Maps Structure
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Transcript of Maps Structure
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Folds, Faults, and other
Records of Rock Deformation
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Deformation of Rocks
Folding and faulting are the mostcommon forms of deformation inthe sedimentary, igneous and
metamorphic rocks thatcompose the Earths crust
Structural Geology is the studyof the deformation of rocks andits effects.
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Orientation of Deformed Rocks
We need some way to describe thegeometry of geologic structures. Sowe use the terms strike and dip.
Strike: compass direction of a rocklayer as it intersects with ahorizontal surface.
Dip: acute angle between the rocklayer and the horizontal surface,measured perpendicular to strike.
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Strike and Dip on a Rooftop
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Strike and Dip in a Rock Structure
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Displaying Strike and Dip
on a Map
35O
N
S
Direction
of strike
E
Direction of
down dip
W
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Creating a Geological
Map and Cross Section
A geological map is a convenient
tool which is commonly used bygeologists for recording suchinformation and the location of
outcrops, the nature of theirrocks, and the dips and strikes oninclined layers
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What determines if a rockFolds (bends)
or faults (breaks)?
Type of force applied
Pressure
Temperature
Rock (mineral) composition
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Strength
Ability of an object to resist
deformation
In lab, marble was tested
Fractured Deformed
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Three Major Types
of Directed Stress
Compression Extension
Shear
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Compression
Action of oppositely directed forcesacting towards each other at thesame time
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Tension
Action of coinciding and oppositelydirected forces acting away from eachother
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Shear
Action of coinciding and oppositelydirected forces acting parallel to eachother across a surface
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Deformation Textures
As rocks along a fault plane shear
past each other, they grind andmechanically fragment solid rockand promote recrystallization
under the high pressures
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Fault breccias:
Along the fault,the rocksfragment and
then are lithifiedinto brecciascomposed ofangularfragments ofvarious sizes
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Mylonites
When the deformation along afault occurs, if the temperatureand/or pressure is high enough,
you can form metamorphic rockscalled mylonites
A mylonite rock can display
minerals that have beentexturally changed by physicalmovement
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Texture deformation in mylonite
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Types of Deformation
Elastic
Ductile (plastic)
Brittle (rupture)
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Elastic Deformation
Temporary change in shape or
size that is recovered when the
deforming force is removed
(Like squeezing a piece of
rubber)
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Ductile (Plastic) Deformation
Permanent changein shape or size
Under goes
smooth andcontinuous plasticdeformation under
stress
Does NOT recover
original shape
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Brittle Deformation (Rupture)
As the appliedforce is increased,the rockundergoes littlechange until itsuddenly breaks
Usually along azone of weakness
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Brittle Versus Ductile
Some rocks are naturally brittle
Some rocks are naturally ductile
A rock that is brittle at shallow
depths can be ductile at greaterdepths
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Factors that Affect Deformation
Temperature
Pressure
Strain rate
Rock type
The variation of these factors determineswhether a rock will fault or fold
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Effects of Rock Type on Deformation
Some rocks arestronger than others.
Competent: rocks
that deform onlyunder great stresses
Incompetent: rocks
that deform undermoderate to lowstresses
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How Rocks Fracture
Joints
Faults
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Definition of Fault
Fractures in rockscreated by
tectonic processes
(Movement of the rock on
both sides of the fault)
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Faults
Regardless of size, all faults
are classified by the directionof relative movement (which iscalled slip)
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Types of Faults
Dip-slip faults
Normal
Reverse Strike-slip faults
Right-lateral
Left-lateral
Oblique-slip faults
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Hanging Wall: Term used byminers. They could hang their
light on this side of the faultbecause it was above them.
Footwall: Also from the
miners, this side of the wall uponwhich they could stand belowthe hanging wall.
Faults
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Dip-slip Faults
Motion of the fault blocks is parallelto the dip direction
Two types:
Normalmovement is down dipReversemovement is up dip
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Hanging wall
Foot wall
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Normal Fault
footwall
hanging wall
cross section
hanging wall
footwall
Before After
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Hanging wall
Foot wall
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R F lt
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Reverse Fault
footwall
hanging wall
cross section
Rift Valley
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Rift Valley
Rift Valley
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Rift Valley
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Thrust Fault
footwall
hanging wall
cross section
Thrust faults are low-angle reverse faults
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Strike-slip Faults
Motion of the fault blocks isparallel to the strike direction
There are 2 types
Right-lateral
Left-lateral
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Left-lateral
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Left-lateral Strike Slip Fault
aerial (map) view
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Right-lateral Strike Slip Fault
aerial (map) view
San Andreas Fault (right-lateral)
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( g )
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Displacementboth verticallyand
horizontally
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Folding of Rocks
Produced by tectonic forces
Very common form ofdeformation that is viable inlayered rock
Scale can be from centimetersto hundreds of kilometers
Fold Terminology
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Fold Terminology
Axial plane: the plane of mirror symmetrydividing the fold into two limbs
Axis: the line formed by the intersection of
the axial plane and a bedding plane
Horizontal fold: fold where the axis ishorizontal
Plunging fold: fold where the axis is nothorizontal
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More Fold Terminology
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More Fold Terminology
Syncline:a sequence of folded rockswith the youngest rocks on the inside
of the fold
Anticline:a sequence of folded rocks
with the oldest rocks on the inside ofthe fold
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S M F ld T i l
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Some More Fold Terminology
Symmetrical folds: have bedsdipping symmetrically on eachside of axial plane
Asymmetrical folds: have thebed on one side of fold dippingmore steeply than other
Overturned folds: one or bothsides of fold dipping beyondvertical
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Asymmetrical folds
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Overturned folds
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Overturned folds
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And Even More Fold Terminology
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And Even More Fold Terminology
Dome: a sequence of folded rocks inwhich all the beds dip away from a
central point
Basin: a sequence of folded rocks in
which all the beds dip towards acentral point
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Syncline exposed in a quarry near Middletown, Virginia
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y p q y , g
Folds are Clues to a Regions
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Folds are Clues to a Region s
Tectonic History
Valley and Ridge Province Near
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Valley and Ridge Province NearAppalachian Mountains: Remnants
of a Folded Mountain Belt
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Dating the Order of Deformation
Use geometry:
Inclusions
Cross-cutting relationships
Combine with fossils andradiometric dating
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