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geophysics > structural geology > Flashcards

structural geology Flashcards

1
Q

for geological purposes we use gps to calculate the movement of plates. what needs to be done to the reciever to use it for this purpose?

A

it is important that the reciever is fitted securely to bed rock so that it is stationary.

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2
Q

generally how much do plates move each year?

A

plates generally move tens of mm a year

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3
Q

whats the formulas for stress, strain and strain rate?

A

stress = F/A

strain = X/L

strain rate = strain/time

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4
Q

high strain rates are associated with what?

A

high strain rates are associated with the edge of plates - whether in extension or comporession.

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5
Q

what do the arrows on a gps vector map show?

A

arrows point in direction of movement

size gives magnitude of movement

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6
Q

describe the tectonics of california.

A

predominantly transverse

western part of california is moving rapidly past the rest of the USA

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7
Q

what are satellite measurement (INSAR) used for and how do they work?

A

are used to view changes in depth e.g. at a normal fault.

works on a phase difference

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8
Q

plates generally form ____________ which predominantly deform at ___________

A

plates generally form rigid blocks that predominantly deform at their edges

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9
Q

small yearly displacement X large amount of time =

A

a potentially large amount of deformation

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10
Q

what does deformation encompass?

A

deformation encompasses:

folding

faulting

shearing

compressing

extension

of rock by tectonic or gavitational forces

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11
Q

deformation at the earth surface is mainly caused by

A

deformation at the earth surface is mainly caused by the horizontal movements of the lithospheric plates relative to one another

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12
Q

tectonic forces that deform rocks at plate boundaries are often horizontally directed and depend on the rate of plate motion

these forces can be:

A

compressive

shear

tensional

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13
Q

we use what we see at the surface to have a best guess of…

A

the underground

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14
Q

define structral geology

A

structral geology

the study of all deformational features in rocks from the small scale to the big scale.

it particularly addresses the geometry, distribution and formation of rock structures such as folds and faults and their links to tectonics.

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15
Q

what is tectonics concerned with?

A

tectonics is concerned with the regional processes that generate a characteristic set of geological structures in an area.

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16
Q

what is the role of structral geology?

A

reconstructing histroy of deformation on earth

earthquakes and other geohazards

vital for petroleum exploration and mining

fluid flow through rocks

understanding the growth and destruction of mountain belts.

set the boundary conditions for surface processes and sedimentation

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17
Q

what is the difference between the compositional and rheological classification of the earth?

A

compositional - based on silica content

rheological - based on strength of rock type

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18
Q

which is permentent elastic or plastic deformation?

A

plastic deformation is permanent

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19
Q

deformation style depends on?

A

type of material (or rock)

whether it is being extended or compressed

temperature

pressure

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20
Q

what to these four samples show you?

A

1) inital, undeformed marble cylinder
2) sample compressed under condition representative of the shallow crust. it has fractured thus brittle deformation
3) sample compressed under condition representative of mid crustal levels. there is evidnce of both bittle and ductile deformation
4) sample compressed under conditions of deep crust. it has deformed and bulged smoothly in a ductile way

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21
Q

what is normal faulting caused by?

A

normal faulting is caused by tensional forces that stretch a rock and tend to pull it apart.

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22
Q

what causes reverse and thrust faulting?

A

caused by compressive forces that squeeze and shorten the rock.

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23
Q

what is the difference between reverse and thrust faulting?

A

reverse = steep dipping fault plane

thrust = shallow dipping fault plane

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24
Q

how do seismic surveys work?

A

waves are reflected depending on acoustic impedance between layers

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25
Q

what methods can be used to learn about structral geology

A

material science and rock mechanics (theory and experiment)

observations and measurments

geological mapping

numerical and analogue modelling

GIS, remote sensing and seismic metodologies

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26
Q

geological map combine …

A

geological map combine topography and geological structure

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27
Q

define strike

A

the direction of the horizontal in any inclined plane.

there is only one such line on any inclined plane

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28
Q

what is the convention of measureing dip to strike?

A

beds dip to the right of the strike

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29
Q

how many figures do we use for strike and dip?

A

strike = 3 figures

dip = 2 figures

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30
Q

define dip

A

the maximum angle of inclination in an inclined plane.

the direction of this inclination is at right angles to the strike

the amount of dip is measured from the horizontal in the line maximum dip in the plane.

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31
Q

what is the convention in writing dip and dip direction?

A

we use dip and then dip direction

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32
Q

how can we use the thickness of beds as seen at the surface to tell the steepness of a bed?

A

vertical beds give the true thickness at the surface.

wide outcrops can be either from a shallow angle or that the bed actually has a large thickness.

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33
Q

what can we say if geological boundaries are parallel to contour lines?

A

we can say that the beds are horizontal

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34
Q

if we see an outcrop in a V shape across a valley what can we say?

if the V is close to a straight line what can we say?

A

outcrop pattern V points in direction of dip

a straight line signifies a vertical line

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35
Q

other than the dip of the bed what else controls the thickness of the bed as seen on a geological map?

A

topography also controls the thickess of outcrop

for low slopes, or those in dip direction = wide outcrops

for steep slopes, or at right angles to dip = narrow outcrops

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36
Q

what shape do you look for on a geological map to see an angular unconformity?

A

a T shape

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37
Q

what do we look for in a anticline on a map?

A

the oldest rocks in the middle

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38
Q

what can we say of the fold axis of an anticline that is plunging?

A

the fold axis is not horizontal

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39
Q

how do we view plunging anticlines on a geological map?

A

outcrop patterns curve in direction of plunge

beds dip away from centre

oldest rocks in centre

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40
Q

how do we view plunging synclines on a geological map?

A

opposite to anticlines

outcrop patterns curve in opposite direction to the plunge

beds dip towards the core of the syncline

rocks youngest in centre.

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41
Q

how do we tell if the V shapes on a geological map are either a plunging syncline or anticline?

A

look at strat column to find the oldest rocks

look at dip direction of rocks

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42
Q

what are outliers?

A

outliers are isolated areas of younger rocks surrounded by older rocks

outliers have oldest on outside

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43
Q

what is meant by an inlier?

A

inliers are isolated areas older rocks surounded by younger rocks

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44
Q

what are ouliers and inliers called when the features are created by faulting?

A

outlier = klippe (pl. klippen)

inlier = window beneath the thrust plane

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45
Q

deformation is where physical changes are produced as a result of

A

deformation is where physical changes are produced as a result of the action of applied forces - such as gravity and tectonics

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46
Q

define shear stress

A

the force per unit area in a direction parallel to the area to which it is applied

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47
Q

if there is no overall acceleration on a body then we can always find…

A

if there is no overall acceleration on a body then we can always find three mutually perpendicular planes where the shear stress is zero

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48
Q

what is meant by principle stress axes?

A

the normal stresses, on axis perpendiculer to planes of zero shear stress are called the principle stress axes

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49
Q

what terms are given to the maximum and minimum stress axes?

A

maximum = σ1

minimum = σ3

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50
Q

strain is deformation produced as a result of ______

A

strain is deformation produced as a result of stress

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51
Q

stress can lead to changes in…

A

stress can lead to changes in the volume, size and orientation of objects

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52
Q

what is meant by homogeneous strain?

A

homogeneous strain

where straight lines remain straight, and strain is the same through the body.

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53
Q

what is meant by heterogeneous strain?

A

hetrogeneous strain

where strain is non-uniform through the body

(looks like a prune)

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54
Q

what is meant by pure shear?

A

pure shear

where strain axes are the same as the original body

all the angles at still at 90o

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55
Q

what is meant by simple shear?

A

simple shear

where the deformation is rotational.

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56
Q

all of the measurements of strain are ___________.

linear strains are often quoted in __________.

A

all of the measurements of strain are dimensionless

linear strains are often quoted in percent

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57
Q

what do the axis on a strain ellipsoid represent?

A

x = σ1

y = σ3

z = σ2

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58
Q

how do we visulaise strain ellipsoids in rocks?

A

depending on the orientation of the rock we get a 2D section through the strain ellipsoid.

we must be careful as some orientations of a strain ellipsoid may look like its never changed ( it can be deformed but some axis are still circular)

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59
Q

in regards to the strain ellipsoid what is the flinn diagram?

A

Flinn diagram

ratios of quadratic elongation in the x versus y and y versus z direction show whether objects have been turned into cigars or pancakes

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60
Q

which axes are larger relative to others to create cigars and pancakes on a flinn diagram

A

cigar x>>y>z

pancake** x~y>>z**

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61
Q

what do we need to apply the strain ellipsoid to real rocks?

A

some form of strain marker in the rock (fossils, clast, dykes, foliations

an initial idea of what the marker looked like

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62
Q

define rheology.

A

rheology

is the study of the deformation and flow of matter. for structral geologists, it refers to the different ways in which rocks can deform in response to an applied stress in varying geological contexts.

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63
Q

what is elastic deformation?

A

in elastic deformation, stress is proportional to strain.

deformation is recoverable.

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64
Q

if stress is proportional to strain then what is the constant?

what is the name given to this law>

A

the constant is the youngs modulus with units of Pa

this is known as hooke’s law

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65
Q

how do rocks deep in the crust behave like when compressed or stretched beyond a relatively small amount?

A

behave plastically

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66
Q

for viscous materials at high temperature, stress is proportional to…

the constant is_________

A

stress is proportional to strain rate

the constant is viscosity

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67
Q

what is meant by plastic deformation?

A

plastic deformation

is permanent change in shape or size of a body without fracture, accumulated over time by a stress beyond the elastic limit of the body.

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68
Q

what happens to perfectly plastic materials once they yield?

A

perfectly plastic materilas deform at a constant stress regardless of strainrate once they yield so technically their viscosity is zero.

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69
Q

what is meant by strain hardening and strain softening?

A

strain hardening

more stress needed to strain rock once is has yielded

strain softening

less stress needed to strain rock once it has yielded

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70
Q

what is meant by ductility?

A

ductility

describes the ability of a sample or rock to derom under stress

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71
Q

describe the three types of deformation in rocks

A
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72
Q

which has the largest youngs modulus?

which has the lowest compressive strength?

A

which has the largest youngs modulus? pyroxenite

which has the lowest compressive strength? dolomite

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73
Q

what are the controls on the rheology of rocks?

A

applied stress

temperature

rock type (and/or orientation of rock)

confining pressure

strain rate

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74
Q

what is the impact of temperature on rheology of a rock?

A

higher temperatures = lower strength (acts more ductile)

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75
Q

what is the impact of orientation of a rock on a stress strain diagram?

A

stronger when σ1 is parallel to the dominant foliation.

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76
Q

what is the impact of confining pressure on rheology of rocks?

A

both strength and plasticity increase with greater confining pressure

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77
Q

what is the impact of strain rate on rheology of rocks?

A

rocks are weaker and deform plastic ductile way when low strain rates are applied (slow)

they flow in a process called creep

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78
Q

in a stress against strain rate graph what does the line =

how do we find the angle the graph makes to the horizontal?

A

line = viscosity

tan-1(viscosity) = angle to horizontal

viscosity = stress/ strain rate

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79
Q

what does e on this graph mean?

what does the graph represent?

A

strain instantaneous when stress applied at time 0.

as it is a constant it is a straight line.

object snaps into original shape when stress is removed at time 1

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80
Q

describe the graphs of a plastic material

A
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81
Q

define brittle

A

brittle

a material can be defined as brittle if it fails (breaks) when subjected to an applied stress, without any significant deformation.

there is little or no permanent strain prior to failure

failure can be compressional or tensional

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82
Q

which is more prone to brittle failure , extension or contraction?

A

when under tension there is a huge range of confining pressure where brittle failue happens

extension needs high pressure and temperature for it to act plastic thus acts brittle more often

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83
Q

what conditions are suited for brittle failure?

A

brittle failure favoured:

low temp

low confining pressure

high strain rates

coarse grained or heterogeneous rocks

in tension rather than compression

high water pressure

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84
Q

what brittle deformation mechanisms are there in terms of grains?

A

include frictional sliding along grain contacts

grain rotation

grain fracture

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85
Q

what do we say if a rock has under gone cataclasis?

A

where the individual grains break (micro fracturing)

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86
Q

what is meant by fracture?

A

fractures

are discontinuities in a rock which are associated with offset in mechanical properties such as strength, and spatial properties such as displacement.

if fractures are present then it has been deformed

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87
Q

what do fractures become if they have any significant displacement?

A

fractures become faults if they have any significant displacement

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88
Q

which two fractures do you normally find together?

A

often find shear and extensional fractures together.

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89
Q

how do fractures grow in uniaxial tensional normal stress?

A

fractures grow perpendicular to σ3

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90
Q

how do fractures grow in uniaxial compressional normal stress?

A

fracture develops parallel to σ1

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91
Q

in both triaxial extension and compression what direction does the shear fracture develop at?

A

shear fracture develops ~30 degrees to σ1

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92
Q

on what plane does a shear fracture develop on?

A

shear fracure develops on a plane which the resolved shear stress has a component of σ1 and σ3

this plane makes an angle alpha with σ1

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93
Q

in theory maximum shear stress should be on a plane of 45 degrees to σ1 and σ3. why isnt this the case?

A

because less normal stress (on a steeper plane) helps shearing to go more readily

94
Q

what is meant by inernal angle?

A

intranal angle = ß

95
Q

what is the formula and graph showing the linear relationship between normal stress and shear stress on failure?

A
96
Q

on the coulomb failure criteria what do we plot on the normal stress axis?

A

we plot σ1 and σ3 with the average normal stress in the middle

97
Q

what is the formula for shear stress

A
98
Q

on the coulomb failure criteria what is the angle between the line and σn?

A

it is equal to ß

99
Q

in the formula for shear stress what is mu equal to?

A
100
Q

in coulombs failure criteria, what is the gradient the tangent of?

A

the gradient is the tangent of the internal angle

101
Q

what is shear stress failure equal to interms of the mohr’s circle?

A
102
Q

what is the formula for the normal stress σn?

A

where k is from the centre of circle to y axis

103
Q

what is the formula for shear stress?

A
104
Q

what does coulombs failure line show?

A

it is an envelope which represents the failure points of several mohr’s circles - for rock bodies stressed by different σ1 and σ3.

if you know the failure envelope for a rock type and you can calculate the applied stress on any plane in the rock body then you can calculate when the rock will break

105
Q

what is meant when a mohr’s circle is:

not overlaping

touching

overlaping

the envelope?

A

not overlaping - stable

touching - critical

overlapping - will fail (not stable)

106
Q

in tension rocks break by propagation of micro-fractures.

stresses are much bigger than applied σ1 on the tips of cracks.

what is the griffith criterion?

A

griffith criterion takes account of crack size and length and thus the coulomb mohr critero changes as it over estimates the tensile strength.

107
Q

what is the effect of pore fluid pressue on the effective normal stress?

A

pore fluids, such as water in between grains of a sandstone, resists the normal stress applied to the rock.

the effective normal stress is lower

108
Q

what is meant by anisotropy?

A

anistropy means a prexisting weakness in a rock

109
Q

what is the effect of anisotropy on a mohrs circle?

A

usually it will fail along the fracture

110
Q

what are the three mohrs circle complexities

A

pore fluid pressure

anistropy

failure in tension

111
Q

define a fold

A

a fold is a product of plastic deformation where layers are warped upwards or downwards, often in response to layer parallel compressive stress.

they have a sense of shortening

112
Q

folds are important structral _____ for the petroleum industry.

A

folds are important structral traps for the petroleum industry

113
Q

anticline and syncline refer to _____ relationships in the rock

A

anticline and syncline refer to age relationships in the rock

114
Q

synform are…

A

synforms are any downward closing structures

115
Q

antiforms are…

A

antiforms are any upward closing structures

116
Q

synclines are..

A

any downward closing structures when the rocks are the right way up

117
Q

anticlines are…

A

any upward closing structures when the rocks are the right way up

118
Q

what is meant by an antiformal syncline?

A

it is an antiform however the rocks have been overturned so the youngest rocks are now in the middle

119
Q

what is fold geometry classification based on?

A

dip of fold axial plane

plunge of hinge line

120
Q

what two types of shaped folds can we get?

A

cylindrical fold (godd for stereonet)

non-cylindrical fold (could be conical or have a nonstraight hinge line)

121
Q

describe a fold labelling the points

axial surface

hinge line

fold axis

inflection point

inflection line

interimb angle

limb

amplitude

wavelength

A
122
Q

what are the four interlimb angles? from little strain to lots of strain?

A
123
Q

what is meant by m, s and z folds?

A

1st is m folds

2 and 3 are either s or z folds

124
Q

what is meant by fold vergence?

A

asymmetric folds have a sense of vergence - whigh way the axial plane or trace tilts

fold vergence often indicates the directionof shear

if the nose of a fold closes to the right while the axial plane dips to the left so the fold verges to the right.

125
Q

refolded folds is the same principle as _______

A

refolded folds is the same principle as the superposition of waves

126
Q

what is meant by dip isogons?

A

making lines by joining up equal dip on the inner and outer arc of a fold

127
Q

what is meant by class 1, 2 and 3 dip isogons?

A

class 1

isogons that converge on inner arc

class 2

vertical dip isogons

class 3

diverging isogons on inner arc

128
Q

what is special about class 1b dip isogons?

A

the lines are parallel to each other

129
Q

what is meant by buckling or active folding?

A

buckling (or active folding)

occurs for layer parallel shortening in a rock when there is a contrast in rheology (viscosity) between different layers

130
Q

what is meant by bending and passive or shear folding?

A
131
Q

what is meant by passive folding?

A

passive folding

the layers in the rock exert no mechanical influence on the folding- they are just passive strain markers

132
Q

what is meant by shear folds?

A

shear folds

are passize folds associated with a significant component of simple shear

133
Q

what type of folding normaly creates class 2 folds?

A

passive folding associated with pure shear typically form class 2 folds.

134
Q

what is meant by pure shear?

what do you need for pure shear?

A

where the axis remain orthogonal to each other

you need a bit of bend in the material already to get it to fold. otherwise it would just contract (squashed)

135
Q

how a layer deforms depends on rheology and layer thickness.

what is the effect of layer thickness?

A

layer thickness - which controls fold wavelength

thin layers have shorter wavelengths

136
Q

what is meant by compentent and incompetent layers?

A

competent

layers maintain thickness round fold (class 1b)

this means the weak layers cant

incompetent

layers change in thickness around fold

137
Q

what is meant by parasitic folding?

A

small high frequancy folds on top of thick low frequancy folds

138
Q

explain how parasitic folding is formed.

A

folds symmetrical at first in thin weak layer

when more competent layer finaly bends, σ1 is oblique to weak layer so there is simple shear strain.

139
Q

smaller scale parasitic fold can verge towards the hinge of the bigger structure thus reflecting …

A

smaller scale parasitic fold can verge towards the hinge of the bigger structure thus reflecting the bigger scal geometry

(axial plane of small waves are the same as the big wave)

140
Q

to maintain layers with uniform thickness as folding continues we either:

A

1) get beam like orthogonal flexure - extensional cracks on outer arc(cold butter) (stretch outer arc, contract inner arc)
2) flexural slip or flow within them (warm butter) (layers slip against each other)

141
Q

why are class 1B folds useful to structral geologists?

A

class 1B folds have the same thickness but changes in length when folded so strain can be worked out.

this makes it a great contraction marker

142
Q

what is an implication of calculating strain from a fold?

A

the layer normally shortens first before it buckles and fold.

calculating strain does not take this into account, thus is wrong

143
Q

what is a geological fault?

A

a geological fault ia a product of brittle failure in response to an applied stress. they are fractures which have a noticeable displacement.

range from ~1m to 100s of km in size

144
Q

a geological fault can be active and seismogenic meaning…

A

is is still accumulating slip at the present day

145
Q

a geological fault can be dead but _________ in the rock record

A

A geological fault can be dead but preserved in the rock record

146
Q

describe a normal fault

A

produced from tension forces.

they are extensional features because they involve a lengthening of the original unfaulted block

angle ~50-60 degrees

147
Q

describe a reverse or thrust fault

A

created from compressional forces

angle ~30 degrees

148
Q

how can you tell what type of strike slip fault it is?

what usually is the angle of the fault?

A

when standing on it the foot that goes backwards gives the type.

e.g. if the left foot goes backwards then it is a left lateral strike slip fault

the angle is usually vertical

149
Q

what is meant by oblique slip faulting?

A

possible to combine strike slip and dip slip fault types e.g. transtensional faults

150
Q

define displacement, throw and heave interms of fault

A

DISPLACEMENT – the relative offset between markers on either side of a fault, measured along the fault plane in the direction of slip (the slip vector)

THROW – the vertical component of displacement on a fault which has some dip-slip movement

HEAVE – the horizontal component of displacement on a fault which has some dip-slip movement.

151
Q

a principle stress axis σn must be _________ to the surface of the earth.

and other stresses are _________ to this and __________ to the surface of the earth

A

a principle stress axis σn must be perpendicular to the surface of the earth.

and other stresses are perpendicular to this and parallel to the surface of the earth

152
Q

why does a normal fault have an angle of 60 degrees?

A

as σ1 is gravity alpha will be 30 degrees then the dip of the fault plane is 60 degrees.

153
Q

why does a reverse fault have an angle of 30 degrees

A

the maximum force is now perpendicular to gravity. gravity is now the minimum compressive force.

as alpha lies in the direction of σ1 which is also the dip.

as alpha = ~30 = dip

154
Q

why does a strike slip fault have a vertical angle?

A

gravity in this one is the intermediate stress σ2.

thus as both σ1 and σ3 are in the earths crust the fault formed is vertical. and has angle alpha to σ1

155
Q

what are the implications that make it hard to describe the geometry of a fault?

A

local stress might not be coaxial

falts have a finite size (they arent just blocks)

geological rock type/ anistropy/ pore fluid pressure

not just brittle deformation

156
Q

the greater the fault length the greater the _________

A

the greater the fault length the greater the displacement

157
Q

describe the displacment along the strike of a fault.

A

displacement of faults varies along the strike on an individual fault.

faults tend to have a finite length, and maximum displacement in the centre

158
Q

faults dont have to be a single plane.

they often have a ___________________ associated with them

A

faults dont have to be a single plane.

they often have a deformation or damage zone associated with them

159
Q

faults consist of one or more ______ that may overlap

A

faults consist of one or more segments that may overlap

160
Q

what is meant by relay ramps?

A

often exploited by rivers draining uplifted footwall blocks

if you sum the displacment on both faults it would be a constant

161
Q

how do faults propagate?

A

fault propagates and grows under the applied stress from the tip of the fault.

162
Q

what happens when several different fault segments grow?

A

as fault segments grow, interact and eventually link their displacement profiles start to overlap

163
Q

when does most displacement on geological faults take place?

A

most of the displacement on geological faults takes place during earthquakes

164
Q

describe the cycle of strain on a fault over time?

A

we get cycles of elastic strain (stress build up) before brittle failure releases the energy.

165
Q

what does the frequancy of earthquakes depend on?

A

local rock strength

applied stress

the extent to which deformation is taken up on other structures

166
Q

what is meant by sesimic moment?

how do you calculate it?

A

fault rupture area and slip distance are related to the energy released. we calculate this using the seismic moment

seismic moment Mo = shear modulus x displacement area X displacement length

units = joules

167
Q

the earthquake magnitude is calculated as the moment magnitude.

what is it proportional to?

A

moment magnitude is proportional to log(seismic moment)

168
Q

where do most earthquakes happen?

A

most happen at plate boundaries and < 50km deep

deep earthquakes can be seen in the cold descending slab in subduction zones

169
Q

when plotting lines and planes on a stereonet what will they look like?

A

lines are dots

planes are curved lines

170
Q

what is meant by poles to planes on a stereonet?

A

any plane can be defined uniquely by a line that runs perpendicular to it.

if we define a line which is perpendicular to a bedding plane, we can plot this line as a point on a stereonet that represents the plane.

we call these features poles to planes

we draw them as 90 degrees opposite on the E W axis

171
Q

what is the formula for working out the trend of pole and plunge of pole?

A

trend of pole = strike of plane - 90

plunge of pole = 90 - dip angle of plane

(we can work this out by its placement compared to the line)

172
Q

what is meant by a rock fabric?

A

a rock fabric is made of minerals or mineral agregates with a preffered orienation, that penetrate the rock from the microscopic to cm scale.

173
Q

foliations, lineations and other types of rock fabric could be primary or secondary. what does this mean and which is most popular?

A

primary: such as depositional environment

secondary: result from the application of stress to the rock body.

secondary type is most common

174
Q

describe linear fabrics

A

many rocks have linear fabrics are because the rocks have been stretched and therefore the strain ellipsoid is prolate (ciger shaped)

175
Q

describe foliated fabrics

A

many rocks with foliated fabrics are caused by deformation field producing an oblate strain ellipsoid (pancake shape)

176
Q

describe rock cleavage

A

**cleavage **

refers to the development of parallel planar (or initially planar) fabrics that pervade rocks at relatively low temperatures and pressures (low grade).

it is particularly common in rocks with abundant micas.

platy minerals line up perpendicular to σ1.

characterized by substantial strain induced volume loss (compaction)

177
Q

what direction does cleavage form at?

A

cleavage typically forms perpendicular to this stress direction.

178
Q

in cleavage formation what do we get bands of?

A

we can get bands of quartz and mica.

179
Q

what are the five types of cleavage?

A

compaction cleavage

pencil cleavage

slaty cleavage

crenulation cleavage

schitosity

180
Q

describe compaction cleavage

A

widely spaced cleavages form parallel to bedding as the rock deforms under its own weight.

greatest force is gravity

181
Q

what is meant by pencil cleavage?

A

formed as a tectonic cleavage as the rock starts to deform due to horizontal shortening

182
Q

describe slaty cleavage

A

if horizontal shortening continues from pencil cleavage, we get pervasive slaty cleavage.

183
Q

what is meant by crenulation cleavage?

A

if a second phase of deformation occurs, the first set of cleavage is crenulated and a new set of cleavage is formed.

this second cleavage can form at any angle to the original cleavage depending on the orientation of the maximum compressive stress

184
Q

describe crenulation cleavage under the microscope.

A

in thin section marked out by small scale folding and concentration of micas. also get pressure solution of minerals and recrystallisation in the solid state.

185
Q

describe the formation of schistosity

A

at higher temperatures and pressures, cleavage begins to form into schistosity.

mica crystals grow larger, solid state diffusion leads to compositional banding of mineral types. foliation becomes less planar as it wraps round newly grown metamorphic minerals.

at this point it is hard to tell its past due to deformation.

186
Q

what is the relationship between folds and cleavage?

A

cleavage line parallel to fold axial plane

187
Q

when does cleavage refract between layers?

A

cleavage is sometimes observed to refract between layers which are silty and not very competent, and those which are sandy and harder to deform.

188
Q

explain this diagram interms of cleavage refraction.

A

relates to the rheology contrast between the different layers

the silty layer undergoes greater shear strain for the same stress during deformation so the cleavage refracts

189
Q

what is meant by ductile shear zones?

A

these occur where shear deformation becomes localised on discrete planes

as the direction of foliation approaches the shear plane orientation, the shear strain reaches a maximum.

the angle theta gives an idea of the amount of strain.

190
Q

what are the three main types of lineation?

A

stretching

intersection

movement on structures (slickensides)

191
Q

what can we get from stretching lineation?

A

we can get a trend and a plunge

192
Q

give an example of stetching lineation.

A

where pebbles have been stretched into cigar shapes.

193
Q

describe intersecting lineation

A

some lineations result as the intersection of foliations of different ages.

these lineations have nothing to do with stretching and nothing to do with the strain ellipsoid.

194
Q

describe slickesides

A

these are a combination of mechanical abrasion, causing grooves, and the growth of mineral fibres in orientation of movement.

195
Q

what can slickensides be used for?

A

the slickensides can be used to deduce the fault slip vector - in this case the trend and plunge of the lineation feature.

rough areas would block movement thus the top surface would have to move in the opposite direction.

196
Q

describe fibre, groove and/or mineral elongation lineations in terms of movement of structures.

A

minerals grow in a preferred orientation in fractures or faults.

197
Q

where on a stereonet would lineation data of a fault be seen?

A

lineation data should plot on the plane representing fault (i.e. the curved line) on stereonet

198
Q

whats the main difference betwee foliations and lineations?

A

foliations = planar stuff

lineations = linear fabrics

199
Q

describe normal fault arrays

A

where each ridge in the image is the footwall of a large normal fault.

200
Q

where would you find normal fault arrays?

what are they the product of?

A

found in continents and continental shelves.

typically a product of fault growth and interaction in areas of crustal extension

201
Q

normal fault arrays are caused by extension.

what causes this extension?

A

1) plate tectonic stresses
2) gravitational slumping of gravity collapse
3) hotspots and areas of high heat flow from warm mantle which stretches the lithosphere

can get a mixture of the three

202
Q

describe where you would find horsts, grabens and half grabens on a normal fault array

A

half grabens are where there is a footwall on one side and a hanging wall on the other

horst are high points

203
Q

what is a rift?

A

rifts are linear zones of localized crustal extension, bounded by normal faults. they range in width from somewhat less than 100km up to several hundred km

rifts can be **active ** and driven by buoyant, rising asthenospheric plumes of warm mantle, or passive due to extensional stresses related to plate tectonics.

204
Q

what does a normal fault array become over time?

A

becomes a rift

205
Q

it is important to relise that all rifting and extension requires you…

A

it is important to relise that all rifting and extension requires you to thin the crust and lithosphere

206
Q

describe the process of normal faults to rifting

A
207
Q

describe the three stages of passive rift geology

A
208
Q

describe active rifting

A

associated with mantle plumes - typically notable volcanism

thinned crust and lithosphere results with volcanoes on footwall flanks of central graben

209
Q

where would you find active rifting on earth?

A

the great rift valley africa.

210
Q

describe from rift to drift

A

if extension continues, and the lithosphere continues to be thinned, then the stretching can become so great that oceanic crust starts to be formed from partial melting of the upwelling asthenosphere

this is the birth of a new ocean. lithosphere is thinned to zero at the mid ocean ridge

211
Q

how does partially melting asthenosphere create oceanic plates?

A

partially melting asthenosphere gives a basalt

212
Q

if crustal extension destroys topography, then it is crustal convergence that creates it through the process of… which has the name________

A

if crustal extension destroys topography, then it is crustal convergence that creates it through the process of mountain building which has the name orogenesis

213
Q

in areas of continental collision, a ________ and _______ belt with a particular shape is created

A

in areas of continental collision, a fold and thrust belt with a particular shape is created

214
Q

what are the key factors of orogenesis?

A

the rheological properties of the wedge material

the dip and friction factor of the basal surface

the convergence rate

any erosion off the top

215
Q

in orogenesis we get failure and shearing along the detatchment (decollement) and along the top of the wedge when…

A

the coulomb failure criteriaion is exceeded

216
Q

converging mountain belts look like

A

a wedge

217
Q

what are the angles alpha and beta of an accretionary wedge a function of?

A

alpha and beta are a function of:

rheology

friction properties

pore fluid pressure

218
Q

why is the accretionary wedge theory a bit wrong?

A

a bit wrong because

mountains arent large pile of sand with bulk homogenous properties

for real stratigraphy we get buckling folds and thrusts formed

219
Q

what are the features found with thin skinned fold and thrust belts?

A

detachment horizon

imbricated thrusts

thrust related buckle folds

verging folds

ramps and flats

a foreland (not deformed area)

piggy back basins

220
Q

describe fault propagation folds

A

fault propagation folds

associated with stratigraphy being deformed as a thrust propagates through the rock stratigraphy. folding forms ahead of, or at the tip of the propagating thrust

221
Q

describe fault bend folds

A

fault bend folds

associated with stratigraphy being transported over ramps and flats. fold is pinned over the ramp flat.

222
Q

describe drag folds

A

drag folds

associated with shearing. get hanging wall anticlines and footwall synclines

223
Q

describe detatchemnt folds

A

detatchment (decollement) folds

these are buckle folds formed between two detachment horizons, particularly when one end is fixed.

224
Q

what is meant by a balanced cross section?

A

if you make the assumption that the beds havent changed their thickness you can restore them to their original form. this is known as a balanced cross section.

225
Q

how do we get low angle thrusts?

A

high pore fluid pressure

pre existing weakness

  • could be bedding, weak layer (salt, shale), or foliation

the basal detatchment of most thrusts is in weak rock and at a low angle.

remember anisotrophies usually win in rock layers such as shale, salt and siltstone with low mechanical strength compared to something like a sandstone.

226
Q

what is the difference between passive and active rift zones?

A

passive caused by magma

active caused by plate tectonics

227
Q

silicon rich rocks tend to be ________ than silicon poor rocks

A

silicon rich rocks tend to be weaker than silicon poor rocks

228
Q

rocks behave stronger as depth increases. why?

how is this not consistent with the rheological division of the earths interior?

A

confining pressure increases with depth

rock hardening through burial

and greater shear stress is required to overcome higher confinging pressures in order to deform the rocks.

it is not consistent as when you go downwards there is a liquid core thus weaker.

229
Q

what is meant by foliation?

A

 A folation is any planar structure that pervades the rock - it might be primary (e.g. bedding) or secondary (e.g. cleavage due to tectonic stresses). Because a foliation is part of the rock fabric it should be repeated throughout the rock. A fault plane, for instance, is not a foliation.

230
Q

Write down the equation for the Coulomb failure criterion and define what the terms are, including units where appropriate.

A

beta in equation is actually C
Where t is the shear stress at failure (N/m2)
sigma n is the normal stress at failure (N/m2)
Mu is the coefficient of friction = tan(beta)where beta is the internal angle.
c is the cohesion (the shear stress to fail when the normal stress is zero (N/m2)

geophysics (9 decks)

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