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Folds & Faults
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content
Introduction
Definition of folds.
Types of folds.
Definition of faults.
Types of faults.
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Introduction
Structural geology :
is the study of three dimensional distribution of rock
units with respect to their deformational histories
The study of geologic structures has been of prime
importance in economic geology
Folded and faulted rock strata commonly forms traps or
the accumulation and concentration of fluids such aspetroleum and natural gas
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Folds The term fold is used in geology when one or a stack of
originally flat and planar surfaces, such as sedimentary strata,are bent or curved as a result of plastic (i.e. permanent)
deformation , Folds form under varied conditions of stress,
hydrostatic pressure, pore pressure
Types :-
Anticline
Syncline
Tight Fold
Overturned fold
Recumbent Fold
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Anticline
In structural geology, ananticline is a fold that is convexup and has its oldest beds at itscore .
Fold mountains are formed
when the crust is pushed up astectonic plates collide. Whenformed, these mountains areusually enormous like the newlyformed Rocky Mountains inWestern Canada and the UnitedStates
Anticlines and structural domesare favored locations for oil andnatural gas drilling
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Syncline
A syncline is similar to an anticline,in that it is formed by thecompression of a tectonic plate.However, a syncline occurs whenthe plate bends in a downwardmotion.
The lowest part of the syncline isknown as the trough.
on a geologic map, synclines are
recognized by a sequence of rocklayers that grow progressivelyyounger, followed by the youngestlayer at the fold's center or hinge , Ifthe fold pattern is circular orelongate circular the structure is a
basin.
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Tight Fold
A tight fold is a sharp peakedanticline or syncline.
It is just a regular anticline orsyncline, but was compressed
with a greater force causing theangle to be much smaller.
Folds such as these occur to formsteep mountain slopes like those
in Whistler, British Columbia.
Angle between the folds limbsrang from 30 to 0
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Overturned fold
An over fold takes place when folding rock becomes bent orwarped.
axial plane is inclined and both limbs in the same direction ,usually at different angle
Sometimes the folds can become so disfigured that they mayeven overlap each other.
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Recumbent Fold
This type of fold is
compressed so much that it
is no longer vertical.
There is a large extent of
overlapping and it can take
the form of an s.
To the right is a diagram
that shows the process of
recumbent folding.
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Faults Fault is planner fracture in rock in which the rock on one
side of the fracture has moved with respect to the rock on the
other side , Earth quakes are caused by energy release
during rapid slippage along a fault
Types :-
Dip-slip fault
- Normal Fault
- Reverse Fault
Strike-slip Fault
Oblique-slip fault
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Normal Fault
In a normal fault, the blockabove the fault moves downrelative to the block below thefault. This fault motion is caused
by tensional forces and results inextension. Other names: normal-
slip fault, tensional fault orgravity fault.
On the top right is the movementof a normal fault. A picture is
also shown below. Notice thedisplacement of the differenttypes of rock on each side of thefault.
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Reverse Fault Reverse faults are the opposite of
normal faults. Rocks arecompressed such that one platemoves up while the other descends
below it.
When plates compress and crack,usually the more dense one isforced under the less dense one.This is similar to the action of thecontinental crust colliding with theoceanic crust. Here the moredense crust, being the oceanic
crust is forced under thecontinental crust.
To the right is an animation of areverse fault. Below that is a real
picture of what a reverse fault
looks like.
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Strike-slip Fault
A strike-slip fault , also known as a transform fault, occurs
when two tectonic plates slide in a lateral motion past eachother.
This type of fault causes the most severe earthquakes becausethey grind against each other. These earthquakes can either be
shallow or deep and cause tremors over a short or long periodof time.
Strike-slip faults can occur frequently, especially along thecoast of California.
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Oblique Fault
Oblique-slip faulting suggests both dip-slip faulting and strike-
slip faulting. It is caused by a combination of shearing and
tension of compressional forces.
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Horst & Graben Fault
A Horst is the opposite of arift valley. The land
between the parallel faults isforced upward because thetwo faults are being pushed
together. This process can take a long
time to occur because theaverage plate movement isone inch per year
There are examples of horstfaults on the left.
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Summary
Folding and faulting has a major influence on
the way the earth looks. Mountains form and
disappear over time, as well as large rift
valleys and other features. This has an impact
on where and how we live.
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Thanks for your
attention