Structural Geology Flashcards
What is structural geology about?
The observation of deformed rocks and the explanation for how and why they ended up in their present state
What are the two types of geologic structures?
Planar: bedding plane, cleavage plane, fault plane
Linear: fold hinge line, stretching line action, worm tube
What are the three types of foliations?
Slaty cleavage
Schistosity
Gneissic foliation
Listed in increasing metamorphic grade
What causes high grade metamorphism?
Higher temperature and pressure
What are the characteristics of slaty cleavage?
Fine bedding
What are the characteristics of schistosity foliation?
Intermediate bedding
Grain size visible to the naked eye
What are the characteristics of gneissic foliation?
Light dark banding
Coarse
What is a lineation?
A set of lines produced by deformation
What are the four types of lineation?
Crenulation lineation - corrugated
Stretching lineation - long axes of stretched grains
Mineral lineation - long axes of aligned metamorphic minerals
Intersection lineation - lines formed by the intersection of two planes (bedding cleavage)
What causes a geological fold?
The result of compressive forces in earths crust
How do you record the geometry of a planar structure?
Strike - orientation of a horizontal structure on a planar structure 000
Dip - angle of inclination of a planar structure (0=horizontal, 90=vertical) 45
Dip direction - direction of dip
For a single deformation, what angle are bedding cleavage intersections to the fold axis?
Parallel to the fold axis
What is foliation?
A plane defined by the alignment of minerals
What way is anti formal and synformal?
Antiformal = hill
Synformal = ditch
What is a stereo net used for? =
To project large quantities of data
A 2d visual representation of 3d data
What are the four types of failure?
Plane
Wedge
Circular
Toppling
Give an example of a rock failure.
Lidong village, china, nov 15
Aberfan disaster, 1966
Rest and be thankful, 2012
What can stereographic projection be applied to?
Landslide hazard/slope failure studies
Earthquake studies
Hydrogeology and/or groundwater pollution potentials
Anything with relative orientations of planes and lines
What are the two types of stereonet?
Equal angle - Wulff
Equal area - Schmidt
What is the benefit of a wulff stereonet?
Preserve area for the analysis of the distribution or density of data
What is the benefit of a Schmidt stereonet?
Preserves equal proportions, essential for distribution analysis
Each 2 degree polygon on the net has an equal area
What is a great and small circle?
Great circle - north to south circular arcs
Small circle - east to west circular arcs
What is the primitive circle?
Perimeter if a stereonet
How do you plot a plane with a stereonet?
Mark north and south on tracing paper
Mark the strike on the paper (000)
Move the strike to the home north
Count dip in from east, if dipping east, or west if dipping west
Follow line up to home north and south
Return to home base
How are planes represented?
000/00 NESW
Lines on a stereonet
How are lines represented on a stereonet?
00 -> 000
Plunge -> plunge direction
Dot on a stereonet
How do you plot a line on a stereonet?
Mark the plunge direction on the compass at angle 000
Position mark at home north
Count plunge and mark on the paper as a dot
How is pitch represented in a stereonet?
000/00 NESW (Pitch 00 NESW)
Strike/dip direction of dip (pitch angle and direction)
Line with a dot
How do you plot pitch on a stereonet?
Mark north and south
Mark strike on compass
Turn compass to so mark is north
Count dip from east or west
Follow line to north and south
Coun from strike north to pitch or south mark pitch as dot
What is a pole?
A projection of a line drawn perpendicular to the surface of a plane
Horizontal plane = pole in centre of stereonet
Vertical plane = pole at 090 or 270
Inclined plane = 90 back from the line
How do you record linear structures?
Method 1: plunge and plunge direction
Method 2: angle of pitch and direction
What is plunge?
The angle of inclination of the lineation, relative to the horizontal
What is plunge direction?
The compass direction in which the lineation is plunging
How do you record planar fabrics, linear fabrics and planar fabrics+pitch in the field and on a map?
Planar fabrics: field= 000/00NESW, map= strike and dip to correct angle
Linear fabrics: field= 00->000, map= directional arrow ->00
Planar fabrics+pitch: field= 000/00NESW, Pitch 00NESW, map: strike dip and plunge
What can we determine from the intersection of two planar surfaces?
Plunge and plunge direction of the intersection
Angle between planar beds
Where do we find intersection locations?
Bedding-cleavage intersections
What angle is bedding cleavage intersections to the hinge line/major fold axis?
Parallel
How do you calculate plunge and plunge direction from the intersection of planes?
Plot planes as lines
Rotate the intersection to the EW axis and mark on the primitive circle
Count the angle from the primitive circle to the intersection = plunge
Return to home base.
Mark on primitive circle = plunge direction
How do you describe the sip of an axial surface?
Recumbent <10 degrees
Gently inclined 10-30 degrees
Moderately inclined 30-60 degrees
Steeply inclined 60-80 degrees
Upright 80-90 degrees
How do you describe the plunge of a hinge line?
Horizontal <10 degrees
Gently plunging 10-30 degrees
Moderately plunging 30-60 degrees
Steeply plunging 60-80 degrees
Vertical 80-90 degrees
How do you measure the interlimb angle?
Poles method
How do you define the shape of the interlimb angle?
Gentle 180-120
Open 120-70
Close 70-30
Tight 30-0
Isoclinal 0
When is a fold classed as non cylindrical?
When you cannot draw a best fit great circle
How do you use the pole method?
Plot the planes
Plot the poles by aligning them NS and counting back 90
Angle poles along a great circle, and draw a dashed line of best fit
Count angle in
Intersection angle = 180 - angle
How do you describe the geometry of a fold?
Symmetry
Interlimb angle
Orientation of axial surface
Orientation of hinge line
Consistency
Aspect ratio
Bluntness
How do you describe the interlimb angle?
Gentle
Open
Close
Tight
Isoclinal
Fan
Involute
How do you describe fold aspect ratio?
Amplitude of the wave vs width of curve
Wide
Broad
Equator
Tall
Elongate
How do you describe fold bluntness?
Sharp or rounded hinge
Chevron
Sharp
Angular
Sub angular
Surrounded
Rounded
Circular
Blunt
How do you describe the orientation of the axial plane and axial hinge?
Axial plane described in first term, hinge line second
Vertical
Upright plunging
Inclined plunging
Reclined
Upright horizontal
Inclined horizontal
Recumbent
What are the types of dip isogons?
Class 1A hinge thinner than limbs
Class 1B equal thickness
Class 1C hinge slightly thicker than limbs
Class 2 skinny but rounded
Class 3 skinny and getting skinnier
What are the three types of folding?
Active folding: buckling, bending
Passive folding
What are the characteristics of passive folding?
Deformation takes place at grain scale in a mechanically isotopic rock
No significant competence
Found in mylontite zones, monomineralic rocks
Asymetric
What are the characteristics of active bending folding?
Buoyancy or mechanical (hard rock, rising magma, salt intrusions)
Between boudins, above thrift ramps, reactivated faults, as igneous intrusions
What is a boudin?
A mechanically strong horizon (ie sandstone) that has expanded and fractured, then fills with a softer material (ie mudstone)
Like a sausage
What are the characteristics of active buckling?
Deformation takes place at layer scale
Strength of layers directly affects the deformation pattern
Compressive stress parallel to layers
Competence controls folding
What is competence?
Relative viscosity of layers of rocks
What controls the wavelength of a deformation?
Thickness of layers
What is the biot-ramberg equation?
Wavelength = 2pi x layer thickness x cubed root of viscosity 1 / 6 viscosity 2
What are the assumptions for the biot-ramberg equation?
Low amplitude
Sinusoidal
Plane strain
No volume loss
Layer thickness small compared to wavelength
Why can we not assume no loss of volume in the biot-ramberg equation?
Compressive forces squeeze air and water out of porous materials
Do thick and high viscosity layers have smaller or larger wavelengths?
Larger
What happens if a low viscosity layer is encompassed by a higher viscosity matrix?
Mullion structures occur
U shaped spikey pattern
What is a fault?
Fractures along which there is a visible offset by a shear displacement parallel to the fracture surface
What is the scale of displacement?
Few cm to 100s km
Ie great glen fold, San Andreas fault
What is a joint?
A fracture that occurs in a rock in response to an applied stress
What is the purpose of joint formation?
To release stress
What are the triangles of the geometry of faults?
Heave, throw and dip slip
Strike slip, dip slip and net slip
Dip and net being hypotenuse
What is heave?
The horizontal displacement
What is throw?
Vertical displacement
What does a fault represent?
Physical discontinuity
Velocity discontinuity
How do rocks move?
- Cracks and slide
- Flexion until an eventual snap = earthquake
What are the characteristics of faults at the surface?
Brittle structures because rock is cold
Where is the brittle plastic transition?
10-15km depth
Changes with the composition of the rock and the strain
What determines whether a rock rearranges itself or snaps?
The rate of the application of stress
Where is displacement at its max and min?
Max at centre of structure
Min at tips - pinching away
What are the assumptions for Anderson’s model?
Earths surface is a principal plane, with a principal stress direction perpendicular to it (gravity)
Conjugate faults develop at 25-30 degrees to sigma 1
If sigma 1,2,3 aren’t the same then there are 3 possible configurations for he principal stressors
What are the three types of faults?
Normal where sigma 1 is vertical
Thrust where sigma 1 is squeezing II
Strike slip where sigma 1 is pushing =
What is the acute angle between fold sets?
60 degrees
What is a reverse fault?
Steep thrust
Acute angle 35 degrees
What are the two explanations for reverse and low angle normal faults?
Reactivation : reverse faults are reactivated normal faults (extension then compression), low angle faults are reactivated thrust faults (compression then extension)
Stress trajectories: fault angles represent the curvature of stress trajectories in deep earth
What determines if a fault will be reactivated?
Faults orientation to sigma 1
Perpendicular and fold plane is at a low angle it is easier to reactivate
How can we model and predict reactivation faults?
Based on Mohr circles
Why is earths crust not homogeneous?
Rigid crust
Temp and pressure increase further down = more ductile
Stress is distributed differently
What is a listeria fault?
Extensional, spoon shaped
Concave upwards, dip decreases with depth
Occurs due to variation in stress trajectories caused by heterogenous rocks
How are cracks linked together?
By ramp and relay structures
What is the relationship between a ramps length and width?
Ramp length is 3-3.5 times the width of the ramp o
Where is stress at its max and min?
Max= tip of the fault structure with no deformation (releases it by bending towards a low stress zone, verging tips)
Min= zone of maximum deformation/strain (already deformed/strained in response to stress therefore stress decreases)
What is a ramp?
The untouched area between tips of strain
What is the damaged zone?
Ramp that has cracks/strain through it
Fractured rocks - high permeability
What is fault geometry dependant on?
Host rock structural and rheological homogeneity
what is a mullion layer?
a weak layer that deforms as points with lower viscosity material in the point (cusp) and higher viscosity material making a bulbous base
when multiple layers are folded, what controls the wavelength of the folds?
the thick layers
how do parasitic folds form?
are a thinner layer that buckles early on in compression, and then is forced to bend with a wavelength determined by the thicker layer
what are the characteristics of parasitic folds?
asymmetric and verging
long limb and shorter limb
/
/ \/
/
what makes a fold disharmonic?
layers do not act symbiotically
fold axial planes dont match up
folding occurring at different times in different materials, thus deforming prefolded structures
what is the difference between disharmonic folds and refolded folds?
disharmonic folds occurs due to different materials folding at different pressures/times
refolded folds occur when there is tectonic activity, a period of rest, then another period of tectonics
what is the identifying feature of harmonic folds?
each layer has roughly the same composition and layers are of equal thickness
what happens if competent layers are close together?
they behave as a single unit and therefore have the same wavelengths and amplitude
what is pumpelly’s rule?
the orientation of small structures is representative of the orientation of regional structures
how is vergence defined?
clockwise or anticlockwise orientation from the normal to the peak
what can vergence be applied to?
parasitic folds in fold analysis
cleavage bedding relationships
asymmetrical folds in shear zones
how do you determine if a structure is a disharmonic fold or a refolded fold?
representation on a stereonet
what is a fabric?
minerals with a preferred orientation that penetrate a rock
what is a primary fabric?
fabric that is formed during sedimentary deposition or igneous crystallisation
what is a secondary fabric?
fabrics that are well developed in strongly deformed rocks i.e. metamorphic rocks
what is a tectonite?
rocks with fabric that clearly displays coordinated geometric features that indicate continuous solid flow during formation
what are the two types of tectonite?
L-tectonite: lineations (linear fabrics) i.e. cigarette shaped
S-tectonite: foliation (planar fabric) schistosity, pancakes
what is a L-S tectonite?
when a rock has both a lineation and a foliation
when does a shale become a schist?
when crystals become visible to the naked eye
what are the two types of protoliths?
orthogneiss - igneous protolith
paragneiss - pelitic/sedimentary protolith
what is cleavage?
the ability of a rock to split or cleave into parallel surfaces
forms below greenschist facies conditions
what happens to muscovite during high grade metamorphism?
reacts to form K-feldspar
what does the development of foliation depend on?
the composition of the parent lithology
metamorphic grade
magnitude of deformation
what are the four types of cleavage development?
compaction cleavage
pencil cleavage
slaty cleavage
crenulation cleavage
what is the primary process in the formation of pencil, slaty and crenulation cleavage?
pressure solution (wet diffusion)
quartz dissolves and micas grow in response to the orientation of the stress field.
by what process does compaction cleavage form?
diagenetic foliation
thin elongate detrital mica grains rotate passively into parallel orientation
what is pencil cleavage and how does it form?
shale fractures along primary (bedding) and secondary (tectonic) foliations
OR
fracturing along 2 tectonic foliations
creates elongate shards
what is the lithology of slate?
pelite that has been metamorphosed to low greenschist grade
what is a cleavage fan?
contrast in competence between folded layers of rock
what is pelite?
a muddy protolith
what is psammite?
a sandy protolith
what determines whether crenulation cleavage is symmetric or asymmetric?
stress parallel to pre existing foliation = symmetric
stress not parallel to pre existing foliation = asymmetric
where do you find quartz and mica in a crenulation cleavage?
quartz rich hinges
mica rich limbs
give an example of a structure that is not a fabric
faults and shear fractures
what promotes brittle deformation?
increased strain rate and cold
what promotes ductile deformation?
increased temperature (depth)
what gives psuedotachylite a glassy/devitrified look?
partial melting due to lots of friction
how are brittle incohesive fault rocks formed?
generally formed at earths surface
what are the two classifications of brittle incohesive fault rock?
fault breccia: >30% fragments, visible angular fragments
fault gouge: <30% fragments, fine powder, hydrothermally altered to clays
what is a brittle cohesive fault rock?
cataclasite
what are the characteristics of cataclasite?
fine grained, glassy with some angular clasts
deformation accomplished by crushing
what are the three classifications of cataclasite?
protocataclasite: crushed matrix forms 10-50% of the rock
cataclasite: crushed matric forms 50-90% of the rock
ultracataclasite: crushed matrix forms >90% of the rock
what is a mylonite?
a fault rock produced as a result of ductile reduction of initial grain size
stretching lineations
what are the three types of mylonite?
protomylonite: 10-50% of the rock has undergone grain size reduction
mylonite: 50-90% of the rock has undergone grain size reduction
ultramylonite: >90% of the rock has undergone grain size reduction
what is mylonitisation?
grain size reduction
