Transcript Folds and Faults

Folds and Faults
This is a plunging anticline
Supplemental Review and
Exercises
Deformation of Rocks
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When rocks have been deformed, folding or faulting
may result
How the deformation manifests its self is dependent
on:
– Temperature
– Pressure
– Rock type
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Rocks under high pressure, warm, and flexible are
more likely to fold
Rocks under low pressure, cold, and brittle are
more likely to break or fault
Driving force of
deformation……
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Plate Tectonics:
– The lithosphere is broken into “plates” free to
move around on the Earth’s asthenosphere
– These plates are free to move with respect to
each other, either slipping along one another,
colliding, or pulling apart
– Forming three different plate boundaries which
can result in various types of faulting and or
folding.
The Plates
http://observe.arc.nasa.gov/nasa/earth/tectonics/Tectonics2.html
The Plate Boundary Types
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Divergent Plate Boundary: plates
move away from each other resulting
in extension (tensional forces)
Convergent Plate Boundary: plates
move toward each other resulting in
shortening (compress ional forces)
Transform Plate Boundary: plates
slide past each other results in lateral
movement (shear forces)
http://observe.arc.nasa.gov/nasa/earth/tectonics/Tectonics3.html
Convergent Boundaries
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Folding
– Anticlines: Arch shaped structure
– Synclines: Basin like structure
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Faulting
– Reverse Faults (shortening)
Vertical movements
Divergent Boundaries
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Normal Faults (Extension) – vertical
movements
Tension
Transform Boundaries
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Strike Slip Faults – lateral movement
and shear forces
Shear
More on folds and
inclined beds….
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The axis of the fold divides it in half,
separating the fold into two limbs
the limbs of the fold have the same
strike for a non-plunging fold but
opposite dips
Describe orientation of the beds of
rocks using strike and dip
– Strike: the trend of a rock with respect
to north, typically parallel to bed
contacts and axis of fold
– Dip: the inclination of the rock away
from the horizontal down slope in a
direction perpendicular to strike.
Strike and Dip Symbol
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The strike and dip symbol resembles a
“T”. However the strike line is longer
than the dip line.
Click to insert strike and dip symbols on the below images.
You may want to think of the dip symbol
as having an arrow on the end pointing in
the direction the rocks are dipping
downward
Rules for Strike and Dip
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Older rocks dip in the direction of younger
rocks
For an anticline, dips point away from each
other on opposite sides of the axis of the
fold
For a syncline, dips point toward each other
on opposite sides of the axis of the fold
Age of the rock units play
a role in interpretation….
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Age is important for determining dip, and fold type.
Use the geologic time scale as follows:
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Q – Quaternary (youngest)
T – Tertiary
K - Cretaceous
J - Jurassic
TR - Triassic
P - Permian
lP - Pennsylvanian
M - Mississippian
D - Devonian
S - Silurian
O - Ordovician
Є - Cambrian
PЄ – Pre Cambrian
–
NOTE: the law of superposition indicates that in undeformed rocks, younger rocks
are on top of older rocks at a single point in a vertical succession
Looking at the Structures
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Geologic Maps
Cross-Sections
Block Diagrams
Geologic Maps
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Geologic maps of course provide geologic information as related to
rock type, age, and structure
Rock ages are described on the maps using the symbols previously
discussed
Different rock types and ages are depicted as different colors on the
map with the contacts between the units represented by a line.
The map symbols and configuration of the units provide structural
information such as fold or fault type.
Geologic maps show the features as seen in “map view” what things
look like on the surface as if you were flying or walking over an area
Geologic Cross-Sections
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Provide information regarding the vertical
configuration of the units
Similar to a topographic profile, but
providing subsurface information.
A slice of the earth/feature to provide a side
view, so what you would see if “cut” a
vertical slice, like cutting a cake in half and
seeing the layers inside
Block Diagrams
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Incorporate the information from the map
view and the cross-sectional view to show
both the surface and subsurface
Seeing a block of the earth illustrated how it
would look if you could cut a piece out like
using a cookie cutter and see the top and
the sides – allow you to get an idea of the
feature in a three dimensional aspect
Map View and Cross
Sectional View to a Block
Diagram
The diagrams above are all illustrating the same features
just from in different views and or dimensions
The Anticline…
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Anticline: an arch like structure with older beds near its center
that are flanked by younger rock
Erosion
S
S
O
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O
S
S
Cross-sectional View
O
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O
O
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O
S
S
Map View
Click to add Strike
And dip
The Syncline
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Є
O
S
Syncline: a basin like structure with younger beds
near its center that are flanked by older beds
O
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Erosion
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O
S
O
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Cross-sectional View
O
S
O
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Map View
Click to add Strike
And dip
Plunging Folds (Map View)
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In map view, non plunging folds have straight contact lines while
plunging folds contact lines curve around axial trace.
For a plunging anticline the bed contacts converge around the fold
axis in the direction the fold plunges and for a syncline the bed
contacts diverge in the direction of dip around the fold axis
Three Types of Faults
Foot
Wall
Foot
Wall
Hanging
Wall
Hanging
Wall
Normal Fault: any fault where the hanging wall has moved down
with respect to its footwall. Results in elongation of rocks on the
surface due to tensional forces such as along a divergent plate
boundary.
Reverse Fault: any fault where the hanging wall moves up with
respect to its footwall. Results in shortening due to compressional
forces such as along a convergent plate boundary
Strike Slip Fault: any fault where adjacent fault blocks move
parallel to the strike or trend of the fault surface. Results in
lateral movement due to shear forces such as along transform
boundaries.
Hanging VS Foot Wall
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Foot
Wall
Hanging
Wall
If a tunnel was
placed along
the fault plane,
a persons feet
would be on
the foot wall
and their head
by the hanging
wall for a
normal or
reverse fault
Fault Rules
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1.) The up thrown side of a normal or reverse fault will be
eroded to the level of the down thrown side of the fault in
time.
2.)Contact lines and associated rock formations (KEY BED)
always shift in the direction of dip as the land surface is
lowered by erosion.
The Key Bed is any layer of rock that may be traced along
the fault even though the faulting may have displaced the
bed tens to hundreds of feet, pick a bed to be the key bed
to determine which fault is the up thrown block
Illustrating Rules 1 and 2
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For a fault cutting inclined beds:
Upthrown
Fault Plane
Erosion
Cross-sectional View of Fault
Erosion
O
Key
Bed
S
D
M
Upthrown
Direction of
apparent
bed
Arrow
migration
showing
dip of fault
plane,
points
down dip
and toward
the
hanging
wall
Map View of Fault
Є
O
o
S
S
D
D
M
Additional Fault
Exercises:
N
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O
O
S
S
D
D
M
Up thrown
Side (West)
1.) Place appropriate
strike and dip symbols
on the diagram
2.) What is the up thrown
side?
3.) What type of fault is
illustrated?
Normal
4.) What is the dip of the
fault plane?
East
Answer the following for
the diagram below:
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D
s
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o
D
s
s
o
– Anticline
D
s
1.) Place appropriate
strike and dip symbols.
2.) Draw in the fold
axis.
3.) What type of fold is
shown?
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– Normal
D
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Southwest is up Thrown
4.) What type fault is
cutting the fold?
5.) What is the up
thrown side?
Answer the following for
the diagram below:
North Side Up Thrown
N
O
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O
S
S
D
D
M
1.) Place appropriate
strike and dip symbols
on the diagram
2.) What is the up thrown
side?
3.) What type of fault is
illustrated?
Reverse
4.) What is the dip of the
fault plane?
North