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1
Q

what does structural geology study

A

the deformation of rocks by looking at their geometrical structures

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2
Q

why is it important to study structural geology

A

mapping and exploitation of resource (oil and gas, ores, groundwater)
geological reconstruction of complex areas

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3
Q

method of structural geology

A
  1. observation and measurement
  2. restoration
  3. causes, mechanisms, timing
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4
Q

what did identification of striations on the sea floor lead to

A

formulate the existence of oceanic detachment faults in the atlantic ocean

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5
Q

what did oceanic detachment faults provide

A

detachment faults providea new mechanism to form ocean basins, not by classic magmatic activity but via tectonic activity (e.g. stretching of the lithosphere)

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6
Q

how much sea floor spreading is caused by detachment faults

A

80%

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7
Q

structural geology vs tectonics

A

structural geology -> study ofdeformationof rocks through the analysis of thegeometries (e.g., faults, folds, striations) observed at a small scale

tectonics -> set ofprocesses that operate at alarge scale(e.g., mountain building, plate motion) and generate a characteristic set of structures

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8
Q

what is scale invariance

A

an element observed at the microscopic scale is reflected exactly identical at the macroscopic (perhaps kilometric) scale

appear exactly identical at any scale

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9
Q

structural geology as a tool for tectonic reconstructions

A

small-scale structures (e.g., a fault) can be used to infer on large-scale processes (e.g. large mountain belt formation)

structural geology → measurement of foliations in gabbro (tibet)
tectonics → india-asia collision zone: himalayas

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10
Q

what is deformation

A
  • transformation from an initial to a final condition
  • this includes change involume, position, orientationorshape
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11
Q

types of deformation

A
  • dilation(change in volume)
  • translation(change in position)
  • rotation(change inorientation)
  • distortion(change in shape) → also called strain
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12
Q

what is dilation

A

changes in volume (both increase and decrease)

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13
Q

what is translation

A

rigid body change in position

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14
Q

what is rotation

A

change in orientation

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15
Q

what is distortion

A

change in shape
also called strain

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16
Q

simple shear

A
  • deformation produced by shear forces
  • produces shortening in one direction and extension in the perpendicular direction
  • lines not parallel to the shear direction rotates
  • orthogonal lines (see black grid) are no longer perpendicular after deformation
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17
Q

pure shear

A
  • deformation produced bycontractionalorextensionalforces
  • producesshorteningin one direction andextensionin the perpendicular direction
  • linesdo not rotateduring deformation
  • orthogonal lines (see black grid) remainperpendicularduring deformation
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18
Q

what is strain

A
  • he change in shape (visible /measurable) of a rock caused by a stress
  • you can see/measure a folded strata
  • strain is a number (dimensionless)
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19
Q

what is stress

A
  • a force (F) applied over a unit area (A) which produces deformation
  • stress has themeasure of a pressure(force/area) and its unit is N/m2or more commonlyPascal(Pa)
  • stress isnot visible → you cannot see the gravity force
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20
Q

what is compaction

A

compaction reduces the space between grains in sedimentary rocks due to lithostatic pressure

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21
Q

how much can sediments be compacted by

A

50% their initial volume

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22
Q

what type of deformation does compaction induce

A

strain and dilation

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23
Q

what is serpentinisation

A

hydration of mantle rocks (peridotites)

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24
Q

how much can volume increase as a result of serpentinisation

A

up to 40%

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25
Q

what type of deformation does serpentinisation induce

A

dilation

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26
Q

what are joints

A

fractures with minimum opening (mm to cm) and no displacement along their walls

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27
Q

why do joints form

A

form in response to extension and perpendicular to the stretching direction

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28
Q

why are joints important

A
  • form perpendicular to extension -> paleostress analysis
  • enhance permeability of rocks -> oil exploration
  • conduits for magma -> volcanic hazard
  • can localise faulting -> seismic hazard
  • control erosion -> landslide hazard
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29
Q

where do joints form in rocks that are strained by simple shear

A

joints are arranges “en echelon” within a shear zone. dip in the direction of the shear

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30
Q

where do joints form in rocks that are strained by pure shear

A

joints are parallel and overlapping

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31
Q

what are veins

A

jointsfilled with minerals that precipitate from fluids circulating through the fractures

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32
Q

what shape do veins have

A

lens

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33
Q

how do veins form

A

perpendicular to stretching direction

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34
Q

why are veins important

A
  • veins may form large ore deposits
    • gold, silver, lead, copper, zinc etc
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35
Q

what is cleavage

A

planar surfaces where rocks can easily split eg slate

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36
Q

how does cleavage form

A

forms as a result of alignment of platy materials (typically phyllosilicates like mica or chlorite) during deformation and metamorphism

forms perpendicular to compression

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37
Q

what is a fault

A
  • aplanar surface or zonealong which one side have beendisplacedrelative to the other
  • displacement may range from few millimetres to kilometres
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38
Q

when does brittle deformation occur

A

<350ºc or high strain rates

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39
Q

when does ductile deformation

A

> 350ºc or low strain rates

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40
Q

what type of deformation causes faults

A

brittle deformation → faults typically form at shallow depths (less than ~15km) where temperature is below ~350ºc

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41
Q

what is the geothermal gradient value

A

~30ºc/km → 350ºc can be achieved in 10km

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42
Q

how do faults form

A

faults from when theinternal friction(FR) of rocks is overcome due to the application of stress.
when stress approaches FRmicro-cracks form in the region where the future fault will develop
a fault forms when stress is larger than FR
pre-existing weaknesses (e.g. joints or faults) play a key role in fault formation or reactivation

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43
Q

where is the hanging wall of a fault

A

above the fault plane

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44
Q

where is the footwall of a fault

A

below the fault plane

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45
Q

kinematics of a normal fault

A

the hanging wall moves down relative to the footwall

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46
Q

kinematics of a reverse fault

A

the hanging wall moves up relative to the footwall

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47
Q

kinematics of a strike-slip fault

A

hanging wall and footwall are displaced laterally (ie in the horizontal plane)

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48
Q

what is a dextral strike-slip fault

A

right lateral

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49
Q

what is a sinistral strike-slip fault

A

left lateral

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50
Q

what are thrust faults

A

low angle (~30°) reverse faults are calledthrust faults (or just thrusts)

thrusts play a key role in the formation of mountain belts (i.e. orogenesis)

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51
Q

what are detachment faults

A

normal faults typically dips by ~60°. Low angle (~30°) normal faults are calleddetachment faults (or just detachments)

detachment faults allow deep rocks to be exhumed at the surface

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52
Q

what is the heave

A

horizontal displacement between two adjacent points

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53
Q

what is throw

A

vertical displacement between two adjacent points

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54
Q

how do faults terminate

A

faults may terminate laterally by decreasing progressively their displacement
- displacement is zero attip points

faults may also terminate against another fault (transfer fault) or atrelay rampswhere two faults overlap

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55
Q

effect of normal faults on stratigraphy

A

stratigraphy is omitted

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56
Q

effect of reverse faults on stratigraphy

A

stratigraphy is repeated

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57
Q

what are mylonites

A

fine grained rocks formed by dynamic recrystallization of new minerals duringstrain at high temperature

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58
Q

where do mylonites form

A

at deep (>15 km) shear zones where deformation is
ductile

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59
Q

what is the stick-slip mechanism

A

as stress increases, elastic strain builds up overyears(stick phase) until eventually the elastic limit is reachedand the rock snaps (slip phase),forming a fault

the elastic energy is released withinsecondsin the shape ofseismic waves

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60
Q

features of p waves

A
  • arrive first
  • compressional waves
  • can travel in any material
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61
Q

features of s waves

A
  • arrive after p waves
  • shear waves
  • only travel in solids
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62
Q

the seismic cycle

A

longinterseismic stage in which energy is stored as elastic deformation
shortcoseismic stage in which this elasticenergy is abruptly released

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63
Q

what are earthquakes caused by

A

sudden slip on a fault

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64
Q

how do faults grow

A

by repeated slip during earthquakes (i.e. stick-slip mechanism), as totaldisplacement is accumulated over hundreds/thousands of earthquakes

this occurs because once the fault is formed it will likely fail again in the future

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65
Q

how are fault length and fault displacement related

A

length of fault = ~10,000 x fault displacement (1 m slip → 10km long)

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66
Q

what is aseismic slip

A

if thefrictional resistance on the fault is lowelastic energy cannot be stored, and displacement is accumulated at a constant ratewithout causing anyearthquake

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67
Q

where is aseismic slip expected

A

generally occurs in theuppermost part of the crust(< 3 km) where normal stress is lower

stick-slip mechanism occurs at deeper levels between ~3 and ~15 km

at large depths aseismic slip is again expected because rocks are ductile due to high temperature

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68
Q

earthquakes at subduction zones

A
  • earthquakes at subduction zones can be deeper because the temperature at the plate interface is < 350°C
  • shallow → <99km depth
  • intermediate → 100-300km
  • deep → 300-700km
69
Q

what phases of the seismic cycle are energy stored during

A

the stick phase and the interseismic phase

70
Q

what is a fold

A

folds are the product ofductile deformation which resulted in bending of originally planar structure

71
Q

what is the hinge of a fold

A

point or zone of greatest curvature

72
Q

what is the limb of a fold

A

connects two hinge points

73
Q

what is the hinge line of a fold

A

line through the hinge along one layer

74
Q

what is the fold axis

A

any line parallel to the hinge line

75
Q

what is the axial plane of a fold

A

divides two limbs, passing through the hinge lines of overlying laters

76
Q

how are folds classified

A

based on the dip ofaxial planeand plunge of hinge line

77
Q

most common fold classifications

A
  • upright
  • plunging upright
  • horizontal inclined
  • recumbent
78
Q

interlimb angle of gentle folds

A

180-120º

79
Q

interlimb angle of open folds

A

120-70º

80
Q

interlimb angle of close folds

A

70-30º

81
Q

interlimb angle of tight folds

A

30-0º

82
Q

interlimb angle of isoclinal folds

A

0º

83
Q

geometry of monocline folds

A

Monoclines are folds with one limb. They are produced by deeper faults that have not reached the surface, calledblind faults

84
Q

geometry of kink band folds

A

Kink bands aresharp angular folds bounded by planar surfaces. Normally kink bands are observed at a small scale.

85
Q

geometry of chevron folds

A

Form in layers with a regular alternation of contrasting (soft – rigid) competences (e.g. claystones and sandstones)

  • v-shape
  • straight limbs
  • sharp hinges
  • 60º interlimb angle
86
Q

geometry of concentric folds

A
  • rounded shape
  • curved limbs
  • broad hinges (hinge zone)
87
Q

geometry of box folds

A
  • box shaped
  • three limbs
  • two hinges and axial planes
  • ~90º interlimb angle
88
Q

geometry of sheath folds

A

Highly non-cylindrical folds formed inhigh-strain shear zones, hence a deep levels of the crust where rocks deform plastically

89
Q

parasitic folds

A
  • form in layers with different competence (soft and rigid layers alternate)
  • Z-shaped parasitic folds indicate that fold core is located to the right
  • S-shaped parasitic folds indicate that fold core is located to the left
  • M-shaped parasitic folds form near fold hinge
90
Q

what does antiform fold mean

A

convex up (n)

91
Q

what does synform fold mean

A

convex down (u)

92
Q

what does anticline fold mean

A

older rock in fold core

93
Q

what does syncline fold mean

A

younger rock in fold core

94
Q

what is flexural slip

A
  • When a layered rock unit is folded, adjacent layers within each limbs experience a simple shear, which has opposite sense of slip in the two limbs.
  • This process is called flexural slip, and can be reproduced by bending a deck of cards or a soft book.
95
Q

where do slickenlines form

A

parallel to the main stress responsible for the folding may form above the layers surfaces

96
Q

what is flexural flow

A

essentially the same process as flexural slip but with deformation distributed more evenly within the limbs. It commonly occur in more plastic conditions (higher temperature).

97
Q

what are the folding mechanisms

A

buckling, bending, passive folding

98
Q

what is buckling

A

layer-parallel shortening

99
Q

what is bending

A

forces applied at a high angle to the layers

100
Q

what is passive folding

A

produced by simple shear

101
Q

what are the fundamental geological principles

A
  • uniformitarianism
  • superposition
  • cross-cutting relationship
102
Q

what is uniformitarianism

A
  • invariance of physic laws in space and time
  • in geology is described by the concept “present is the key to the past”
103
Q

implications of uniformitarianism

A
  • we can look at modern geological systems (sedimentary, mountain-building, volcanoes etc.) and use them to explain past geological processes
  • similarly, we can look at ancient events (e.g., climate changes) to understand current processes
104
Q

principal of superposition

A
  • younger sediments are deposited horizontally over older sediments in a “layer cake” style
  • this principle is the basis ofstratigraphy.
105
Q

principal of cross-cutting relationship

A
  • younger rocks cut across older rocks
  • very important in structural geology, as it alsoapplies to faults
106
Q

what is angular unconformity

A

Older rocks below were tilted and eroded before the deposition of the younger layers above. Angle between the beds.

107
Q

what is non-conformity

A

Younger layers were deposited over metamorphic/igneous basement where layering is absent.

108
Q

what is disconformity

A

Uplift and erosion of layered sedimentary rocks is followed by down thrown and new deposition: all beds are parallel

109
Q

what are contour lines

A

ines connecting points of equal elevation. A given contour line (or just contour) is the intersection between a 3D object (i.e., topography) and a horizontal plane with a given elevation (in meters)

110
Q

what do different colours represent on geological maps

A

the lines dividing areas of differentcolour areunit boundaries, which are planes

111
Q

what controls the shape of lines on geological maps

A
  • dip of planes
  • topographic profile
112
Q

when do you see the true thickness on a geological map

A

Thetrue thicknessof a rock unit is visible only along a sectionperpendicular to its layering/bedding.
Along any other section you see theapparent thickness, which is always larger than the true one.

113
Q

when is true dip observed on a geological map

A

only along a section normal to the strike (or parallel to the dip direction).
Any other section shows anapparent dip,which is alwaysshallowerthan the true one.

114
Q

how do vertical plane unit boundaries appear on geological maps

A

straight line cutting across contour lines

115
Q

how do horizontal plane unit boundaries appear on geological maps

A

curved line running parallel to a specific contour line

116
Q

how do inclined plane unit boundaries appear on geological maps

A

curved line cutting across contour lines

117
Q

how do folds appear on geological maps

A

marked by arepetition of units, which are symmetric with respect to the hinge line

118
Q

where is the hinge line of a fold located on a geological map

A

between two areas of the map showingopposite dips(convergent or divergent)

119
Q

unit boundaries of upright folds on geological maps

A

parallel to each other

120
Q

plunging folds on geological maps

A
  • Plunging folds in geological maps produceV-shaped unit boundaries
  • The two limbs of plunging fold havedifferent strike
121
Q

plunging folds on maps: the v rule

A
  • if plunging folds form chevron-shaped unit boundaries:
  • Plunging antiform: plunge is in the direction of the chevron
  • Plunging synform: plunge towards the core of the chevron
122
Q

faults in geological maps

A
  • In geological maps faultscut and offset unit boundaries.
  • The offset observed at the topographic surface may be deceiving on the true kinematics of the fault.
  • Lateral offset of unit boundaries in geological maps donot always indicate strike-slip faults.
  • Lateral offset is created also whendipping bedsare faulted by normal or reverse faults.
123
Q

what are stereographic projections used for

A
  • Stereographic projections (orStereonets) are the simplest and most effective tools torecord, plot, and analyse 3-D data on 2-D systems(maps, field books).
  • Stereonetsallow us to recordand view easilythousandsor data in one small plot
124
Q

what planar features can be plotted on stereonets

A
  • bedding planes
  • fault planes
  • axial planes of folds
  • veins
  • joints
  • cleavage/foliation
125
Q

what linear features can be plotted on stereonets

A
  • fault plane striations (eg slickenlines)
  • fold axes
  • hinge lines
  • paleocurrent directions
126
Q

how are linear structures measured

A
  • plunge (angle of line from horizontal; 2 digits)
  • plunge azimuth (measured down plunge; 3 digits)
  • plunge first then plunge azimuth (the arrows or slash reads as “towards”)
  • eg: 30º→068º
127
Q

how are planar features measured

A
  • strike (angle of horizontal line on the plane to north; 3 digits)
  • dip (angle of plane from horizontal; 2 digits)
  • dip direction (direction of maximum inclination of the plane)
  • strike first then dip, followed by dip direction
  • eg: 235º/24º NW
128
Q

how are strikes and azimuths read on stereonets

A

outer circle of the stereonet (primitive) as on a compass, with north east south and west being at 0º, 90º, 180º and 270º respectively
longitude and latitudes have a 2º spacing

129
Q

how are dips and plunges read on a stereonet

A

along a line connecting the primitive to the centre of the stereonet, with values being 0º at the primitive and 90º at the centre

130
Q

how is the magnetic field produced

A
  • by a magnetic dipole (magnet with N and S pole) that is roughly parallel to the earth’s spin axis
  • generated into the outer liquid core upon its rotation around the earths spin axis, based on a mechanism called geodynamo
131
Q

function of the magnetic field

A

acts as a shield for the earth against dangerous particles projected from the sun towards the earth, known as the solar wind

132
Q

shape of magnetic field

A

distorted by the solar wind, forming a tail behind the earth

133
Q

key features of tectonic plates (5)

A
  • solid, rigid, large portion of the earths outer layer
  • composed of crust + upper mantle
  • coincide with the lithosphere (30-150km thick)
  • float over a thin (50-100km) zone of partially molten mantle called asthenosphere
  • although rigid, plates deform at their margins where they interact with each other, while other interiors, called craton, remain undeformed (and typically old)
134
Q

how do tectonic plates move

A
  • at high temperatures, solid-state flow can occur in the mantle (~1cm/yr), allowing plates to penetrate it
  • note the convection currents model is an old, invalid idea
  • plate motion is then supported by a lubricating zone below them called asthenosphere
135
Q

why are continental and oceanic crust at different elevations

A

continental crust is thicker, less dense, and more buoyant then oceanic crust, eg it floats more on the mantle (3.3g/cm3)

136
Q

describe crust that is above sea level

A
  • continental crust
  • thicker (30-70km)
  • granodioritic mean composition (felsic)
  • density of 2.7g/cm3
137
Q

describe crust that is below sea level

A
  • oceanic crust
  • thinner (5-6km)
  • basaltic mean composition (mafic)
  • density of 3.0g/cm3
138
Q

what theory explains why continents are much older than oceans

A

the wilson cycle

139
Q

wilson cycle year

A

1966

140
Q

who coined the term “plate”

A

tuzo wilson

141
Q

when was plate tectonics theory formulated

A

1967

142
Q

what occurs at divergent plate boundaries

A
  • crust is formed
  • normal faults
143
Q

what occurs at convergent plate boundaries

A
  • crust is consumed
  • reverse/normal faults
144
Q

what occurs at transform plate boundaries

A
  • strike-slip faults
  • plates slide laterally
145
Q

plate tectonics effect on volume of crust

A
  • constant volume
    • crust produced = crust consumed
146
Q

structure of oceanic crust

A

layer 1: sediments (cherts), semail ophiolite (oman)

layer 2: pillow lavas, semail ophiolite (oman)

layer 3: sheeted dykes, mirdita ophiolite (albania)

layer 4: gabbro, semail ophiolite (oman)

layer 5 (mantle): mirdita ophiolite (albania)

147
Q

how was the structure of oceanic crust discovered

A

geophysics and fragments of oceanic crust on land called ophiolites

148
Q

fast spreading ridges

A
  • (>4cm/yr)
    • sufficient melt production for all spreading to be taken up by creation of new crust
    • steady-state magma chamber feeding dykes, which feed pillow lavas; gabbros form out of the remaining melt in the magma chamber
149
Q

slow spreading ridges

A
  • (<4cm/yr)
    • upwelling mantle cools
    • less melt produced
    • faults cut deep
    • pronounced rift valley results
    • magma chamber only intermittently present
150
Q

ultra-slow spreading ridges

A
  • oceanic detachments form at slow and ultra-slow spreading ridges (<4cm/yr)
  • low angle normal faults with a curved shape rooted below the spreading ridge
151
Q

example of continental rifting

A
  • the east african rift is a region experiencing tectonic extension, lithospheric break up and volcanism as a consequence of this
  • a new ocean will be generated at this rift within a few million years
152
Q

earthquake mechanism on transform faults

A

earthquake mechanism → strike-slip mechanism

153
Q

what are fracture zones

A

the lateral continuation of transform faults, but unlike transform faults they are inactive (no relative displacement along them)

154
Q

what do fracture zones show

A
  • indicate the direction of plates in the past
  • show the orientation of transform faults at the time of crust formation
  • record relative motion of plates through time
155
Q

what types of convergent margin are subduction

A
  • ocean-ocean (eg tonga, antilles)
  • ocean-continent (eg andes, rocky mountains)
156
Q

what types of convergent margin are under-thrusting

A
  • continent-continent (eg tibet, himalaya)
  • always starts with an ocean-continent subduction
157
Q

what is a subduction zone slab

A

portion of the down-going plate sank into the mantle

158
Q

what is a subduction zone trench

A

depression where plate bends into subduction zone

159
Q

what is a subduction zone volcanic arc

A

100-150km above slab where fluids are expelled causing overlying mantle to melt (slab itself does not melt)

160
Q

what is a subduction zone forearc

A

region located between the trench and the volcanic arc

161
Q

what is a subduction zone accretionary wedge

A

pile of sediments scraped off (bulldozed) subducting plate

162
Q

what is a subduction zone back-arc

A

region located behind the volcanic arc where spreading/extension may occur

163
Q

what is slab-roll back

A
  • if the sinking of the slab is faster than the convergence between the two plates the slab ‘rolls back’ and the trench retreats backwards
  • this may cause extension in the overriding plate, leading to the opening of a back-arc basin
164
Q

where do volcanoes mainly form

A

at convergent margins (subduction zones)

165
Q

what is the average distance of a volcanic arc-trench

A

150-200km

166
Q

continent-continent subduction

A
  • continent-ocean subduction may eventually lead to continent-continent subduction
  • neither will subduct easily (they are both buoyant)
  • both plate margins are deformed and faulted, leading to orogeny (mountain building)
167
Q

what are the main forces acting on plates to cause movement

A
  • slab pull
    • weight of cool, dense subducting slab pulls plate along
  • ridge push
    • like the plate sliding downhill from the high ridge
168
Q

what is ridge push force

A
  • the elevation difference between the ridge and the adjacent deeper seafloor triggers a force called ridge push
  • directed horizontally, perpendicular to and away from the ridge
  • like a wedge of honey with a sloping surface, the mass of the ridge pushes sideways
  • considered one of the main driving forces of plate tectonics
169
Q

what is slab pull force

A
  • the negative buoyancy of the sense slab (vertical downward) applied along an inclined surface (subduction zone) drags the plate down
  • produces a horizontal motion of the plate towards the trench (yet not necessarily perpendicular to it)

earth structures and tectonics (1 decks)

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