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Earth structure and tectonics - 33740 > Fossen pre reading- structural geology > Flashcards

Fossen pre reading- structural geology Flashcards

1
Q

What are the 2 definitions of deformation?

A

difference in position of points before and after deformation
or
the strain history from undeformed to deformed stage

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2
Q

What components are included in deformation from an external co-ordinate system?

A

Translation
Rigid rotation
Internal deformation

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3
Q

What components can internal deformation be further split into?

A

Rigid rotation
Strain

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4
Q

What is Re?

A

rotation of a rigid body relative to an external co-ordinate system

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5
Q

What is Ri?

A

Rotation of axes of the strain ellipsoid (main strain axes)

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6
Q

What is non-coaxial deformation?

A

Process of internal rotation involved in internal deformation

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7
Q

What is strain?

A

Defines a change in size and or shape

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8
Q

What are some examples of what strain can be like?

A

Change in original shape
Change in volume (dilation)
Rotation of planes and lines
Change of original length of lines

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9
Q

What 2 types of strain are there?

A

Homogenous
Heterogenous

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10
Q

What is homogenous strain characterised by?

A

No strain gradient
Linear transformation (straight line straight, parallel stay parallel)

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11
Q

Will natural strains be more heterogenous or homogenous? (with example)

A

Exhibit some heterogenity
Shear zone show increasing strain from side wall to centre

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12
Q

What is isotropic dilation?

A

Equal lengthening or shortening in any direction

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13
Q

What is anisotropic dilation?

A

Shortening or lengthening in only 1 or 2 directions

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14
Q

What is positive and negative dilation?

A

Positive = increased volume
Negative = decreased volume

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15
Q

How is progressive strain described in terms of?

A

Infinitesimal or Instantaneous strain parameters

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16
Q

What are the infinitesimal strain parameters?

A

Infinitesimal stretching axes
Velocity field
Flow apophyses
Vorticity and Wk
Steady state/ non-steady state deformation

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17
Q

How are the infinitesimal stretching axes (ISA) aligned?

A

perpendicular to each other

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18
Q

What does ISA1 describe?

A

direction of max stretch rate

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19
Q

What will happen to physical lines on ISA1?

A

experience fastest stretching during deformation

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20
Q

What is ISA3?

A

minimum stretching rate

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21
Q

What are the two ISAs?

A

ISA1
ISA3

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22
Q

What will occur at ISA3?

A

slowest (usually) negative stretching rates (fastest shortening)

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23
Q

What is the velocity field?

A

the velocity and direction of motion of the particles as strain progresses

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24
Q

What is vorticity?

A

measures angular velocity

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25
Q

What are flow apophyses?

A

theoretical planes that compartmentalise the flow pattern in which particles cannot cross

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26
Q

What is simple shear?

A

non-coaxial deformation with the long axis of deformation rotating towards but not through the shear plane

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27
Q

What is pure shear?

A

planar coaxial deformation

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28
Q

What is subsimple shear?

A

non-coaxial constant area that combine with simple or pure shear

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29
Q

Does knowing the stress field during deformation reveal the progression of deformation?

A

ultimately no

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30
Q

What are some examples of strain markers?

A

Pebbles
mineral grains
ooids
vesicles
Pillows in pillow lava
Ammonites
Belemnites
Graptolites
Worm burrows
Brachiopods/ trilobites
Crinoid strems
Xenoliths
Reduction spots

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31
Q

What are passive strain markers?

A

markers with the same rheologic properties as the surroundings (no viscosity difference so deforms with rock entirely)

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32
Q

What is the perfect marker for deformation?

A

reduction spots

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33
Q

Why are reduction spots the “perfect” marker for deformation?

A

due to extremely small amounts of colour pigment

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34
Q

What are active deformation markers?

A

markers which will deform differently to the surrounding rock

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35
Q

When would you use the Breddin graph for deformation?

A

where we have pairs of lines in deformed rock where we know original angle

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36
Q

What does the Rf/(oval with line through) diagram handle? (deformation)

A

markers with initial elliptical shapes
or
markers with different original shape

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37
Q

What does the Fry diagram use? (deformation)

A

centerpoints of objects with similar initial size

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38
Q

What does the normalised Fry method handle? (deformation graphing)

A

objects with different initial sizes and involves object centerpoints + shapes and orientations

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39
Q

How is 3D strain presented?

A

Flinn or Hsu diagrams to express the geometry of the strained ellipsoid

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40
Q

What is force?

A

push or pull on an object that results from the interaction with another object (physical or in a force field)

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41
Q

What is Newtons first law?

A

An object remains at rest or moves at constant velocity when no net force is exerted in the object

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42
Q

What is Newtons second law?

A

the change in velocity (acceleration) of an object with mass is equally directed and proportional to the applied net force

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43
Q

What is Newtons third law?

A

for every force there is an equal and opposite force

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44
Q

What is the context of pressure?

A

hydrostatic stress field
materials with negligible shear strength
Fluid or gas

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45
Q

What does normal stress indicate for compression and tension?

A

Compression is positive
Tension is negative

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46
Q

What does shear stress indicate for counter-clockwise and clockwise rotation?

A

Counter-clockwise is positive
Clockwise is negative

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47
Q

What is normal stress?

A

stress vector orientated normal to a surface (i.e. a fault)

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48
Q

What is shear stress?

A

stress vector which parallels the surface

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49
Q

When is shear stress highest?

A

When at a 45* angle to the surface

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50
Q

What is an example of an area where the rock in the crust has been tectonically inactive for 10 or hundreds of millions of years?

A

Baltic shield

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51
Q

What is the state of stress in crustal rock that has been tectonically inactive?

A

lithostatic (equal in all directions) and increases with burial depth

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52
Q

What are wide joints called?

A

Veins

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53
Q

What are joints?

A

fractures without visible offset perpendicular to the fracture surface

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54
Q

What is a joint set?

A

composed of joints with similar orientation and morphology

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55
Q

What is a joint system?

A

when there is a combined pattern of 2 or more jointed rocks

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56
Q

What are the different types of joint intersections?

A

T- orthogonal (regular, rectangle)
X- Conjugate (diagonal)
Y- polygonal (many sides i.e. hexagon)

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57
Q

What are conjugate joints?

A

2 individual sets formed as result of deformation during different stress

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58
Q

Where are joints most common?

A

Brittle upper crust in stiff rocks like well-lithified sandstone and limestone, granitic rocks and lavas

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59
Q

How can joints be found within the environment?

A

scattered or in zone of fracture corridors

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60
Q

What stress is needed to form joints?

A

Tensile stress

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61
Q

How are joints and orogenic processes linked?

A

think either the orogeny itself causes joints or the orogenic stress is stored in the rock and controls joint orientation

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62
Q

What are orogenic processes?

A

linked to continental orogen formation

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63
Q

What happens to rocks as they cool?

A

volume reduction (contraction) which is taken up (grainscale) if flexible but if not extension fractures form

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64
Q

What 2 things happen during cooling of rock?

A

Decompaction and cooling

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65
Q

What happens during decompaction?

A

Any cement (in porous sedimentary rock) that formed at depth locked in some of the elastic strain at those conditions change as rock gets near surface can cause cement to break and joints to form

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66
Q

How does exfoliation occur?

A

during the last part of exhumation when there is a removal of overburden (vertical stress)

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67
Q

What is exfoliation?

A

where joints more or less parallel to the surface form

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68
Q

What rocks is exfoliation common in?

A

massive rocks like granites and thick sandstones

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69
Q

What zone are exfoliation joints restricted to?

A

upper 100-200m of the crust

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70
Q

How does hydraulic fracturing occur?

A

Where fluid is trapped and over pressurised by overburden, tectonic stress or fluid pulses

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71
Q

What are most vein systems a result of?

A

Repeated hydraulic fracturing during periods of elevated pore fluid pressure

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72
Q

How do crack-seal veins form?

A

when the vein grows by repeated fracture events at random locations in vein material

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73
Q

What is seismic pumping?

A

sufficient pulses of overpressure which can cause large volumes of fluid to be expelled from fluid-filled fractures

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74
Q

How do dike intrusions typically occur?

A

hydraulic fracturing (magma is the over pressurised fluid)

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75
Q

What is artificial hydraulic fracturing called?

A

Fracking

76
Q

Why is fracking done?

A

to acquire hydrocarbons from wells, especially gas from shales

77
Q

How does fracking done?

A

horizontal part of wellbore is sealed and pressure increased until the minimum horizontal stress is exceeded

78
Q

Why is fracking used to release hydrocarbons?

A

its hoped the extension fractures will increase rock permeability making it easier for fluid flow

79
Q

Why is sand (propants) added to fracking fluid?

A

to stop the fractures from sealing and becoming joints when pressure normalises

80
Q

What are wingcracks?

A

Faults, fractures and other weak structures that are reactivated to produce joints in their walls

81
Q

How do wingcracks form?

A

in extensional quadrants around the reactivated structure

82
Q

What controls the distance between joints?

A

the thickness of the stiff layer (thicker = larger)

83
Q

How can Dikes form joints?

A

Joints increase with spacing away from dike and thought to form ahead of dike tip due to the tensile stress of the magma

84
Q

What are joint corridors?

A

wide zones of joints that can extent 100’s of km

85
Q

How can fracture corridors be recognised?

A

one dominant orientation (but might have different fracture populations)

86
Q

What is the Colatina fracture system (Brazil)?

A

Fracture corridor with many populations (many mafic dikes)

87
Q

How do mineral veins form from a single event?

A

Form as the mineral grows from fluids in an already open fracture cavity

88
Q

How do mineral grains form from a multi-event growth?

A

form periodically as the veins open

89
Q

What is syntaxial mineral growth?

A

growth along centre fracture
Vein youngest in centre
Same mineral as wall rock
Crystallographic continuity with wall

90
Q

What is Antitaxial mineral growth?

A

growth along 2 walls
central part of vein is oldest
fibrous veins
crystallographically different from wall

91
Q

What is crack-seal growth veins?

A

veins that grow by repeated cracking of existing vein material

92
Q

What are sigmodial veins?

A

form at shear zones in echelon style and rotate systematically as shear accumulates

93
Q

What is the oldest of the sigmodial vein?

A

the central part as a result it is more rotated due to the longer strain exposure

94
Q

How do joints have a positive influence on fluid flow?

A

the joints are like pathways allowing increased permeability

95
Q

Where are joints particularly important for fluid flow?

A

non- to low-porosity rocks (basement rocks) and high porosity limestone reservoirs with poor permeability

96
Q

What is apeture?

A

the ‘openness’ of veins at depth (distance between the 2 walls)

97
Q

where are faults found they found?

A

brittle deformation structures commonly forming in the upper 10-15km of earths crust

98
Q

What are faults?

A

fracture zones of localised deformation that accommodate movement parallel to fracture surface.

99
Q

What is the common movement of faults?

A

1 meter +

100
Q

What are smaller movement fault-like structure called?

A

shear fractures

101
Q

What is a fissure?

A

fracture where movement normal to fracture surface is easily recognised

102
Q

What are the most common vein materials?

A

Quartz and Calcite

103
Q

How are non-vertical faults defined?

A

hanging wall defines the overlying fault block (footwall is underlying fault block)

104
Q

What are the 3 main classes of faults based on the movement of the hanging wall relative to footwall?

A

Normal faults
Reverse faults
Strike-slip faults

105
Q

What are normal faults?

A

Down dip (dip-slip) displacement of hanging wall relative to footwall

106
Q

What are reverse faults?

A

Up dip (dip-slip) displacement of hanging wall relative to footwall

107
Q

What are strike-slip faults?

A

Strikes-parallel displacement of hanging wall relative to footwall

108
Q

What are oblique-slip faults?

A

Combination of reverse or normal-slip with strike slip

109
Q

What are wrench faults?

A

sub-vertical
steeper than normal or reverse

110
Q

What are low and high angle faults?

A

Low angle- <30 degrees
High- steeper than 60 degrees

111
Q

What are synthetic faults?

A

dip in the same direction as primary fault

112
Q

What are antithetic faults?

A

dip in the opposite direction to the primary fault

113
Q

What do oppositely forming faults produce?

A

Graben (lower area)
and
Horsts (raised area)

114
Q

What is displacement (net slip)?

A

the distance between 2 originally contigous points on fault surface

115
Q

What defines displacement?

A

Displacement vector (split into dip-slip or strike-slip component)

116
Q

What is the Pitch (rake)?

A

angle between the displacement vector and the strike of the fault

117
Q

What is separation? (faults)

A

apparent displacement observed in any given section (only if with true displacement vector)

118
Q

What is strike separation?

A

apparent displacement as measured parallel to strike of the fault

119
Q

What is Dip separation?

A

dip-slip component of actual displacement observed in vertical section perpendicular to fault

120
Q

What is heave?

A

horizontal component of dip separation

121
Q

What is throw?

A

vertical component of dip separation

122
Q

What is stratigraphic separation?

A

the isopach thickness of strata between 2 bedding horizons bought into contact at a fault

123
Q

What is isopach thickness?

A

Thickness in direction of normal bedding

124
Q

What 2 architectural elements do most faults contain?

A

interior fault core
enveloping fault damage zone

125
Q

What material do fault cores contain?

A

cataclastic material (gouge and breccias)

126
Q

What are lenses (horses) in the fault core?

A

bodies of host rock ripped from sidewalls and incorporated into the fault

127
Q

How does faulting initiate in non-porous rocks?

A

development of a isolated shear fracture

128
Q

What does Griffths model say about shear fractures?

A

they nucleate by growth and linkage of pre-existing micro-discontinuities

129
Q

How do faults grow in porous rocks?

A

fractures develop as deformation bands
with the pores collapsing in shear

130
Q

How will material change with faulting of sandstone?

A

Non-cataclastic bands at shallow burial
Cataclastic bands for deeply buried

131
Q

What are the 4 main types of fault intersection?

A

Intersection (overprinting)
Mutual interaction
Single-tip interaction
Double tip interaction

132
Q

What is the function of overlap zones?

A

to accommodate transfer of displacement between overlapping structures

133
Q

What are unlinked/ isolated faults?

A

faults that do not mechanically interact with other faults

134
Q

What are soft-linked faults

A

Faults that form an overlap zone but do not physically connect but instead mechanical or geometric coherence obtained by elastic/ ductile strain between faults

135
Q

What are hard linked faults?

A

Faults which are physically connected

136
Q

Where do eye structures form in faults?

A

form between doubled linked faults

137
Q

What are transfer faults?

A

ones orientated normal to overlapping faults and exhibit substantial stroke-slip movement

138
Q

What are branch lines?

A

define lines along which 2 faults are hard linked

139
Q

What is the Wasatch fault?

A

major extensional fault marking boundary between Wasatch mountain range to the east and basin range system to the west this area is seismically active with several minor Earthquakes a year

140
Q

What are foliations?

A

planar structures that penetrate metamorphic rocks

141
Q

What is tectonic foliation?

A

general term about penetrative and cohesive that involve shortening across the structure

142
Q

What are primary foliations?

A

non-tectonic penetrative planar structures in rock

143
Q

What are some examples of primary foliations?

A

Layering or lamination (sedimentary)
Flow banding (volcanic rocks)
Cumulate (intrusive rocks)

144
Q

What is cleavage?

A

foliation in very low grade metamorphic rocks (rock easily split or cleaved)

145
Q

What is schiostocity?

A

tectonic foliations in more coarse grained recrystallised rocks (quartz schist)

146
Q

What are cleavage domains?

A

domain concentrated in insoluble materials (mostly phyllosilicates)

147
Q

What is spaced cleavage?

A

cleavage domains spaced at cm-scale

148
Q

What is continuous cleavage?

A

cleavage domain at mm scale or less

149
Q

What is crenulation cleavage?

A

cleavage formed by microfolding

150
Q

What is disjunctive cleavage?

A

no evidence of microfolding (cleavage cut straight across earlier planar structures)

151
Q

What are M-domains? (cleavage)

A

Mica-rich domain

152
Q

What are QF-domains? (cleavage)

A

Quartz-feldspar rich domains

153
Q

How does a pencil cleavage form?

A

when tectonic strain is added to the compaction strain so that the 2 strains are approx equal to magnitude at a high angle

154
Q

Where is pencil cleavage usually found?

A

foreland of orogenic belts

155
Q

What is slaty cleavage?

A

First forming cleavage occurring at onset of metamorphic conditions around 200*c

156
Q

What is phyllitic cleavage?

A

continuous larger mineral then slaty

157
Q

What does it mean if folds form actively?

A

instability generated by mechanical and rheological properties of layering versus the layers of surrounding matrix

158
Q

What is buckling?

A

when the active folds nucleate and grow as the competent layer shortens

159
Q

What is passive folding?

A

folding which occurs during flow in rocks without internal viscosity contrasts

160
Q

What do the layers function as in passive folding?

A

like markers that “go along for the ride”

161
Q

Where do passive folds form?

A

Mylonite zones (shear zones)

162
Q

What is bending/ forced folding?

A

when layers competent or not bend as forces act across layering

163
Q

When does buckling occur?

A

when a layer is shortened parallel to its length

164
Q

What does buckling require?

A

A layer with higher competence (viscosity) then surroundings
Layer-parallel to shortening
Plastic deformation

165
Q

How will shortening occur id there is no competence contrast?

A

By thickening

166
Q

What might shortening lead to in a brittle regime?

A

Brittle failure

167
Q

What happens when shortening direction is somewhat oblique compared to layering?

A

Formation of asymmetric folds (potentially reflecting obliquity)

168
Q

How can closeness of layers affect buckling?

A

Far apart will act as individual layers

Close together will act as singular layer which the thickness is the sum of the individual layers

169
Q

What is an example of multilayer buckling?

A

Finnmark NE Norway

170
Q

What happens when buckling occurs on alternating thick and thin layers?

A

Thin layers will start to fold first, while thick layers go through longer thickening period

171
Q

What will fold be like in a thin layer?

A

small folds that later get folded by the thicker layer

172
Q

What are first and second order folds?

A

Largest fold is first order
Smaller folds related to the same fold are second order

173
Q

Why do smaller folds become asymmetric?

A

Occurs along the limbs because of shear and vergence reflects position of origin

174
Q

When do flexural flow and flexural slip occur?

A

During multilayer buckling combined with other processes

175
Q

What is flexural slip similar to?

A

Bending a paper back book

176
Q

What might flexural slip leave evidence for?

A

Bedding parallel to slip (slicklines, shear fractures or shear deformation) on bedding plane

177
Q

What is the difference between flexural slip and flexural flow?

A

There is a discrete slip layering parallel surfaces are replaced by disturbed shear

178
Q

What properties are required for flexural slip and flow?

A

Constant layer thickness
Zero strain at hinge point
Orthogonal lines don’t typically remain orthogonal

179
Q

What occurs with orthogonal flexure?

A

Line initially orthogonal stay so
Neutral surface separates (outer arc extension and inner arc contraction)
Parallel folds

180
Q

When does large scale bending occur?

A

when salt or magma rise towards the surface

181
Q

What are laccoliths?

A

Shallow extrusions that force the overlying rocks and sediments upwards

182
Q

What are kink folds?

A

Deformation of pre-existing foliation

183
Q

What are the characteristics of kink folds?

A

Typically small
Band thickness of a few cm
Angular hinges
Straight limbs
Asymmetric fold geometry

184
Q

What might kink folds grow into?

A

chevron (angular) fold (but usually stop developing before that level of strain required is met)

185
Q
A

Earth structure and tectonics - 33740 (7 decks)

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