do folds die out?
What does this mean for nearby layers?
How can layers be forced to fold as one?

A

Over a depth less than their wavelength
Nearby layers fold independently
When layers are very close they act as a single layer

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51
Q

How does viscosity contrast effect buckling? (2)

A

Decreasing viscosity/strength contrast reduces wavelength and changes the type of folding
In general, decreasing strength contrasts makes folding less important and layer parallel shortening more so

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52
Q

What kind of folds are produced with different strength contrasts? (3)

A 
High = ptygmatic fold
Intermediate = general folding
Small = cuspate-lobate folds
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53
Q 
When does bending occur to produce folds?
Give examples (3)
A

Forces act across layers at a high angle
Fault bend folds in extensional grabens or above contractional thrust ramps
Monoclinal fold above fault propagation
Forceful intrusions of magma or salt diapir exhumation

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54
Q 
When does passive folding occur?
What does layering serve as?
Which fold Class is produced?
When is it more common?
Where is it common?
A

The layering exerts no mechanical influence on the folding (i.e. no competence contrast)
Layering is a visual expression of strain
Class 2 similar folds
More common at higher temperatures and in monomineralic rocks
Common in ductile shear zones

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55
Q

The strain within single layer folds is taken up in which three ways? (3)

A

Flexural slip: layer-parallel shear on discrete internal bedding or lamination surfaces
Flexural flow: distributed layer-parallel simple shear where lamination is weaker
Tangential-longitudinal strain: layer is homogeneous and only weakly ductile

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56
Q

What does tangential-longitudinal strain lead to?

A

Tension in outer arc and compression in the inner arc

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57
Q

In flexural slip, what happens to the strain near the hinge zone?
When is it more typical?

A

Strain dies out

Anisotropic layers such as shale

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58
Q

Where does flexural flow typically occur?

A

Deeper in the crust at higher temperatures

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59
Q

What are the special fold shapes? (5)

A 
Kink band
Chevron folds
Monocline
Concentric folds
Box fold
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60
Q

Define rock fabric

A

The geometrical arrangement of its constituent grains or structures that is penetrative at most scales of observation

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61
Q

How do primary and secondary rock fabrics differ? (2)

A

Primary: sedimentary or igneous in origin
Secondary: tectonic in origin

62
Q

What are the three general types of rock fabric? (3)

A

Planar
Linear
Random

63
Q

What are the three special types of homogeneous strain? (3)

A

Axially symmetric extension
Plane strain
Axially symmetric shortening

64
Q

How do L and S tectonites differ? (2)

A

L-tectonite: Axially symmetric extension, constrictional strain, L = lineation
S-tectonite: axially symmetric shortening, flattening strain, S = schist

65
Q

What is the purpose of the Flinn plot?

What is the distance from the origin indicative of?

A

Input the ratios of principal strain axes to plot the strain ellipsoid shapes
Intensity

66
Q

How is the K parameter in a Flinn plot used to define strain?

A

K = infinity: axially symmetric extension

1

67
Q

What do foliations describe?
What are the ways to distinguish foliations?
What are spaced foliations split up into?

A

Any planar-curviplanar structure in a rock
Primary vs secondary, continuous vs spaced
Cleavage and microlithon domains

68
Q

In spaced foliations:
What are the varying relations between cleavage domains? (3)
What are the various shapes? (4)

A

Parallel, anastomosing, conjugate

Rough, smooth, wiggly, stylolytic

69
Q

Outline pressure solution cleavage

A

Anastomosing zones of stress removal of soluble grains

70
Q

Outline slaty cleavage

A

A planar realignment of existing grains with limited recrystallisation

71
Q

Outline crenulation cleavage

A

Microfolding of a pre-existing foliation or bedding

72
Q

Outline phyllitic cleavage

A

A surface sheen of recrystallised minerals but no visible new grains

73
Q

Outline schistosity

A

Recrystallised grains visible to naked eye

74
Q

Outline gneissosity

A

Spaced compositional banding due to diffusion or partial melting

75
Q

What is foliation development dependent on? (2)

A

Protolith composition

Temperature

76
Q

Outline the progressive foliation development of a typical mudstone (7)

A 
Compaction cleavage
Pencil cleavage
Slaty cleavage
Crenulation cleavage
Phyllitic cleavage
Schistosity
Gneissosity
77
Q

Define cleavage

A

The ability of a rock to split into parallel surfaces

78
Q

Give three examples of primary foliations (3)

A

Bedding
Fissility
Flow banding

79
Q

Define lineation

A

A fabric element with one dimension considerably longer than the other two

80
Q

How does lineation and foliation formation differ in response to stress? (2)

A

Foliations form at high angles to shortening

Lineations tend to form parallel to shear direction

81
Q

Give two examples of primary lineations (2)

A

Parting lineation

Flow lineation

82
Q

Name the three types of deformation-induced secondary lineations and define them (3)

A

Mineral: a continuous grain alignment fabric
Crenulation: a microfolding in which fold hinges are more prominent than axial planes
Intersection: intersection of two planar fabrics

83
Q

What determines the cleavage refraction in a folded structure?

A

The contrast in competency between layers

84
Q

Define composite fabric

A

Planar and linear components are both prominent

85
Q

Define isotropic fabric

When can these occur?

A

Randomly oriented grains or structures

Undeformed rocks or those recrystallised in a hydrostatic stress field

86
Q

Outline brittle and ductile deformation (3)

A

Sub-divisions of plastic deformation (permanent)
Brittle is discontinuous
Ductile is continuous

87
Q

Define creep

A

Slow ductile deformation by various lattice-scale mechanisms

88
Q

Define fractures

When do fractures form in the brittle regime?

A

Cracks across which the cohesion of material is lost

When a rock fails at a critical stress level

89
Q

What is a Mohr circle?

A

A way to describe σ_n and σ_s acting on planes of all possible orientations

90
Q

What can multiple stress circles on a Mohr diagram be used to define? (2)

A

A failure envelope

Which separates stable stress states from unstable ones

91
Q

In which two ways can brittle failure occur? (2)

A

A single set of tensile fractures (at low σ_1 and σ_3)

Paired sets of shear fractures (at higher σ_1 and σ_3)

92
Q

Outline the Coulomb failure criterion (3)

A

Predicts the state of stress at which a given rock under compression is at the verge of failure
Gives a linear failure envelope
σ_s = C + μ σ_n

93
Q

Outline the Griffith failure criterion (3)

A

Based on microscopic cracks, pores and flaws weakening rocks
Gives a parabolic failure envelope
σ_s^2 = 4T(T - σ_n)

94
Q

Outline the von Mises failure criterion (3)

A

Failure envelope flattens as ductile regime approached
Constant shear stress criterion
σ_s = constant

95
Q

Why is a composite failure envelope model necessary? (2)

A

Coulomb’s matches the compressive σ field, but not the tensile field
Griffith’s matches the tensile field but not the compressive field

96
Q

Why does the Mohr diagram rationalise why normal faults form at 60 degrees? (3)

A

Maximum shear stress when angle is 45, but normal stress is also large
Normal stress decreases faster than shear as the angle decreases
Optimal balance at 60 degrees

97
Q

Why do faults reactivate? (3)

A

Reactivation criterion is different to that of unfractured rocks
Reactivation stress is much smaller
So faults grow/accumulate displacement

98
Q

What is the effect of anisotropy on the failure envelope? (2)

A

Whether a rock fails along a foliation or a new fracture forms depends on foliation orientation relative to the stress field
Across-foliation > along-foliation

99
Q

What is the effect of increased pore fluid pressure on rock failure? (3)

A

Pore fluid pressure decreases the effective normal stress
Moves the Mohr circle left on the Mohr diagram
Can induce hydraulic failure

100
Q

Define joints

A

Brittle fractures with little or no visible displacement across them

101
Q

What types of joint are there? (3)

A

Extension: form perpendicular to the minimum principal stress σ_3
Shear: form conjugate with their acute bisector parallel σ_1
Hybrid joints

102
Q

How are joints often related to folds? (3)

A

Normal to bedding
Shear joints bisector is perpendicular to fold axis
Extension joints parallel to fold axis

103
Q

How is a fault structure formed in the brittle regime? (3)

A

Start as R and R’ shear fractures
Progressive deformation
Merge via P fractures to form through-going fault structures

104
Q

What is the brittle-ductile regime characterised by?

A

A duality of behaviours

105
Q

What is the ductile regime characterised by? (2)

A

Sigmoidal cleavage patterns that form at 45 degrees to the zone boundary
With a continuous variation in strain across the shear zone

106
Q

How do en echelon tension gashes form? (4)

A

During prograde metamorphism:
fluids expelled from rocks
pore fluid pressure builds up locally until hydraulic fracturing
ductile deformation while pore fluid pressure builds up again

107
Q

What are the consequences of en echelon tension gashes forming? (2)

A

Strain hardening

Widening of shear zone over time

108
Q

How do strain hardening and strain softening give different growth histories for shear zones? (2)

A

Strain hardening: deformation in centre slows, shear zone thickens, central part shows early deformation, marginal part shows final deformation
Strain softening: deformation localised in central part, margins are inactive, thin shear zones with high shear strain gradients

109
Q

When does strain hardening occur?
What does it mean for the stress needed to deform?
What can it result in?

A

When dislocations accumulate
Increased stress needed for a given strain rate
A transition to the brittle regime

110
Q

When does strain softening occur?

How does it affect deformation mechanisms?

A

Grain size is reduced, recrystallisation of new weaker minerals, the introduction of fluids or increase in temperature
More effective leading to strain localisation

111
Q

How can fault rocks be classified? (5)

A 
If >30% large clasts (>2mm) = fault breccia
If <30% large clasts and:
incohesive = fault gouge
cohesive and glassy = pseudotachylite
cohesive and non-foliated = cataclasite
cohesive and foliated = mylonite
112
Q

What is the main difference between mylonites and other fault rock types?

A

Mylonite formation involves ductile deformation

113
Q

Fault gouges are often seen with multiple colours, how did this come about?

A

Mineralisation as different fluids travelled through the loose and permeable material

114
Q

How does a pseudotachylyte form?

A

Forms by frictional melting during faulting

115
Q

How do S-C-C’ fabrics form in a mylonite? (6)

A

Strain accumulation
Set of slip surfaces develop parallel to walls of shear zone = C-planes
Foliations labelled S
Shear strain increases
S rotates into C-plane
New set of shear bands develop = C’ planes

116
Q

What is brittle flow?

What are the necessary conditions?

A

Deformation involving frictional sliding along grain contacts, grain rotation and grain fracture
Low T, high differential stress

117
Q

How do granular flow and cataclastic flow differ in mechanisms and in where they occur? (4)

A

Granular flow:
rotation + frictional sliding
very shallow levels deforming porous rocks/sediments
Cataclastic flow:
rotation + frictional sliding + microfracturing
non-porous and consolidated rocks

118
Q

What are deformation twins an expression of? (2)

A

Relatively low T, low finite strain mechanism

Stress results in mechanical bending of crystal lattice

119
Q 
Pressure solution mechanism:
Conditions?
What happens?
What does it depend on?
Where does it occur?
A

Low T
Mineral dissolved, ions carried in fluid and precipitated elsewhere
Higher stress = faster
Occurs along thin films of fluid along grain boundaries

120
Q

What are the two kinds of crystal defects? (2)

What is the term for the movement of those defects? (2)

A

Point defect: vacancies/impurities
movement = diffusion creep
Line defect: edge/screw dislocations
movement = dislocation creep

121
Q

Define dislocation

A

A mobile line defect that contributes to intracrystalline deformation by mechanisms of slip

122
Q

How do dislocation creep and diffusion creep differ in their T conditions? (2)

A

Diffusion is high T

Dislocation is medium T

123
Q

What are the two types of diffusion creep? (2)

A

Diffusion along grain boundaries = Coble creep

Diffusion within grains = Nabarro-Herring creep

124
Q

How can deformation of quartzo-feldspathic rocks be used as a guide for deformation temperature? (3)

A

Quartz deforms ductilely above 300
Feldspar deforms ductilely above 500
Interim T = contrasting behaviours

125
Q

Outline Anderson’s theory of faulting (principal stresses) (5)

A 
Earth's surface has no shear stress
So one principal stress is vertical
σ_v = σ_1 = extensional
σ_v = σ_2 = strike-slip
σ_v = σ_3 = contractional
126
Q

What are the main tectonic settings hosting crustal contraction? (4)

A

Continental collision zones
Foreland thrust belts
Accretionary prisms
Compressional subduction arcs

127
Q

What are the more localised settings hosting crustal contraction? (2)

A

Transpression zones

Toe of advancing deltas/glaciers/gravity slides

128
Q

How can shortening be accommodated in crustal contraction? (4)

A

Volume loss (dissolution, compaction)
Pure shear (no viscosity contrast)
Buckling (viscosity contrast)
Thrust/reverse fault formation

129
Q

What is the difference between thick-skinned and thin-skinned crustal deformation? (2)

A

Thick-skinned involves the basement

Thin-skinned is underlain by low-dip detachment

130
Q

Which contractional structures are common in the upper crust? (3)

A

Reverse faults
Folds (often flexural slip)
Low-grade foliations (slate/phyllite)

131
Q

Which contractional structures are common in the lower crust? (3)

A

Ductile shear zones
Folds (often passive folds)
High-grade foliations (schist/gneiss) and lineations

132
Q

What is an imbrication zone?

Which way are the horses younging?

A

Series of similarly oriented reverse faults connected through a floor thrust
Younging in the foreland direction

133
Q

What is a thrust duplex?
Which way do they propagate?
How do the tectonic horses form?

A

An imbrication zone with a roof thrust
Towards the foreland
Successive formation of ramps in competent layers that act as stress guides

134
Q

Where are the main tectonic settings hosting crustal extension? (4)

A

Intracontinental rift zones
Oceanic ridges
Back-arc basins
Upper crustal levels of orogens

135
Q

Where are the more localised settings hosting crustal extension? (2)

A

Transtension zones

Heads of gravity slides

136
Q

Which extensional structures are common in the upper crust? (3)

A

Planar-listric normal faults
Extensional vein systems
Folds related to faults

137
Q

Which extensional structures are common in the lower crust? (2)

A

Ductile shear zones

High-grade foliations and lineations

138
Q

What are the purposes of transfer zones (relays)? (2)

A

A way to transfer deformation between fault structures

Control drainage

139
Q

What is shown in zones with planar normal faults? (2)

A

Major tilt blocks with uplifted footwalls and basins in the hangingwalls
Looks like fallen dominoes

140
Q

When a weak layer at base is absent, which system is favoured for extension? (3)

A

Symmetric horst-and-graben
Total extension and crustal thinning are the same
Pure shear

141
Q

Where are the main tectonic settings hosting crustal strike-slip? (4)

A

Plate boundary transform zones
Oceanic ridge transforms
Lateral to continental collision zones
Above subduction zones with oblique subduction

142
Q

How does strike-slip deformation differ when thick- or thin-skinned? (2)

A

Most is thick-skinned and involves basement

Thin-skinned occur as lateral ramps in thrust systems

143
Q

Which strike-slip structures are common in the upper crust? (3)

A

Strike-slip/oblique-slip faults and shear zones
Folds with oblique cleavage
Low-grade foliations and lineations

144
Q

Which strike-slip structures are common in the lower crust? (2)

A

Ductile strike-slip shear zones

High-grade fabrics, particularly low-plunge lineations

145
Q

How can strike-slip dominated zones by subdivided? (3)

A

Pure transcurrent zones
Transpressional zones
Transtensional zones

146
Q

What is shown in cross-section of a transpressional zone? (2)

A

Positive flower structure

Reverse-oblique faults that converge downwards

147
Q

What is shown in cross-section of a transtensional zone? (2)

A

Negative flower structure

Normal-oblique faults that converge downwards

148
Q

In map view, what can often be seen in low displacement transcurrent zones? (2)

A

En echelon folds

Simple Riedel shear patterns

149
Q

In map view, what can often be seen in high displacement transcurrent zones? (2)

A

Throughgoing displacment zone

With subsiding releasing bends and uplifting restraining bends

150
Q

In map view, what can sometimes be seen in mature transcurrent zones? (2)

A

Pull-apart basins at releasing bends

Push-up horsts at restraining bends

1B ESA (10 decks)

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