Structural Geology Flashcards
Definition of Structural Geology
The observation of deformed rocks, and the explanation for how [the process(es)] and why [cause(s)] they ended up in their present state
What is the important of structural geology?
Crucial for the exploration, mapping and exploitation of a wide range of Earth materials/resources
Materials/Resources that structural geology helps exploits
Building stones, ores, groundwater, oil and gas
What are the two types of geologic structure which can be described and measured?
Planar and Linear
Examples of Planar structures
Bedding plane, Cleavage plane, Fault Plane etc.
Examples of Linear structures
Fold Hinge Line, Stretching lineation, Worm tube etc
How does increasing the metamorphic grade effect foliations?
Change, with increasing coarsisity
What is the order of foliation with increasing metamorphic grade?
Slaty cleavage, Schistosity, Gneissic foliation
How does compositional variation effect bedding plane?
Changes bedding plan visibility
What are lineations?
A set of lines produced by deformation
What four types of lineations?
Crenulation, Stretching, Mineral, Intersection
What is stretching lineation?
the long axes of stretched grains e.g. quartz
What is mineral lineation?
long axes of aligned metamorphic minerals which grew in a set orientation
How are folds created?
Compressive forces in earth’s crust
Name anatomy of a fold
Limbs, hing zone/fold axis, hinge points, axial trace, profile plane, inflection points, antiform/synform, anticline/syncline, symmetric/asymmetric, plunge, upright/inclined/recumbent
What are the three main features needed to define geometry of planar structure?
Strike, Dip angle, Dip direction
Definition of Strike
orientation of horizontal line
Definition of Dip angle
angle of inclination, which is perpendicular to strike
An example of strike, dip angle & direction as seen in fieldwork
090/45 S
What is intersection lineation?
lines formed by the intersection of two planes e.g. bedding/cleavage
What are bedding-cleavage intersections like at a single deformation event?
parallel to fold axis
What are the two main features required to define geometry of linear structure?
plunge and plunge direction
What is the difference between plunge and plunge direction?
Plunge is angle of inclination relative to the horizontal while plunge direction is the compass direction
What are stereonets used for?
Understanding slope stability
What can poles be used to study?
plate/tectonic processes
What are the two structures plotted on a stereonet?
plane and line
TRUE or FALSE: Pitch is the angle between strike and lineation
TRUE
What is used to further analyse the relationship between planar surfaces?
Pole to the plane
What is ‘Pole to the plane’?
the projection of a line drawn normal (perpendicular) to the surface of a plane
What are terms used to describe folds observed in the field?
Fold hinge, Axial - planar cleavage, intersections of cleavage planes on folded bedding plane
What are terms used to describe folds determined using a stereonet?
Fold axial plane, fold axis, fold profile plane
What can be determined of intersecting planes using a stereonet?
The plunge/plunge direction of intersection and angle between planar beds
What allows you to measure fold axis without the seeing it?
bedding and cleavage planes, as they are parallel to the fold axis
What can you do if you don’t have cleavage measurements when determining fold axis?
just using bedding data on a stereonet
What can you determine through plotting of bedding data alone?
Strike, dip and dip direction of profile plane.
Plunge and plunge direction of fold hinge.
Cylindrical nature of the fold.
Inter-limb angle of the fold.
What are uses of poles and planes?
observation of mineral alignment and determination of cause
What are potential causes of mineral alignment determined through poles and planes?
tectonic activity after magma cools evidenced by quartz infill in biotie.
fabric broadly already parallel to rim of a pluton and some parallel to a fault.
What does stereonets help with?
processing large quantities of data e.g. reconstructing kinematics of mountain forming
Why are sterenoets used to model folds?
They are less susceptible to human error
Define pi-plot/girdle
Plotting bedding from the fold as poles
What are poles for cylindrical expressed as?
constant orientation of the hinge line which defines a single great circle
Define cylindrical fold
Fold profile constructed without ambiguity
Define ambiguity of fold profile
lack of error when plotting line of best fit on stereoplot
when is a fold classified as non-cylindrical?
If a best fit line great circle cannot be drawn
What can be inferred from the interlimb angle?
the degree of shortening a fold has experienced
What can be used to determine the inter limb angle?
Maths or a stereonet
TRUE or FALSE: There are techincal names for degree range of inter limb angle
TRUE
give examples of techincal terms for inter limb angles
great: 180 -120
open: 120 - 70
close: 70 - 30
tight: 30 - 0
isoclinal: 0
elastica: negative values
What are inflections points?
Where the angle of bed inclination changes
How do you measure the inter-limb angle?
Examine the shape of the fold along the fold profile
Method for inter-limb angle measuring?
Poles method
TRUE or FALSE: there are techincal classifications for plunging ranges
TRUE
What causes failure of a rock face?
the orientation of the cracks
What does folding involve?
low composition contrast between successive layers and/or shear or folding at high temps
What does fold/shear at high temps allow?
Plasticity
Define fold bluntness
describes the shape of the fold hinge
What is fold classification based on?
orientation of hinge line and axial surface
What is needed to describe fold orientation?
inclination and plunging of fold
What is used to describe fold geometry?
symmetry, interlimb angle, orinetation of axial surface and hinge line, consistency, aspect ratio and bluntness
Examples of terms for fold tightness
Gentle, open, close, tight, isoclinal, fan, involute
What folds are obtuse?
Fan and involute
Examples of fold aspect ratio terms
Wide, broad, equant, tall, elongate
examples of fold bluntness terms
Chevron, sharp, angular, subangular, subrounded, circular fold, blunt
Examples of fold classification terms
Vertical, upright plunging, upright horizontal, inclined plunging, inclined horizontal, reclined, recumbent
What are Dip Isogons?
these compare fold styles of the two surfaces of the layer
What is the origin of Dip Isogons?
Ramsey’s classification (1967)
TRUE or FALSE: Materials of different competency don’t behave differently when folded
FALSE
How do you find the class via dip isogons?
Draw lines between tanferental points
What are the classes of folds?
Class 1A
Class 1B Parallel
Class 1C
Class 1C Similar
Class 3
TRUE or FALSE: There is a relationship between stress, rheology and fold mechanisms
TRUE
What are the types of folding?
Active folding; buckling, bending.
Passive folding
Define Passive folding
Folded layers have no little or no rheological influnence as there is no competence between folding laters and host rock
Define competence
Viscosity contrast between successive layers
Define Active buckling folding
Competence between folding layers and host rock
Define bending folding
Forces applied across the layer at a high angle, may or may not be competence contrast
In passive folding where does deformation take place?
Grain scale in a mechanically isotropic rock
What do layers serve as in passive folding?
Geometrical strain marker only
Where is passive folding found?
Mylonite zones, paraticitically in monomineralic
Example of passive folding
Moine rocks in the highlands
Where does active bending folding occur?
Between boudins
Above thrust ramps
Above reactivated faults
In host igneous/salt instructions
Where does deformation take place for active buckling folding?
at the layer scale
What affects the deformation pattern of active buckling folding?
strength of layers
What is buckling folding?
Compressive stress parallel to layers
Where does buckling folding occur?
Parallel to orogeny
Where does buckling instability develop?
sites of irregularity on the layer interface
TRUE or FALSE: relative viscosity of layers of rock involved control folding
TRUE
Define vicsoity
the ability of a material to resist flowing
What does no mechanical contrast between layers generate?
Only thickening of material, no folding
What does mechainal contrast between layers generate?
creates folds, which vary in ar length as a function of thickness of layer
TRUE or FALSE: the thicker the strongest layer the broader the wavelength of the fold
TRUE
TRUE or FALSE: Thickness of layers control wavelength (where competency contrast exists)
TRUE
What is the mathematical expression of the relationship between wavelength and layer thickness?
Biot-Ramberg equation
What are the asumptions of the Biot-Ramberg model?
No volume loss, isn’trelaistic
What can create volume loss in the feild that constrats the bio-ramberg model?
Compression reducing pore space, can cause ~30 - 40% volume loss
What does mechanical or thickness contrast between successive layers causes?
Change in fold geometry
What does high viscosity and thickness of layers create
Large wavelengths (km scale)
What forms when a layer has a lower viscosity than the matrix?
Mullion structures
What are mullion strcutures
High viscoity material makes bulbous folds and the lower viscoity mater is pushed into the cusp
What controls the wavelength of bigs folds when there are multi-layers?
The thicker layers
What do the thing layers in a multilayer fold create?
Parasitic folds
What forms when folding multilayers which have scpaed apar competent layers?
Disharmonic folds
What is the distint features of disharmonic folds?
The axial surfaces is discontinues with dissimilar fold axial plan and amplitude
What trumps layer thickness in fold control?
vinicity
Where do parasitic fold form?
The back of larger folds
What is fomred is multilayers scpaed togther are close?
they behave as a single layer and form harmonic folds
Define Pumpelly’s Rule
the fold axes and aial surfaces of minor folds of an area are congrugent of the major fold structures of the same phase of deformation
What is the orientation of the small fold structures representative off?
The orientation of regional structures
What does the horizon with lower and high visocity do when stres is imparted?
lower; compresses
higher; folds
Define fold envelope
Macroscale fold geology defined by connecting all the crests of the parasitic folds
Define Vergence
the rotation of axial surface from a symmetrical fold into a asymmetrical fold without changing orientation of enveloping surface
Define Fold vergence
Sense of shear towards a geographic direction
How are asymmetrical folds formed?
Symmetrical folds are bent and distorted by larger scale fold envelope
What are fold vergence types indicators of?
area of a fold
Define S wave
Long limb and short limb with a asymmetrical fold and is verging
TRUE or FALSE: Fold vergence is on all scales and is formed by competent constraints between successive layers
TRUE
What can vergence be applied to?
parasitic folds and asymmetrical folds in shears zones
What can fold vergence be determined by?
cleavage-bedding structure
What can be used to how if a folded is refolded or a disharmonic fold?
Stereonets and map patterns
Define faults
fractures along which there is a visible offset (physical discontiniuty) by shear displacemnt parapllel to the fracture surface
What are faults denominated by?
brittle deformation mechanisms
what is the scale of displacement for faults?
shear fracture (micro-fault) cm’s or less
name the two main features of faults
hanging wall and footwall
Define Net slip
Total amount of displacement
Define heave
The anount of hoziontal displace ment
Define throw
the amount of vertical displacemnt
What do faults represent?
Physical discontinuty and discontinuty in velocity feild
what is limited concenous regarding plate-boundary fault zones?
the width and structure below seismigenic layer
What is the fault zone widths in the lower crust?
<10 - 10’s km
What is the fault zone widths in the lithospheric mantle?
<100 - 100’s km
TRUE or FALSE: faults aren’t continuous planar surfaces and the magnitude of displacement across the fault plane isn;t consant
TRUE
Are these terms for faults areas?
Maximum displacement zone
Tip Point
Tip Line
Yes
What decreases from the maximum displacement zone to tip point?
decreasing displacement
What rule does the Anderson model explain?
fault propagation
What do faults do?
Particular orientation and well-defined angles
What are the assumptions of the anderson model?
- Earth is a prinipal plane with a principal stress direction perpendicular to gravity
- two sets of shear planes (conjugate faults) develop at ~25-30o to O1
- If O1 =/= O2 =/= O3, there are three possible configurations for the prinipal stresses (min, mas and intermediate) relative to the earths surface
Are normal dip-slip faults more steep than thrust dip-slip faults?
Yes
Which faults are generally low angle?
Thrust
What do anderson’s rule form the basis of?
The basis of structural analysis of faults
What does anderson’s rules not explain?
all possible fault types
Examples of faults outwith the anderson model
When O1 is vertical, its assumed normal fault will drip 60 -65o ut <60o faults do form
When O3 is vertical its assumed thrust fault will dip 25-30o but >35o faults form
What are the explanations for reverse faults and low angle normal faults?
Reactivation, stress-trajectories and listric faults
What is the exaplation for reactivation?
reverse faults are reactered normal faults
(compression fafter extension)
low angle normal faults are reactiveated thrust faults
(extension after compression)
Define stress -trajectories
Fault angles reprentthe curvature of stress strajectores with depth in the arths crust
What is an issue with the anderson model and stress trajectories?
some faults from at depth where elevated T & P plus changing differential stress exists so the application of the simiplifed anderson model doesn’t work
If the crust was homogenous what would the stress trajectories be?
Planar at all levels in rock body
As the crust is hertorgenous what will the orinetation of stress trajectores do?
curve
What assumption can be made about cauving stress jacterotes and faults?
If prinipal stress directions vary in orinetaion then the angle of faults relative to the earth’s surface will not conform to predictions made by anderson model
What is listric fault?
A fault surface which is concave upwards with dip that decreases with depth
Where are litric faults common?
extened (strecthed) crust
What could explain listric faults?
variation in the stress trajectories caused by heterogenous rock
Define strain envelope
area around fault where strain is concentrated
Where is the area of lowest stress in a fault?
The area of max strain
What does the fault tip do as stress is released in low stress areas
Bend towards each other
What happens to a ramp when the rocks have compressive stress?
Becomes the damage zone
What is fault geometry dependent on?
Host rock structural and rheological hemogeneity
What are the three types of faults joining?
uniform, basement and variation
What is a uniform fault?
Uniform extension generating a population of incipient faults of which some grow, interact and link up to longer faults
What is a basement fault?
Basement controlled extension generating an achelon faults in the cover that link up to a non-planar large fault
What is a variation fault?
Variation of basement controlled extension where the preexisting fault is inclined with respect to the extension direction
What happens to the ramp as the fault tips propagate?
It twists as the relay lengthens
What is ramp length on average?
3 - 3.5x the width
What does rock flour do?
Act as seal against fluid movement
What is a fabric?
minerals and mineral aggregates with a preferred orientation that penetrate the rock at the microscopic to cm spacing scale
What is a Primary Fabric?
Fabric formed during sedimentary deposition or igneous crystallisation
What fabric does igneous crystallisation create?
A random crystalline fabric
What are secondary fabrics?
Fabrics are well developed in strongly deformed rocks e.g. metamorphic rocks
What is Tectonite?
Rocks with fabric that clearly display coordinated geometric features that indicate continuous solid (ductile) flow during formation
What is lineation?
a linear fabric
examples of lineation
elongate minerals, worms burrows, cigar shaped i.e. prolate
What is the term for Tectonite with lineation?
L - Tectonite
What is foliation?
a planar fabric
examples of foliation
mica; grains are pancake shape, e.g. oblate
What is the term for tectonite with foliation?
S - Tectonite
What is the term for tectonite with foliation and lineation?
L - S Tectonite
What is the increasing order of change with metamorphic grade?
Slaty cleavage (fine) -> Schistosity (intermediate) -> Gneissic foliation (Coarse)
Whats the difference between Schistosity and Gneissic foliation?
Schistosity is grain size visible to naked eye while Gneissic foliation has light - dark banding
Why do crystals tend towards hexagonal shape when recrysallsing?
its a very energy efficient shape which is thermodynamically stable
What are the two types of protolith?
Orthogneiss and Paragneiss
Which protolith is igneous and the other is pelitic?
Ortho = igneous
Para = pelitic
What is an order of protolith to high grade metamorphism?
Shale -> slate -> schist -> gneiss
(proto) (LG) (HG)
Define cleavage?
The ability of a rock to spilt or cleave into ~parallel surfaces along foliation
When does cleavage form?
At temps <350oC, below greenschist facies conditions
What happens to grain size as metamorphic grade increases?
increases
What happens to minerals during deformation?
grow in a preferred alignment according to the stress feild
What happens during schistosity grade metamorphism?
Other metamorphic minerals may grow, depends one composition of protolith e.g. garnet
What must the schistose foliation do with non-elongate xls?
curve around it which forms wavy foliation
How is gneissic foliation defines?
compositional banding
What happens to muscovite in high grade metamorphic rocks?
breakdowns and reacts to form K-feldspar
Rocks can contain more than one foliation, where?
Areas which have experiences polyphase deformation
How are foliations labelled?
So = primary
S1, S2 = subsequent foliation-forming deformation events
What is the development of tectonic foliation depend on?
composition parent lithology, metamorphic grade and magnitude of deformation
What does the specific requirements for faliation development create?
characteristic foliation types
What are the stages of cleavage development?
compaction -> pencil -> slaty -> crenulation
What stages of cleavage development is diagenetic or tectonic?
Diagenetic; compaction
Tectonic; pencil, salty, crenulation
How is diagenetic foliation created?
Thin elongated detritcal mica grains rotate passively into parallel orientation
How are tectonic foliation created?
Pressure solution
define pressure solution
wet diffusion where qtz dissolves and micas grow in response the orientation of the stress feild
Where does tectonic cleavage pronouced and cross cut primary foliation?
Pencil cleavage
How does pencil cleavage create elongated shards?
shale fractures along both primary and secondary foliations
or
fracturing along 2 tectonic foliations e.g. thrust ramp,formed during same deformation event
What happens between pencil and slaty cleavage?
Continued tectonic shortening and tectonic foliation development
What is slaty cleavage?
metamorphosed low greenschist grade
What does salty cleavage create?
quartz and Mica rich domains
What does Q & M domain create with compression?
Axial-planar cleavage; dine planar fabric parallel to the axial planes of the folds
What is axial planar cleavage like in mudstones and sandstones?
mudstone: strong
sandstone: weak/absent
What can axial planar cleavage tell you about?
fold struture where folding is obsercue
What can axial planar cleavage and competece contrats create?
Cleavage refraction
define cleavage refraction
contrast in competence between folded layers of rock
Formation created by cleavage refraction?
cleavage fan
How is cleavage fan formed?
more ductile horizon slides when folding creating contrast in cleavage orientation
Names of sedimentary protoliths?
Psammite; sandy protolith
Pelite; muddy protolith
What happens in transition to crenulation?
tectonic foliation develops at high angle to earlier tectonic foliation and new minerals form in preferred alignment
Define difference between symmetric and asymmetric crenulation cleavage?
Sym; o1 parallel to pre-existing foliation
Asy; o1 not parallel to pre-existing foliation
In crenulation cleavage, what does the axial planes to the microfolds create?
new foliation
What is the physical shapes fromed by crenulation cleavage?
quatrz hinges; older tectontic foliation
mic limbs; younger tectonic foliation
What terms used to classify fault rock?
Brittle, cohesive (incohesive) and ductile
What promotes ductile deformation?
temp increase with depth
What promotes brittle deformation?
strain rate increa
Where are brittle incohesive fault rocks formed?
earth’s surface, and is subdivided based on the volume of angualur clasts vs powdery matrix
What is the subdivision of brittle incohesive?
Fault breccia >30% frags.
shattered with visible angular fragments
Fault gouge <30% frags.
ground down to fine powder and ofter hydrothermally altered to clays
What is brittle cohesive fault rock defined as?
Deformation accomplished by crushing (fracturing, rotation and frictional sliding)
- fine grained, almost glassy with angular clasts
What does brittle cohesive under go which defines subdivsions?
cataclastic and contains angular porphyroclasts in finer matrix
What are the subdivisions of brittle cohesive?
Protocataclasite; crushed matrix 10-50%
Cataclasite; C.m 50-90%
Ultracataclasite; C.m >90%
What is ductile cohesive fault rocks define by?
rocks produced by ductile reduction of initial grain size which develop penetrative foliation and mineral stretching lineation
What are the subdivisions of ductile cohesion?
Protomylonite; 10-50% grain size reduction
Mylonite; 50-90% grain size reduction
Ultramylonite; <90% grain size reduction (few to no porphyroclasts remain)
What is brittle and ductile cohesion called?
Brittle: cataclasite
Ductile: Mylonite
Where was mylonite first applied?
Moine thrust, scotland
