1
Q
what does structural geology study
A
the deformation of rocks by looking at their geometrical structures
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
3
Q
method of structural geology
A
- observation and measurement
- restoration
- causes, mechanisms, timing
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
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)
6
Q
how much sea floor spreading is caused by detachment faults
A
80%
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
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
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
10
Q
what is deformation
A
- transformation from an initial to a final condition
- this includes change involume, position, orientationorshape
11
Q
types of deformation
A
- dilation(change in volume)
- translation(change in position)
- rotation(change inorientation)
- distortion(change in shape) → also called strain
12
Q
what is dilation
A
changes in volume (both increase and decrease)
13
Q
what is translation
A
rigid body change in position
14
Q
what is rotation
A
change in orientation
15
Q
what is distortion
A
change in shape
also called strain
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
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
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)
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
20
Q
what is compaction
A
compaction reduces the space between grains in sedimentary rocks due to lithostatic pressure
21
Q
how much can sediments be compacted by
A
50% their initial volume
22
Q
what type of deformation does compaction induce
A
strain and dilation
23
Q
what is serpentinisation
A
hydration of mantle rocks (peridotites)
24
Q
how much can volume increase as a result of serpentinisation
A
up to 40%
25
what type of deformation does serpentinisation induce
dilation
26
what are joints
fractures with minimum opening (mm to cm) and no displacement along their walls
27
why do joints form
form in response to extension and perpendicular to the stretching direction
28
why are joints important
- 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
29
where do joints form in rocks that are strained by simple shear
joints are arranges “en echelon” within a shear zone. dip in the direction of the shear
30
where do joints form in rocks that are strained by pure shear
joints are parallel and overlapping
31
what are veins
joints filled with minerals that precipitate from fluids circulating through the fractures
32
what shape do veins have
lens
33
how do veins form
perpendicular to stretching direction
34
why are veins important
- veins may form large ore deposits
- gold, silver, lead, copper, zinc etc
35
what is cleavage
planar surfaces where rocks can easily split eg slate
36
how does cleavage form
forms as a result of alignment of platy materials (typically phyllosilicates like mica or chlorite) during deformation and metamorphism
forms perpendicular to compression
37
what is a fault
- a planar surface or zone along which one side have been displaced relative to the other
- displacement may range from few millimetres to kilometres
38
when does brittle deformation occur
<350ºc or high strain rates
39
when does ductile deformation
>350ºc or low strain rates
40
what type of deformation causes faults
brittle deformation → faults typically form at shallow depths (less than ~15km) where temperature is below ~350ºc
41
what is the geothermal gradient value
~30ºc/km → 350ºc can be achieved in 10km
42
how do faults form
faults from when the internal friction (FR) of rocks is overcome due to the application of stress.
when stress approaches FR micro-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
43
where is the hanging wall of a fault
above the fault plane
44
where is the footwall of a fault
below the fault plane
45
kinematics of a normal fault
the hanging wall moves down relative to the footwall
46
kinematics of a reverse fault
the hanging wall moves up relative to the footwall
47
kinematics of a strike-slip fault
hanging wall and footwall are displaced laterally (ie in the horizontal plane)
48
what is a dextral strike-slip fault
right lateral
49
what is a sinistral strike-slip fault
left lateral
50
what are thrust faults
low angle (~30°) reverse faults are called thrust faults (or just thrusts)
thrusts play a key role in the formation of mountain belts (i.e. orogenesis)
51
what are detachment faults
normal faults typically dips by ~60°. Low angle (~30°) normal faults are called detachment faults (or just detachments)
detachment faults allow deep rocks to be exhumed at the surface
52
what is the heave
horizontal displacement between two adjacent points
53
what is throw
vertical displacement between two adjacent points
54
how do faults terminate
faults may terminate laterally by decreasing progressively their displacement
- displacement is zero at tip points
faults may also terminate against another fault (transfer fault) or at relay ramps where two faults overlap
55
effect of normal faults on stratigraphy
stratigraphy is omitted
56
effect of reverse faults on stratigraphy
stratigraphy is repeated
57
what are mylonites
fine grained rocks formed by dynamic recrystallization of new minerals during strain at high temperature
58
where do mylonites form
at deep (>15 km) shear zones where deformation is
ductile
59
what is the stick-slip mechanism
as stress increases, elastic strain builds up over years (stick phase) until eventually the elastic limit is reached and the rock snaps (slip phase), forming a fault
the elastic energy is released within seconds in the shape of seismic waves
60
features of p waves
- arrive first
- compressional waves
- can travel in any material
61
features of s waves
- arrive after p waves
- shear waves
- only travel in solids
62
the seismic cycle
long interseismic stage in which energy is stored as elastic deformation
short coseismic stage in which this elastic energy is abruptly released
63
what are earthquakes caused by
sudden slip on a fault
64
how do faults grow
by repeated slip during earthquakes (i.e. stick-slip mechanism), as total displacement is accumulated over hundreds/thousands of earthquakes
this occurs because once the fault is formed it will likely fail again in the future
65
how are fault length and fault displacement related
length of fault = ~10,000 x fault displacement (1 m slip → 10km long)
66
what is aseismic slip
if the frictional resistance on the fault is low elastic energy cannot be stored, and displacement is accumulated at a constant rate without causing any earthquake
67
where is aseismic slip expected
generally occurs in the uppermost 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
68
earthquakes at subduction zones
- 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
what phases of the seismic cycle are energy stored during
the stick phase and the interseismic phase
70
what is a fold
folds are the product of ductile deformation which resulted in bending of originally planar structure
71
what is the hinge of a fold
point or zone of greatest curvature
72
what is the limb of a fold
connects two hinge points
73
what is the hinge line of a fold
line through the hinge along one layer
74
what is the fold axis
any line parallel to the hinge line
75
what is the axial plane of a fold
divides two limbs, passing through the hinge lines of overlying laters
76
how are folds classified
based on the dip of axial plane and plunge of hinge line
77
most common fold classifications
- upright
- plunging upright
- horizontal inclined
- recumbent
78
interlimb angle of gentle folds
180-120º
79
interlimb angle of open folds
120-70º
80
interlimb angle of close folds
70-30º
81
interlimb angle of tight folds
30-0º
82
interlimb angle of isoclinal folds
0º
83
geometry of monocline folds
Monoclines are folds with one limb. They are produced by deeper faults that have not reached the surface, called blind faults
84
geometry of kink band folds
Kink bands are sharp angular folds bounded by planar surfaces. Normally kink bands are observed at a small scale.
85
geometry of chevron folds
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
geometry of concentric folds
- rounded shape
- curved limbs
- broad hinges (hinge zone)
87
geometry of box folds
- box shaped
- three limbs
- two hinges and axial planes
- ~90º interlimb angle
88
geometry of sheath folds
Highly non-cylindrical folds formed in high-strain shear zones, hence a deep levels of the crust where rocks deform plastically
89
parasitic folds
- 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
what does antiform fold mean
convex up (n)
91
what does synform fold mean
convex down (u)
92
what does anticline fold mean
older rock in fold core
93
what does syncline fold mean
younger rock in fold core
94
what is flexural slip
- 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
where do slickenlines form
parallel to the main stress responsible for the folding may form above the layers surfaces
96
what is flexural flow
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
what are the folding mechanisms
buckling, bending, passive folding
98
what is buckling
layer-parallel shortening
99
what is bending
forces applied at a high angle to the layers
100
what is passive folding
produced by simple shear
101
what are the fundamental geological principles
- uniformitarianism
- superposition
- cross-cutting relationship
102
what is uniformitarianism
- invariance of physic laws in space and time
- in geology is described by the concept “present is the key to the past”
103
implications of uniformitarianism
- 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
principal of superposition
- younger sediments are deposited horizontally over older sediments in a “layer cake” style
- this principle is the basis of stratigraphy.
105
principal of cross-cutting relationship
- younger rocks cut across older rocks
- very important in structural geology, as it also applies to faults
106
what is angular unconformity
Older rocks below were tilted and eroded before the deposition of the younger layers above. Angle between the beds.
107
what is non-conformity
Younger layers were deposited over metamorphic/igneous basement where layering is absent.
108
what is disconformity
Uplift and erosion of layered sedimentary rocks is followed by down thrown and new deposition: all beds are parallel
109
what are contour lines
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
what do different colours represent on geological maps
the lines dividing areas of different colour are unit boundaries, which are planes
111
what controls the shape of lines on geological maps
- dip of planes
- topographic profile
112
when do you see the true thickness on a geological map
The true thickness of a rock unit is visible only along a section perpendicular to its layering/bedding.
Along any other section you see the apparent thickness, which is always larger than the true one.
113
when is true dip observed on a geological map
only along a section normal to the strike (or parallel to the dip direction).
Any other section shows an apparent dip, which is always shallower than the true one.
114
how do vertical plane unit boundaries appear on geological maps
straight line cutting across contour lines
115
how do horizontal plane unit boundaries appear on geological maps
curved line running parallel to a specific contour line
116
how do inclined plane unit boundaries appear on geological maps
curved line cutting across contour lines
117
how do folds appear on geological maps
marked by a repetition of units, which are symmetric with respect to the hinge line
118
where is the hinge line of a fold located on a geological map
between two areas of the map showing opposite dips (convergent or divergent)
119
unit boundaries of upright folds on geological maps
parallel to each other
120
plunging folds on geological maps
- Plunging folds in geological maps produce V-shaped unit boundaries
- The two limbs of plunging fold have different strike
121
plunging folds on maps: the v rule
- 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
faults in geological maps
- In geological maps faults cut 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 do not always indicate strike-slip faults.
- Lateral offset is created also when dipping beds are faulted by normal or reverse faults.
123
what are stereographic projections used for
- Stereographic projections (or Stereonets) are the simplest and most effective tools to record, plot, and analyse 3-D data on 2-D systems (maps, field books).
- Stereonets allow us to record and view easily thousands or data in one small plot
124
what planar features can be plotted on stereonets
- bedding planes
- fault planes
- axial planes of folds
- veins
- joints
- cleavage/foliation
125
what linear features can be plotted on stereonets
- fault plane striations (eg slickenlines)
- fold axes
- hinge lines
- paleocurrent directions
126
how are linear structures measured
- 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
how are planar features measured
- 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
how are strikes and azimuths read on stereonets
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
how are dips and plunges read on a stereonet
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
how is the magnetic field produced
- 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
function of the magnetic field
acts as a shield for the earth against dangerous particles projected from the sun towards the earth, known as the solar wind
132
shape of magnetic field
distorted by the solar wind, forming a tail behind the earth
133
key features of tectonic plates (5)
- 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
how do tectonic plates move
- 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
why are continental and oceanic crust at different elevations
continental crust is thicker, less dense, and more buoyant then oceanic crust, eg it floats more on the mantle (3.3g/cm3)
136
describe crust that is above sea level
- continental crust
- thicker (30-70km)
- granodioritic mean composition (felsic)
- density of 2.7g/cm3
137
describe crust that is below sea level
- oceanic crust
- thinner (5-6km)
- basaltic mean composition (mafic)
- density of 3.0g/cm3
138
what theory explains why continents are much older than oceans
the wilson cycle
139
wilson cycle year
1966
140
who coined the term “plate”
tuzo wilson
141
when was plate tectonics theory formulated
1967
142
what occurs at divergent plate boundaries
- crust is formed
- normal faults
143
what occurs at convergent plate boundaries
- crust is consumed
- reverse/normal faults
144
what occurs at transform plate boundaries
- strike-slip faults
- plates slide laterally
145
plate tectonics effect on volume of crust
- constant volume
- crust produced = crust consumed
146
structure of oceanic crust
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
how was the structure of oceanic crust discovered
geophysics and fragments of oceanic crust on land called ophiolites
148
fast spreading ridges
- (>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
slow spreading ridges
- (<4cm/yr)
- upwelling mantle cools
- less melt produced
- faults cut deep
- pronounced rift valley results
- magma chamber only intermittently present
150
ultra-slow spreading ridges
- 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
example of continental rifting
- 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
earthquake mechanism on transform faults
earthquake mechanism → strike-slip mechanism
153
what are fracture zones
the lateral continuation of transform faults, but unlike transform faults they are inactive (no relative displacement along them)
154
what do fracture zones show
- 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
what types of convergent margin are subduction
- ocean-ocean (eg tonga, antilles)
- ocean-continent (eg andes, rocky mountains)
156
what types of convergent margin are under-thrusting
- continent-continent (eg tibet, himalaya)
- always starts with an ocean-continent subduction
157
what is a subduction zone slab
portion of the down-going plate sank into the mantle
158
what is a subduction zone trench
depression where plate bends into subduction zone
159
what is a subduction zone volcanic arc
100-150km above slab where fluids are expelled causing overlying mantle to melt (slab itself does not melt)
160
what is a subduction zone forearc
region located between the trench and the volcanic arc
161
what is a subduction zone accretionary wedge
pile of sediments scraped off (bulldozed) subducting plate
162
what is a subduction zone back-arc
region located behind the volcanic arc where spreading/extension may occur
163
what is slab-roll back
- 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
where do volcanoes mainly form
at convergent margins (subduction zones)
165
what is the average distance of a volcanic arc-trench
150-200km
166
continent-continent subduction
- 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
what are the main forces acting on plates to cause movement
- slab pull
- weight of cool, dense subducting slab pulls plate along
- ridge push
- like the plate sliding downhill from the high ridge
168
what is ridge push force
- 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
what is slab pull force
- 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
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