Maps and Structures (L1-8) Flashcards
Define drift lithologies
Quaternary deposits that obscure the bedrock geology
Define solid lithologies
Pre-Quaternary units
In the generalised vertical section of a map, what do wavy lines indicate?
An unconformity
In the generalised vertical section of a map, what is the meaning of a wedge shape unit?
The unit is only present over part of the map area
What are the kinds of stratigraphy? (6)
Litho Bio Chrono Magneto Chemo Cyclo
Which structures are looked for in ‘structural geology’? (6)
Faults Folds Bedding orientations Foliations Lineations Lines of intersection between other structures
How is planar data represented? (2)
Strike/dip
OR
dip direction/dip
How is linear data represented? (1)
Plunge/trend
How can the profile shape of folds be estimated from bedding pole dispersion on stereonets? (2)
Poles in two clusters = angular fold
Poles spread along a great circle = rounded fold
How can the interlimb angle of a fold be estimated from bedding pole dispersion on stereonets? (2)
ILA = 180 - x
Where x is the largest angle between two bedding poles on the great circle
How can the fold axial plane be estimated on a stereonet? (1)
When may this be very wrong? (1)
A great circle containing the fold axis and the ILA bisector
Wrong for asymmetric folds and overturned limbs
What is stress?
Stress = force/area
Unit: Pa
What happens to stress when a force acts obliquely to the plane? (2)
Normal component: maximum at 0, minimum at 90
Shear component: minimum at 0 and 90, maximum at 45
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?
Hydrostatic (shear stress = 0)
Causes volume change, not shape change
In rocks at depth, where does the stress come from? (2)
Weight of overlying rocks
Stress = ρgh
What is a system with unequal principal stresses broken down into? (2)
Mean stress P = (σ1 + σ2 + σ3)/3 = hydrostatic component stress field
Remainder = deviatoric stress
What does deviatoric stress control?
Shape change
Define strain
Change in size and shape of a body resulting from an applied stress field
Define homogeneous strain
What is the result?
What is the converse?
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
Define deformation
The transformation from an initial to a final geometry by means of a rigid body translation, rigid body rotation and strain
What is extensional strain (extension)?
How is extension related to stretch?
e = (l - l(0))/l(0) s = 1 + e
What is shear strain (γ)? (2)
γ = tan(ψ)
ψ is the deflection of an original right angle
What does the strain ellipsoid represent? (2)
Homogeneous deformation in 3D
Deformed shape of an imaginary sphere
Give an example of a strain marker
Deformed reduction spots in slate
Assuming they start life as a sphere
What is pure shear?
The orientation of the principal strain axes don’t change during deformation
What is simple shear?
Lines parallel to the principal strain axes rotate away
Define rheology (2)
Study of flow
Linking the response of a material to the forces acting upon it
Outline elastic rheology (4)
Linear stress-strain relationship
Gradient = E = Young’s modulus
Instant response to stress
Non-permanent strain
Outline viscous rheology (4)
Linear stress-strain rate
Gradient = η = viscosity
Time-dependent response to stress: more time = more strain
Permanent strain
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
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
What can folds be viewed as?
A stacked series of deformed surfaces
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
What is maintained in a cylindrical fold?
The same shape in successive profile planes perpendicular to the fold axis
What is fold size specified by?
Amplitude and wavelength of the fold train
What is fold attitude specified by? (2)
Dip and strike of the axial surface
Plunge and trend of the fold hinge
Define closure direction
Define facing direction
The direction in which the limbs converge towards the hinge
The direction in which the strata get younger
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
How do an antiform and a synform differ?
Antiforms close upwards, synforms close downwards
Outline the quantitative descriptors of interlimb angle (4)
Isoclinal: 0-30
Tight: 30-70
Open: 70-120
Gentle: 120-180
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
Define reclined folds
Fold hinges plunging down dip in the axial plane
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
Alternating Class 1B and Class 3 folds can be seen as what?
Class 2 overall (which is stackable)
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
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
What are the three mechanisms of fold formation? (3)
Buckling
Bending
Passive folding
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
How does layer thickness effect buckling?
Thicker layers fold with longer wavelength
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
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
What kind of folds are produced with different strength contrasts? (3)
High = ptygmatic fold Intermediate = general folding Small = cuspate-lobate folds
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
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
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
What does tangential-longitudinal strain lead to?
Tension in outer arc and compression in the inner arc
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
Where does flexural flow typically occur?
Deeper in the crust at higher temperatures
What are the special fold shapes? (5)
Kink band Chevron folds Monocline Concentric folds Box fold
Define rock fabric
The geometrical arrangement of its constituent grains or structures that is penetrative at most scales of observation
How do primary and secondary rock fabrics differ? (2)
Primary: sedimentary or igneous in origin
Secondary: tectonic in origin
What are the three general types of rock fabric? (3)
Planar
Linear
Random
What are the three special types of homogeneous strain? (3)
Axially symmetric extension
Plane strain
Axially symmetric shortening
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
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
How is the K parameter in a Flinn plot used to define strain?
K = infinity: axially symmetric extension
1
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
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
Outline pressure solution cleavage
Anastomosing zones of stress removal of soluble grains
Outline slaty cleavage
A planar realignment of existing grains with limited recrystallisation
Outline crenulation cleavage
Microfolding of a pre-existing foliation or bedding
Outline phyllitic cleavage
A surface sheen of recrystallised minerals but no visible new grains
Outline schistosity
Recrystallised grains visible to naked eye
Outline gneissosity
Spaced compositional banding due to diffusion or partial melting
What is foliation development dependent on? (2)
Protolith composition
Temperature
Outline the progressive foliation development of a typical mudstone (7)
Compaction cleavage Pencil cleavage Slaty cleavage Crenulation cleavage Phyllitic cleavage Schistosity Gneissosity
Define cleavage
The ability of a rock to split into parallel surfaces
Give three examples of primary foliations (3)
Bedding
Fissility
Flow banding
Define lineation
A fabric element with one dimension considerably longer than the other two
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
Give two examples of primary lineations (2)
Parting lineation
Flow lineation
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
What determines the cleavage refraction in a folded structure?
The contrast in competency between layers
Define composite fabric
Planar and linear components are both prominent
Define isotropic fabric
When can these occur?
Randomly oriented grains or structures
Undeformed rocks or those recrystallised in a hydrostatic stress field
Outline brittle and ductile deformation (3)
Sub-divisions of plastic deformation (permanent)
Brittle is discontinuous
Ductile is continuous
Define creep
Slow ductile deformation by various lattice-scale mechanisms
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
What is a Mohr circle?
A way to describe σ_n and σ_s acting on planes of all possible orientations
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
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)
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
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)
Outline the von Mises failure criterion (3)
Failure envelope flattens as ductile regime approached
Constant shear stress criterion
σ_s = constant
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
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
Why do faults reactivate? (3)
Reactivation criterion is different to that of unfractured rocks
Reactivation stress is much smaller
So faults grow/accumulate displacement
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
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
Define joints
Brittle fractures with little or no visible displacement across them
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
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
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
What is the brittle-ductile regime characterised by?
A duality of behaviours
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
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
What are the consequences of en echelon tension gashes forming? (2)
Strain hardening
Widening of shear zone over time
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
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
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
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
What is the main difference between mylonites and other fault rock types?
Mylonite formation involves ductile deformation
Fault gouges are often seen with multiple colours, how did this come about?
Mineralisation as different fluids travelled through the loose and permeable material
How does a pseudotachylyte form?
Forms by frictional melting during faulting
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
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
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
What are deformation twins an expression of? (2)
Relatively low T, low finite strain mechanism
Stress results in mechanical bending of crystal lattice
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
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
Define dislocation
A mobile line defect that contributes to intracrystalline deformation by mechanisms of slip
How do dislocation creep and diffusion creep differ in their T conditions? (2)
Diffusion is high T
Dislocation is medium T
What are the two types of diffusion creep? (2)
Diffusion along grain boundaries = Coble creep
Diffusion within grains = Nabarro-Herring creep
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
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
What are the main tectonic settings hosting crustal contraction? (4)
Continental collision zones
Foreland thrust belts
Accretionary prisms
Compressional subduction arcs
What are the more localised settings hosting crustal contraction? (2)
Transpression zones
Toe of advancing deltas/glaciers/gravity slides
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
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
Which contractional structures are common in the upper crust? (3)
Reverse faults
Folds (often flexural slip)
Low-grade foliations (slate/phyllite)
Which contractional structures are common in the lower crust? (3)
Ductile shear zones
Folds (often passive folds)
High-grade foliations (schist/gneiss) and lineations
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
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
Where are the main tectonic settings hosting crustal extension? (4)
Intracontinental rift zones
Oceanic ridges
Back-arc basins
Upper crustal levels of orogens
Where are the more localised settings hosting crustal extension? (2)
Transtension zones
Heads of gravity slides
Which extensional structures are common in the upper crust? (3)
Planar-listric normal faults
Extensional vein systems
Folds related to faults
Which extensional structures are common in the lower crust? (2)
Ductile shear zones
High-grade foliations and lineations
What are the purposes of transfer zones (relays)? (2)
A way to transfer deformation between fault structures
Control drainage
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
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
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
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
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
Which strike-slip structures are common in the lower crust? (2)
Ductile strike-slip shear zones
High-grade fabrics, particularly low-plunge lineations
How can strike-slip dominated zones by subdivided? (3)
Pure transcurrent zones
Transpressional zones
Transtensional zones
What is shown in cross-section of a transpressional zone? (2)
Positive flower structure
Reverse-oblique faults that converge downwards
What is shown in cross-section of a transtensional zone? (2)
Negative flower structure
Normal-oblique faults that converge downwards
In map view, what can often be seen in low displacement transcurrent zones? (2)
En echelon folds
Simple Riedel shear patterns
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
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
