Maps and Structures (L1-8) Flashcards
1
Q
Define drift lithologies
A
Quaternary deposits that obscure the bedrock geology
2
Q
Define solid lithologies
A
Pre-Quaternary units
3
Q
In the generalised vertical section of a map, what do wavy lines indicate?
A
An unconformity
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
5
Q
What are the kinds of stratigraphy? (6)
A
Litho Bio Chrono Magneto Chemo Cyclo
6
Q
Which structures are looked for in ‘structural geology’? (6)
A
Faults Folds Bedding orientations Foliations Lineations Lines of intersection between other structures
7
Q
How is planar data represented? (2)
A
Strike/dip
OR
dip direction/dip
8
Q
How is linear data represented? (1)
A
Plunge/trend
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
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
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
12
Q
What is stress?
A
Stress = force/area
Unit: Pa
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
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
15
Q
In rocks at depth, where does the stress come from? (2)
A
Weight of overlying rocks
Stress = ρgh
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
17
Q
What does deviatoric stress control?
A
Shape change
18
Q
Define strain
A
Change in size and shape of a body resulting from an applied stress field
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
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
21
Q
What is extensional strain (extension)?
How is extension related to stretch?
A
e = (l - l(0))/l(0) s = 1 + e
22
Q
What is shear strain (γ)? (2)
A
γ = tan(ψ)
ψ is the deflection of an original right angle
23
Q
What does the strain ellipsoid represent? (2)
A
Homogeneous deformation in 3D
Deformed shape of an imaginary sphere
24
Q
Give an example of a strain marker
A
Deformed reduction spots in slate
Assuming they start life as a sphere
25
What is pure shear?
The orientation of the principal strain axes don't change during deformation
26
What is simple shear?
Lines parallel to the principal strain axes rotate away
27
Define rheology (2)
Study of flow
| Linking the response of a material to the forces acting upon it
28
Outline elastic rheology (4)
Linear stress-strain relationship
Gradient = E = Young's modulus
Instant response to stress
Non-permanent strain
29
Outline viscous rheology (4)
Linear stress-strain rate
Gradient = η = viscosity
Time-dependent response to stress: more time = more strain
Permanent strain
30
In viscoelastic rheology, how do elastic and viscous components compare as a function of time? (4)
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
31
What does power law creep match?
What is power law creep?
What is the significance of n? (2)
Ductile deformation behaviour of rocks
ė = A σ^n exp[-Q/RT]
n = 3 in lithospheric mantle
n = 1 in asthenospheric mantle
32
What can folds be viewed as?
A stacked series of deformed surfaces
33
In a 2D cut of a fold, what are the key points on each surface?
What happens to these points in 3D?
The hinge points (maximum curvature) and inflexion points (curvature changes sense)
Points join to form hinge lines and inflexion lines
34
What is maintained in a cylindrical fold?
The same shape in successive profile planes perpendicular to the fold axis
35
What is fold size specified by?
Amplitude and wavelength of the fold train
36
What is fold attitude specified by? (2)
Dip and strike of the axial surface
| Plunge and trend of the fold hinge
37
Define closure direction
| Define facing direction
The direction in which the limbs converge towards the hinge
| The direction in which the strata get younger
38
How do an anticline and a syncline differ?
Anticline has the oldest rocks in the centre, a syncline has the youngest rocks in the centre
39
How do an antiform and a synform differ?
Antiforms close upwards, synforms close downwards
40
Outline the quantitative descriptors of interlimb angle (4)
Isoclinal: 0-30
Tight: 30-70
Open: 70-120
Gentle: 120-180
41
Outline the quantitative descriptors of fold attitude (3)
Based on dip of axial surface and plunge of fold axis
Plunge variation: horizontal, plunging, vertical
Dip variation: Upright, inclined, recumbent
42
Define reclined folds
Fold hinges plunging down dip in the axial plane
43
Outline the rigorous scheme of fold classification (5)
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
44
Alternating Class 1B and Class 3 folds can be seen as what?
Class 2 overall (which is stackable)
45
What are parasitic folds? (2)
| Outline their relationships (3)
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
46
How are parasitic folds formed? (3)
Less competent layer above is previously folded by buckling
Competent layer below is folded by buckling
Layer above presents as parasitic folds
47
What are the three mechanisms of fold formation? (3)
Buckling
Bending
Passive folding
48
When does buckling occur?
What does buckling require?
Buckled folds of competent layers are usually which Class?
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
49
How does layer thickness effect buckling?
Thicker layers fold with longer wavelength
50
When do folds die out?
What does this mean for nearby layers?
How can layers be forced to fold as one?
Over a depth less than their wavelength
Nearby layers fold independently
When layers are very close they act as a single layer
51
How does viscosity contrast effect buckling? (2)
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
52
What kind of folds are produced with different strength contrasts? (3)
```
High = ptygmatic fold
Intermediate = general folding
Small = cuspate-lobate folds
```
53
```
When does bending occur to produce folds?
Give examples (3)
```
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
54
```
When does passive folding occur?
What does layering serve as?
Which fold Class is produced?
When is it more common?
Where is it common?
```
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
55
The strain within single layer folds is taken up in which three ways? (3)
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
56
What does tangential-longitudinal strain lead to?
Tension in outer arc and compression in the inner arc
57
In flexural slip, what happens to the strain near the hinge zone?
When is it more typical?
Strain dies out
| Anisotropic layers such as shale
58
Where does flexural flow typically occur?
Deeper in the crust at higher temperatures
59
What are the special fold shapes? (5)
```
Kink band
Chevron folds
Monocline
Concentric folds
Box fold
```
60
Define rock fabric
The geometrical arrangement of its constituent grains or structures that is penetrative at most scales of observation
61
How do primary and secondary rock fabrics differ? (2)
Primary: sedimentary or igneous in origin
Secondary: tectonic in origin
62
What are the three general types of rock fabric? (3)
Planar
Linear
Random
63
What are the three special types of homogeneous strain? (3)
Axially symmetric extension
Plane strain
Axially symmetric shortening
64
How do L and S tectonites differ? (2)
L-tectonite: Axially symmetric extension, constrictional strain, L = lineation
S-tectonite: axially symmetric shortening, flattening strain, S = schist
65
What is the purpose of the Flinn plot?
| What is the distance from the origin indicative of?
Input the ratios of principal strain axes to plot the strain ellipsoid shapes
Intensity
66
How is the K parameter in a Flinn plot used to define strain?
K = infinity: axially symmetric extension
| 1
67
What do foliations describe?
What are the ways to distinguish foliations?
What are spaced foliations split up into?
Any planar-curviplanar structure in a rock
Primary vs secondary, continuous vs spaced
Cleavage and microlithon domains
68
In spaced foliations:
What are the varying relations between cleavage domains? (3)
What are the various shapes? (4)
Parallel, anastomosing, conjugate
| Rough, smooth, wiggly, stylolytic
69
Outline pressure solution cleavage
Anastomosing zones of stress removal of soluble grains
70
Outline slaty cleavage
A planar realignment of existing grains with limited recrystallisation
71
Outline crenulation cleavage
Microfolding of a pre-existing foliation or bedding
72
Outline phyllitic cleavage
A surface sheen of recrystallised minerals but no visible new grains
73
Outline schistosity
Recrystallised grains visible to naked eye
74
Outline gneissosity
Spaced compositional banding due to diffusion or partial melting
75
What is foliation development dependent on? (2)
Protolith composition
| Temperature
76
Outline the progressive foliation development of a typical mudstone (7)
```
Compaction cleavage
Pencil cleavage
Slaty cleavage
Crenulation cleavage
Phyllitic cleavage
Schistosity
Gneissosity
```
77
Define cleavage
The ability of a rock to split into parallel surfaces
78
Give three examples of primary foliations (3)
Bedding
Fissility
Flow banding
79
Define lineation
A fabric element with one dimension considerably longer than the other two
80
How does lineation and foliation formation differ in response to stress? (2)
Foliations form at high angles to shortening
| Lineations tend to form parallel to shear direction
81
Give two examples of primary lineations (2)
Parting lineation
| Flow lineation
82
Name the three types of deformation-induced secondary lineations and define them (3)
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
What determines the cleavage refraction in a folded structure?
The contrast in competency between layers
84
Define composite fabric
Planar and linear components are both prominent
85
Define isotropic fabric
| When can these occur?
Randomly oriented grains or structures
| Undeformed rocks or those recrystallised in a hydrostatic stress field
86
Outline brittle and ductile deformation (3)
Sub-divisions of plastic deformation (permanent)
Brittle is discontinuous
Ductile is continuous
87
Define creep
Slow ductile deformation by various lattice-scale mechanisms
88
Define fractures
| When do fractures form in the brittle regime?
Cracks across which the cohesion of material is lost
| When a rock fails at a critical stress level
89
What is a Mohr circle?
A way to describe σ_n and σ_s acting on planes of all possible orientations
90
What can multiple stress circles on a Mohr diagram be used to define? (2)
A failure envelope
| Which separates stable stress states from unstable ones
91
In which two ways can brittle failure occur? (2)
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
Outline the Coulomb failure criterion (3)
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
Outline the Griffith failure criterion (3)
Based on microscopic cracks, pores and flaws weakening rocks
Gives a parabolic failure envelope
σ_s^2 = 4T(T - σ_n)
94
Outline the von Mises failure criterion (3)
Failure envelope flattens as ductile regime approached
Constant shear stress criterion
σ_s = constant
95
Why is a composite failure envelope model necessary? (2)
Coulomb's matches the compressive σ field, but not the tensile field
Griffith's matches the tensile field but not the compressive field
96
Why does the Mohr diagram rationalise why normal faults form at 60 degrees? (3)
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
Why do faults reactivate? (3)
Reactivation criterion is different to that of unfractured rocks
Reactivation stress is much smaller
So faults grow/accumulate displacement
98
What is the effect of anisotropy on the failure envelope? (2)
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
What is the effect of increased pore fluid pressure on rock failure? (3)
Pore fluid pressure decreases the effective normal stress
Moves the Mohr circle left on the Mohr diagram
Can induce hydraulic failure
100
Define joints
Brittle fractures with little or no visible displacement across them
101
What types of joint are there? (3)
Extension: form perpendicular to the minimum principal stress σ_3
Shear: form conjugate with their acute bisector parallel σ_1
Hybrid joints
102
How are joints often related to folds? (3)
Normal to bedding
Shear joints bisector is perpendicular to fold axis
Extension joints parallel to fold axis
103
How is a fault structure formed in the brittle regime? (3)
Start as R and R' shear fractures
Progressive deformation
Merge via P fractures to form through-going fault structures
104
What is the brittle-ductile regime characterised by?
A duality of behaviours
105
What is the ductile regime characterised by? (2)
Sigmoidal cleavage patterns that form at 45 degrees to the zone boundary
With a continuous variation in strain across the shear zone
106
How do en echelon tension gashes form? (4)
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
What are the consequences of en echelon tension gashes forming? (2)
Strain hardening
| Widening of shear zone over time
108
How do strain hardening and strain softening give different growth histories for shear zones? (2)
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
When does strain hardening occur?
What does it mean for the stress needed to deform?
What can it result in?
When dislocations accumulate
Increased stress needed for a given strain rate
A transition to the brittle regime
110
When does strain softening occur?
| How does it affect deformation mechanisms?
Grain size is reduced, recrystallisation of new weaker minerals, the introduction of fluids or increase in temperature
More effective leading to strain localisation
111
How can fault rocks be classified? (5)
```
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
What is the main difference between mylonites and other fault rock types?
Mylonite formation involves ductile deformation
113
Fault gouges are often seen with multiple colours, how did this come about?
Mineralisation as different fluids travelled through the loose and permeable material
114
How does a pseudotachylyte form?
Forms by frictional melting during faulting
115
How do S-C-C' fabrics form in a mylonite? (6)
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
What is brittle flow?
| What are the necessary conditions?
Deformation involving frictional sliding along grain contacts, grain rotation and grain fracture
Low T, high differential stress
117
How do granular flow and cataclastic flow differ in mechanisms and in where they occur? (4)
Granular flow:
rotation + frictional sliding
very shallow levels deforming porous rocks/sediments
Cataclastic flow:
rotation + frictional sliding + microfracturing
non-porous and consolidated rocks
118
What are deformation twins an expression of? (2)
Relatively low T, low finite strain mechanism
| Stress results in mechanical bending of crystal lattice
119
```
Pressure solution mechanism:
Conditions?
What happens?
What does it depend on?
Where does it occur?
```
Low T
Mineral dissolved, ions carried in fluid and precipitated elsewhere
Higher stress = faster
Occurs along thin films of fluid along grain boundaries
120
What are the two kinds of crystal defects? (2)
| What is the term for the movement of those defects? (2)
Point defect: vacancies/impurities
movement = diffusion creep
Line defect: edge/screw dislocations
movement = dislocation creep
121
Define dislocation
A mobile line defect that contributes to intracrystalline deformation by mechanisms of slip
122
How do dislocation creep and diffusion creep differ in their T conditions? (2)
Diffusion is high T
| Dislocation is medium T
123
What are the two types of diffusion creep? (2)
Diffusion along grain boundaries = Coble creep
| Diffusion within grains = Nabarro-Herring creep
124
How can deformation of quartzo-feldspathic rocks be used as a guide for deformation temperature? (3)
Quartz deforms ductilely above 300
Feldspar deforms ductilely above 500
Interim T = contrasting behaviours
125
Outline Anderson's theory of faulting (principal stresses) (5)
```
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
What are the main tectonic settings hosting crustal contraction? (4)
Continental collision zones
Foreland thrust belts
Accretionary prisms
Compressional subduction arcs
127
What are the more localised settings hosting crustal contraction? (2)
Transpression zones
| Toe of advancing deltas/glaciers/gravity slides
128
How can shortening be accommodated in crustal contraction? (4)
Volume loss (dissolution, compaction)
Pure shear (no viscosity contrast)
Buckling (viscosity contrast)
Thrust/reverse fault formation
129
What is the difference between thick-skinned and thin-skinned crustal deformation? (2)
Thick-skinned involves the basement
| Thin-skinned is underlain by low-dip detachment
130
Which contractional structures are common in the upper crust? (3)
Reverse faults
Folds (often flexural slip)
Low-grade foliations (slate/phyllite)
131
Which contractional structures are common in the lower crust? (3)
Ductile shear zones
Folds (often passive folds)
High-grade foliations (schist/gneiss) and lineations
132
What is an imbrication zone?
| Which way are the horses younging?
Series of similarly oriented reverse faults connected through a floor thrust
Younging in the foreland direction
133
What is a thrust duplex?
Which way do they propagate?
How do the tectonic horses form?
An imbrication zone with a roof thrust
Towards the foreland
Successive formation of ramps in competent layers that act as stress guides
134
Where are the main tectonic settings hosting crustal extension? (4)
Intracontinental rift zones
Oceanic ridges
Back-arc basins
Upper crustal levels of orogens
135
Where are the more localised settings hosting crustal extension? (2)
Transtension zones
| Heads of gravity slides
136
Which extensional structures are common in the upper crust? (3)
Planar-listric normal faults
Extensional vein systems
Folds related to faults
137
Which extensional structures are common in the lower crust? (2)
Ductile shear zones
| High-grade foliations and lineations
138
What are the purposes of transfer zones (relays)? (2)
A way to transfer deformation between fault structures
| Control drainage
139
What is shown in zones with planar normal faults? (2)
Major tilt blocks with uplifted footwalls and basins in the hangingwalls
Looks like fallen dominoes
140
When a weak layer at base is absent, which system is favoured for extension? (3)
Symmetric horst-and-graben
Total extension and crustal thinning are the same
Pure shear
141
Where are the main tectonic settings hosting crustal strike-slip? (4)
Plate boundary transform zones
Oceanic ridge transforms
Lateral to continental collision zones
Above subduction zones with oblique subduction
142
How does strike-slip deformation differ when thick- or thin-skinned? (2)
Most is thick-skinned and involves basement
| Thin-skinned occur as lateral ramps in thrust systems
143
Which strike-slip structures are common in the upper crust? (3)
Strike-slip/oblique-slip faults and shear zones
Folds with oblique cleavage
Low-grade foliations and lineations
144
Which strike-slip structures are common in the lower crust? (2)
Ductile strike-slip shear zones
| High-grade fabrics, particularly low-plunge lineations
145
How can strike-slip dominated zones by subdivided? (3)
Pure transcurrent zones
Transpressional zones
Transtensional zones
146
What is shown in cross-section of a transpressional zone? (2)
Positive flower structure
| Reverse-oblique faults that converge downwards
147
What is shown in cross-section of a transtensional zone? (2)
Negative flower structure
| Normal-oblique faults that converge downwards
148
In map view, what can often be seen in low displacement transcurrent zones? (2)
En echelon folds
| Simple Riedel shear patterns
149
In map view, what can often be seen in high displacement transcurrent zones? (2)
Throughgoing displacment zone
| With subsiding releasing bends and uplifting restraining bends
150
In map view, what can sometimes be seen in mature transcurrent zones? (2)
Pull-apart basins at releasing bends
| Push-up horsts at restraining bends
