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.
2
Q
generally how much do plates move each year?
A
plates generally move tens of mm a year
3
Q
whats the formulas for stress, strain and strain rate?
A
stress = F/A
strain = X/L
strain rate = strain/time
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.
5
Q
what do the arrows on a gps vector map show?
A
arrows point in direction of movement
size gives magnitude of movement
6
Q
describe the tectonics of california.
A
predominantly transverse
western part of california is moving rapidly past the rest of the USA
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
8
Q
plates generally form ____________ which predominantly deform at ___________
A
plates generally form rigid blocks that predominantly deform at their edges
9
Q
small yearly displacement X large amount of time =
A
a potentially large amount of deformation
10
Q
what does deformation encompass?
A
deformation encompasses:
folding
faulting
shearing
compressing
extension
of rock by tectonic or gavitational forces
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
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
13
Q
we use what we see at the surface to have a best guess of…
A
the underground
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.
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.
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
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
18
Q
which is permentent elastic or plastic deformation?
A
plastic deformation is permanent
19
Q
deformation style depends on?
A
type of material (or rock)
whether it is being extended or compressed
temperature
pressure
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
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.
22
Q
what causes reverse and thrust faulting?
A
caused by compressive forces that squeeze and shorten the rock.
23
Q
what is the difference between reverse and thrust faulting?
A
reverse = steep dipping fault plane
thrust = shallow dipping fault plane
24
Q
how do seismic surveys work?
A
waves are reflected depending on acoustic impedance between layers
25
what methods can be used to learn about structral geology
material science and rock mechanics (theory and experiment)
observations and measurments
geological mapping
numerical and analogue modelling
GIS, remote sensing and seismic metodologies
26
geological map combine ...
geological map combine **topography** and **geological structure**
27
define strike
the direction of the horizontal in any inclined plane.
there is only one such line on any inclined plane
28
what is the convention of measureing dip to strike?
beds dip to the right of the strike
29
how many figures do we use for strike and dip?
strike = 3 figures
dip = 2 figures
30
define dip
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.
31
what is the convention in writing dip and dip direction?
we use dip and then dip direction
32
how can we use the thickness of beds as seen at the surface to tell the steepness of a bed?
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.
33
what can we say if geological boundaries are parallel to contour lines?
we can say that the beds are horizontal
34
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?
outcrop pattern V points in direction of dip
a straight line signifies a vertical line
35
other than the dip of the bed what else controls the thickness of the bed as seen on a geological map?
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
36
what shape do you look for on a geological map to see an angular unconformity?
a T shape
37
what do we look for in a anticline on a map?
the oldest rocks in the middle
38
what can we say of the fold axis of an anticline that is plunging?
the fold axis is not horizontal
39
how do we view plunging anticlines on a geological map?
outcrop patterns curve in direction of plunge
beds dip away from centre
oldest rocks in centre
40
how do we view plunging synclines on a geological map?
opposite to anticlines
outcrop patterns curve in opposite direction to the plunge
beds dip towards the core of the syncline
rocks youngest in centre.
41
how do we tell if the V shapes on a geological map are either a plunging syncline or anticline?
look at strat column to find the oldest rocks
look at dip direction of rocks
42
what are outliers?
outliers are isolated areas of younger rocks surrounded by older rocks
**_o_**utliers have **_o_**ldest on **_o_**utside
43
what is meant by an inlier?
inliers are isolated areas older rocks surounded by younger rocks
44
what are ouliers and inliers called when the features are created by faulting?
outlier = **klippe** (pl. klippen)
inlier = **window** beneath the thrust plane
45
deformation is where physical changes are produced as a result of
deformation is where physical changes are produced as a result of the action of applied forces - such as gravity and tectonics
46
define shear stress
the force per unit area in a direction parallel to the area to which it is applied
47
if there is no overall acceleration on a body then we can always find...
if there is no overall acceleration on a body then we can always find three mutually perpendicular planes where the shear stress is zero
48
what is meant by principle stress axes?
the normal stresses, on axis perpendiculer to planes of zero shear stress are called the principle stress axes
49
what terms are given to the maximum and minimum stress axes?
maximum = σ1
minimum = σ3
50
strain is deformation produced as a result of \_\_\_\_\_\_
strain is deformation produced as a result of stress
51
stress can lead to changes in...
stress can lead to changes in the volume, size and orientation of objects
52
what is meant by homogeneous strain?
**homogeneous strain**
where straight lines remain straight, and strain is the same through the body.
53
what is meant by heterogeneous strain?
**hetrogeneous strain**
where strain is non-uniform through the body
(looks like a prune)
54
what is meant by pure shear?
**pure shear**
where strain axes are the same as the original body
all the angles at still at 90o
55
what is meant by simple shear?
**simple shear**
where the deformation is rotational.
56
all of the measurements of strain are \_\_\_\_\_\_\_\_\_\_\_.
linear strains are often quoted in \_\_\_\_\_\_\_\_\_\_.
all of the measurements of strain are dimensionless
linear strains are often quoted in percent
57
what do the axis on a strain ellipsoid represent?
x = σ1
y = σ3
z = σ2
58
how do we visulaise strain ellipsoids in rocks?
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)
59
in regards to the strain ellipsoid what is the flinn diagram?
**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
60
which axes are larger relative to others to create cigars and pancakes on a flinn diagram
cigar **x\>\>y\>z**
pancake** x~y\>\>z**
61
what do we need to apply the strain ellipsoid to real rocks?
some form of strain marker in the rock (fossils, clast, dykes, foliations
an initial idea of what the marker looked like
62
define rheology.
**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.
63
what is elastic deformation?
in elastic deformation, stress is proportional to strain.
deformation is recoverable.
64
if stress is proportional to strain then what is the constant?
what is the name given to this law\>
the constant is the youngs modulus with units of Pa
this is known as hooke's law
65
how do rocks deep in the crust behave like when compressed or stretched beyond a relatively small amount?
behave plastically
66
for viscous materials at high temperature, stress is proportional to...
the constant is\_\_\_\_\_\_\_\_\_
stress is proportional to strain rate
the constant is viscosity
67
what is meant by plastic deformation?
**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.
68
what happens to perfectly plastic materials once they yield?
perfectly plastic materilas deform at a constant stress regardless of strainrate once they yield so technically their viscosity is zero.
69
what is meant by strain hardening and strain softening?
**strain hardening**
more stress needed to strain rock once is has yielded
**strain softening**
less stress needed to strain rock once it has yielded
70
what is meant by ductility?
**ductility**
describes the ability of a sample or rock to derom under stress
71
describe the three types of deformation in rocks
72
which has the largest youngs modulus?
which has the lowest compressive strength?
which has the largest youngs modulus? **pyroxenite**
which has the lowest compressive strength? **dolomite**
73
what are the controls on the rheology of rocks?
applied stress
temperature
rock type (and/or orientation of rock)
confining pressure
strain rate
74
what is the impact of temperature on rheology of a rock?
higher temperatures = lower strength (acts more ductile)
75
what is the impact of orientation of a rock on a stress strain diagram?
stronger when σ1 is parallel to the dominant foliation.
76
what is the impact of confining pressure on rheology of rocks?
both strength and plasticity increase with greater confining pressure
77
what is the impact of strain rate on rheology of rocks?
rocks are weaker and deform plastic ductile way when low strain rates are applied (slow)
they flow in a process called creep
78
in a stress against strain rate graph what does the line =
how do we find the angle the graph makes to the horizontal?
line = viscosity
tan-1(viscosity) = angle to horizontal
viscosity = stress/ strain rate
79
what does e on this graph mean?
what does the graph represent?
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
80
describe the graphs of a plastic material
81
define brittle
**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
82
which is more prone to brittle failure , extension or contraction?
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
83
what conditions are suited for brittle failure?
**brittle failure favoured:**
low temp
low confining pressure
high strain rates
coarse grained or heterogeneous rocks
in tension rather than compression
high water pressure
84
what brittle deformation mechanisms are there in terms of grains?
include frictional sliding along grain contacts
grain rotation
grain fracture
85
what do we say if a rock has under gone cataclasis?
where the individual grains break (micro fracturing)
86
what is meant by fracture?
**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
87
what do fractures become if they have any significant displacement?
fractures become **faults** if they have any significant displacement
88
which two fractures do you normally find together?
often find shear and extensional fractures together.
89
how do fractures grow in uniaxial tensional normal stress?
fractures grow perpendicular to σ3
90
how do fractures grow in uniaxial compressional normal stress?
fracture develops parallel to σ1
91
in both triaxial extension and compression what direction does the shear fracture develop at?
shear fracture develops ~30 degrees to σ1
92
on what plane does a shear fracture develop on?
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
93
in theory maximum shear stress should be on a plane of 45 degrees to σ1 and σ3. why isnt this the case?
because less normal stress (on a steeper plane) helps shearing to go more readily
94
what is meant by inernal angle?
intranal angle = ß
95
what is the formula and graph showing the linear relationship between normal stress and shear stress on failure?
96
on the coulomb failure criteria what do we plot on the normal stress axis?
we plot σ1 and σ3 with the average normal stress in the middle
97
what is the formula for shear stress
98
on the coulomb failure criteria what is the angle between the line and σn?
it is equal to ß
99
in the formula for shear stress what is mu equal to?
100
in coulombs failure criteria, what is the gradient the tangent of?
the gradient is the tangent of the internal angle
101
what is shear stress failure equal to interms of the mohr's circle?
102
what is the formula for the normal stress σn?
where k is from the centre of circle to y axis
103
what is the formula for shear stress?
104
what does coulombs failure line show?
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
what is meant when a mohr's circle is:
not overlaping
touching
overlaping
the envelope?
not overlaping - **stable**
touching - **critical**
overlapping - **will fail** (not stable)
106
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?
griffith criterion takes account of crack size and length and thus the coulomb mohr critero changes as it over estimates the tensile strength.
107
what is the effect of pore fluid pressue on the effective normal stress?
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
what is meant by anisotropy?
anistropy means a prexisting weakness in a rock
109
what is the effect of anisotropy on a mohrs circle?
usually it will fail along the fracture
110
what are the three mohrs circle complexities
pore fluid pressure
anistropy
failure in tension
111
define a fold
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
folds are important structral _____ for the petroleum industry.
folds are important structral **traps** for the petroleum industry
113
anticline and syncline refer to \_\_\_\_\_ relationships in the rock
anticline and syncline refer to age relationships in the rock
114
synform are...
synforms are any downward closing structures
115
antiforms are...
antiforms are any upward closing structures
116
synclines are..
any downward closing structures when the rocks are the right way up
117
anticlines are...
any upward closing structures when the rocks are the right way up
118
what is meant by an antiformal syncline?
it is an antiform however the rocks have been overturned so the youngest rocks are now in the middle
119
what is fold geometry classification based on?
dip of fold axial plane
plunge of hinge line
120
what two types of shaped folds can we get?
**cylindrical fold** (godd for stereonet)
**non-cylindrical fold** (could be conical or have a nonstraight hinge line)
121
describe a fold labelling the points
axial surface
hinge line
fold axis
inflection point
inflection line
interimb angle
limb
amplitude
wavelength
122
what are the four interlimb angles? from little strain to lots of strain?
123
what is meant by m, s and z folds?
1st is m folds
2 and 3 are either s or z folds
124
what is meant by fold vergence?
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 a**xial plane dips to the left** so the fold **verges to the right.**
125
refolded folds is the same principle as \_\_\_\_\_\_\_
refolded folds is the same principle as the superposition of waves
126
what is meant by dip isogons?
making lines by joining up equal dip on the inner and outer arc of a fold
127
what is meant by class 1, 2 and 3 dip isogons?
**class 1**
isogons that converge on inner arc
**class 2**
vertical dip isogons
**class 3**
diverging isogons on inner arc
128
what is special about class 1b dip isogons?
the lines are parallel to each other
129
what is meant by buckling or active folding?
**buckling (or active folding)**
occurs for layer parallel shortening in a rock when there is a contrast in rheology (viscosity) between different layers
130
what is meant by bending and passive or shear folding?
131
what is meant by passive folding?
**passive folding**
the layers in the rock exert no mechanical influence on the folding- they are just passive strain markers
132
what is meant by shear folds?
**shear folds**
are passize folds associated with a significant component of simple shear
133
what type of folding normaly creates class 2 folds?
passive folding associated with pure shear typically form class 2 folds.
134
what is meant by pure shear?
what do you need for pure shear?
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
how a layer deforms depends on rheology and layer thickness.
what is the effect of layer thickness?
layer thickness - which controls fold wavelength
thin layers have shorter wavelengths
136
what is meant by compentent and incompetent layers?
**competent**
layers maintain thickness round fold (class 1b)
this means the weak layers cant
**incompetent**
layers change in thickness around fold
137
what is meant by parasitic folding?
small high frequancy folds on top of thick low frequancy folds
138
explain how parasitic folding is formed.
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
smaller scale parasitic fold can verge towards the hinge of the bigger structure thus reflecting ...
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
to maintain layers with uniform thickness as folding continues we either:
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
why are class 1B folds useful to structral geologists?
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
what is an implication of calculating strain from a fold?
the layer normally shortens first before it buckles and fold.
calculating strain does not take this into account, thus is wrong
143
what is a geological fault?
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
a geological fault can be active and seismogenic meaning...
is is still accumulating slip at the present day
145
a geological fault can be dead but \_\_\_\_\_\_\_\_\_ in the rock record
A geological fault can be dead but preserved in the rock record
146
describe a normal fault
produced from tension forces.
they are extensional features because they involve a lengthening of the original unfaulted block
angle ~50-60 degrees
147
describe a reverse or thrust fault
created from compressional forces
angle ~30 degrees
148
how can you tell what type of strike slip fault it is?
what usually is the angle of the fault?
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
what is meant by oblique slip faulting?
possible to combine strike slip and dip slip fault types e.g. transtensional faults
150
define displacement, throw and heave interms of fault
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
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 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
why does a normal fault have an angle of 60 degrees?
as σ1 is gravity alpha will be 30 degrees then the dip of the fault plane is 60 degrees.
153
why does a reverse fault have an angle of 30 degrees
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
why does a strike slip fault have a vertical angle?
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
what are the implications that make it hard to describe the geometry of a fault?
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
the greater the fault length the greater the \_\_\_\_\_\_\_\_\_
the greater the fault length the greater the **displacement**
157
describe the displacment along the strike of a fault.
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
faults dont have to be a single plane.
they often have a ___________________ associated with them
faults dont have to be a single plane.
they often have a deformation or damage zone associated with them
159
faults consist of one or more ______ that may overlap
faults consist of one or more segments that may overlap
160
what is meant by relay ramps?
often exploited by rivers draining uplifted footwall blocks
if you sum the displacment on both faults it would be a constant
161
how do faults propagate?
fault propagates and grows under the applied stress from the tip of the fault.
162
what happens when several different fault segments grow?
as fault segments grow, interact and eventually link their displacement profiles start to overlap
163
when does most displacement on geological faults take place?
most of the displacement on geological faults takes place during earthquakes
164
describe the cycle of strain on a fault over time?
we get cycles of elastic strain (stress build up) before brittle failure releases the energy.
165
what does the frequancy of earthquakes depend on?
local rock strength
applied stress
the extent to which deformation is taken up on other structures
166
what is meant by sesimic moment?
how do you calculate it?
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
the earthquake magnitude is calculated as the moment magnitude.
what is it proportional to?
moment magnitude is proportional to log(seismic moment)
168
where do most earthquakes happen?
most happen at plate boundaries and \< 50km deep
deep earthquakes can be seen in the cold descending slab in subduction zones
169
when plotting lines and planes on a stereonet what will they look like?
lines are dots
planes are curved lines
170
what is meant by poles to planes on a stereonet?
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
what is the formula for working out the trend of pole and plunge of pole?
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
what is meant by a rock fabric?
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
foliations, lineations and other types of rock fabric could be primary or secondary. what does this mean and which is most popular?
**primary**: such as depositional environment
**secondary**: result from the application of stress to the rock body.
secondary type is most common
174
describe linear fabrics
many rocks have linear fabrics are because the rocks have been stretched and therefore the strain ellipsoid is prolate (ciger shaped)
175
describe foliated fabrics
many rocks with foliated fabrics are caused by deformation field producing an oblate strain ellipsoid (pancake shape)
176
describe rock cleavage
**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
what direction does cleavage form at?
cleavage typically forms perpendicular to this stress direction.
178
in cleavage formation what do we get bands of?
we can get bands of quartz and mica.
179
what are the five types of cleavage?
compaction cleavage
pencil cleavage
slaty cleavage
crenulation cleavage
schitosity
180
describe compaction cleavage
widely spaced cleavages form parallel to bedding as the rock deforms under its own weight.
greatest force is gravity
181
what is meant by pencil cleavage?
formed as a tectonic cleavage as the rock starts to deform due to horizontal shortening
182
describe slaty cleavage
if horizontal shortening continues from pencil cleavage, we get pervasive slaty cleavage.
183
what is meant by crenulation cleavage?
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
describe crenulation cleavage under the microscope.
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
describe the formation of schistosity
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
what is the relationship between folds and cleavage?
cleavage line parallel to fold axial plane
187
when does cleavage refract between layers?
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
explain this diagram interms of cleavage refraction.
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
what is meant by ductile shear zones?
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
what are the three main types of lineation?
stretching
intersection
movement on structures (slickensides)
191
what can we get from stretching lineation?
we can get a trend and a plunge
192
give an example of stetching lineation.
where pebbles have been stretched into cigar shapes.
193
describe intersecting lineation
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
describe slickesides
these are a combination of mechanical abrasion, causing grooves, and the growth of mineral fibres in orientation of movement.
195
what can slickensides be used for?
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
describe fibre, groove and/or mineral elongation lineations in terms of movement of structures.
minerals grow in a preferred orientation in fractures or faults.
197
where on a stereonet would lineation data of a fault be seen?
lineation data should plot on the plane representing fault (i.e. the curved line) on stereonet
198
whats the main difference betwee foliations and lineations?
foliations = planar stuff
lineations = linear fabrics
199
describe normal fault arrays
where each ridge in the image is the footwall of a large normal fault.
200
where would you find normal fault arrays?
what are they the product of?
found in continents and continental shelves.
typically a product of fault growth and interaction in areas of crustal extension
201
normal fault arrays are caused by extension.
what causes this extension?
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
describe where you would find horsts, grabens and half grabens on a normal fault array
half grabens are where there is a footwall on one side and a hanging wall on the other
horst are high points
203
what is a rift?
**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
what does a normal fault array become over time?
becomes a rift
205
it is important to relise that all rifting and extension requires you...
it is important to relise that all rifting and extension requires you to **thin the crust and lithosphere**
206
describe the process of normal faults to rifting
207
describe the three stages of passive rift geology
208
describe active rifting
associated with mantle plumes - typically notable volcanism
thinned crust and lithosphere results with volcanoes on footwall flanks of central graben
209
where would you find active rifting on earth?
the great rift valley africa.
210
describe from rift to drift
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
how does partially melting asthenosphere create oceanic plates?
partially melting asthenosphere gives a **basalt**
212
if crustal extension destroys topography, then it is crustal convergence that creates it through the process of... which has the name\_\_\_\_\_\_\_\_
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
in areas of continental collision, a ________ and \_\_\_\_\_\_\_ belt with a particular shape is created
in areas of continental collision, a **fold** and **thrust** belt with a particular shape is created
214
what are the key factors of orogenesis?
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
in orogenesis we get failure and shearing along the detatchment (decollement) and along the top of the wedge when...
the coulomb failure criteriaion is exceeded
216
converging mountain belts look like
a wedge
217
what are the angles alpha and beta of an accretionary wedge a function of?
alpha and beta are a function of:
rheology
friction properties
pore fluid pressure
218
why is the accretionary wedge theory a bit wrong?
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
what are the features found with thin skinned fold and thrust belts?
detachment horizon
imbricated thrusts
thrust related buckle folds
verging folds
ramps and flats
a foreland (not deformed area)
piggy back basins
220
describe fault propagation folds
**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
describe fault bend folds
**fault bend folds**
associated with stratigraphy being transported over ramps and flats. fold is pinned over the ramp flat.
222
describe drag folds
**drag folds**
associated with shearing. get hanging wall anticlines and footwall synclines
223
describe detatchemnt folds
**detatchment** (decollement) **folds**
these are buckle folds formed between two detachment horizons, particularly when one end is fixed.
224
what is meant by a balanced cross section?
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
how do we get low angle thrusts?
**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
what is the difference between passive and active rift zones?
passive caused by magma
active caused by plate tectonics
227
silicon rich rocks tend to be ________ than silicon poor rocks
silicon rich rocks tend to be weaker than silicon poor rocks
228
rocks behave stronger as depth increases. why?
how is this not consistent with the rheological division of the earths interior?
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
what is meant by foliation?
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
Write down the equation for the Coulomb failure criterion and define what the terms are, including units where appropriate.
**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)
