Folds and FaultsFolds and Faults

Supplemental Review and Supplemental Review and ExercisesExercises

This is a plunging anticline

Deformation of RocksDeformation of Rocks

When rocks have been deformed, folding or When rocks have been deformed, folding or faulting may result faulting may result

How the deformation manifests its self is How the deformation manifests its self is dependent on:dependent on:– TemperatureTemperature– PressurePressure– Rock typeRock type

Rocks under high pressure, warm, and Rocks under high pressure, warm, and flexible are more likely to foldflexible are more likely to fold

Rocks under low pressure, cold, and brittle Rocks under low pressure, cold, and brittle are more likely to break or faultare more likely to break or fault

Driving force of Driving force of deformation……deformation…… Plate Tectonics:Plate Tectonics:

– The lithosphere is broken into “plates” free The lithosphere is broken into “plates” free to move around on the Earth’s to move around on the Earth’s asthenosphere asthenosphere

– These plates are free to move with respect These plates are free to move with respect to each other, either slipping along one to each other, either slipping along one another, colliding, or pulling apart another, colliding, or pulling apart

– Forming three different plate boundaries Forming three different plate boundaries which can result in various types of which can result in various types of faulting and or folding. faulting and or folding.

The PlatesThe Plates

http://observe.arc.nasa.gov/nasa/earth/tectonics/Tectonics2.html

The Plate Boundary The Plate Boundary TypesTypes

http://observe.arc.nasa.gov/nasa/earth/tectonics/Tectonics3.html

Divergent Plate Boundary: plates Divergent Plate Boundary: plates move away from each other move away from each other resulting in extension (tensional resulting in extension (tensional forces)forces)

Convergent Plate Boundary: plates Convergent Plate Boundary: plates move toward each other resulting in move toward each other resulting in shortening (compress ional forces)shortening (compress ional forces)

Transform Plate Boundary: plates Transform Plate Boundary: plates slide past each other results in slide past each other results in lateral movement (shear forces) lateral movement (shear forces)

Convergent Convergent BoundariesBoundaries

FoldingFolding– Anticlines: Arch shaped structureAnticlines: Arch shaped structure– Synclines: Basin like structureSynclines: Basin like structure

FaultingFaulting– Reverse Faults (shortening)Reverse Faults (shortening)Vertical movementsVertical movements

Divergent BoundariesDivergent Boundaries

Normal Faults (Extension) – Normal Faults (Extension) – vertical movementsvertical movements

Tension

Transform BoundariesTransform Boundaries

Strike Slip Faults – lateral Strike Slip Faults – lateral movement and shear forcesmovement and shear forces

Shear

More on folds and More on folds and inclined beds….inclined beds….

The axis of the fold divides it in The axis of the fold divides it in half, separating the fold into half, separating the fold into two limbs the limbs of the fold two limbs the limbs of the fold have the same strike for a non-have the same strike for a non-plunging fold but opposite dipsplunging fold but opposite dips

Describe orientation of the beds Describe orientation of the beds of rocks using strike and dip of rocks using strike and dip – Strike: the trend of a rock with Strike: the trend of a rock with

respect to north, typically parallel respect to north, typically parallel to bed contacts and axis of foldto bed contacts and axis of fold

– Dip: the inclination of the rock Dip: the inclination of the rock away from the horizontal down away from the horizontal down slope in a direction perpendicular slope in a direction perpendicular to strike. to strike.

Strike and Dip SymbolStrike and Dip Symbol

The strike and dip symbol resembles The strike and dip symbol resembles a “T”. However the strike line is a “T”. However the strike line is longer than the dip line. longer than the dip line.

Click to insert strike and dip symbols on the below images.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 Rules for Strike and DipDip Older rocks dip in the direction of Older rocks dip in the direction of

younger rocksyounger rocks For an anticline, dips point away from For an anticline, dips point away from

each other on opposite sides of the each other on opposite sides of the axis of the foldaxis of the fold

For a syncline, dips point toward each For a syncline, dips point toward each other on opposite sides of the axis of other on opposite sides of the axis of the foldthe fold

Age of the rock units Age of the rock units play a role in play a role in interpretation….interpretation…. Age is important for determining dip, and fold type. Age is important for determining dip, and fold type. Use the geologic time scale as follows:Use the geologic time scale as follows:

– Q – Quaternary (youngest)Q – Quaternary (youngest)– T – Tertiary T – Tertiary – K - CretaceousK - Cretaceous– J - JurassicJ - Jurassic– TTR R - Triassic- Triassic– P - PermianP - Permian– lP - PennsylvanianlP - Pennsylvanian– M - MississippianM - Mississippian– D - DevonianD - Devonian– S - SilurianS - Silurian– O - OrdovicianO - Ordovician– Є - CambrianЄ - Cambrian– PЄ – Pre CambrianPЄ – Pre Cambrian

– NOTE: the law of superposition indicates that in undeformed rocks, 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 younger rocks are on top of older rocks at a single point in a vertical successionsuccession

Looking at the Looking at the StructuresStructures Geologic MapsGeologic Maps Cross-SectionsCross-Sections Block DiagramsBlock Diagrams

Geologic MapsGeologic Maps Geologic maps of course provide geologic information as Geologic maps of course provide geologic information as

related to rock type, age, and structurerelated to rock type, age, and structure Rock ages are described on the maps using the symbols Rock ages are described on the maps using the symbols

previously discussedpreviously discussed Different rock types and ages are depicted as different Different rock types and ages are depicted as different

colors on the map with the contacts between the units colors on the map with the contacts between the units represented by a line.represented by a line.

The map symbols and configuration of the units provide The map symbols and configuration of the units provide structural information such as fold or fault type.structural information such as fold or fault type.

Geologic maps show the features as seen in “map view” Geologic maps show the features as seen in “map view” what things look like on the surface as if you were flying or what things look like on the surface as if you were flying or walking over an areawalking over an area

.

Geologic Cross-Geologic Cross-SectionsSections Provide information regarding the Provide information regarding the

vertical configuration of the unitsvertical configuration of the units Similar to a topographic profile, but Similar to a topographic profile, but

providing subsurface information.providing subsurface information. A slice of the earth/feature to provide A slice of the earth/feature to provide

a side view, so what you would see if a side view, so what you would see if “cut” a vertical slice, like cutting a “cut” a vertical slice, like cutting a cake in half and seeing the layers cake in half and seeing the layers insideinside

Block DiagramsBlock Diagrams

Incorporate the information from the Incorporate the information from the map view and the cross-sectional view map view and the cross-sectional view to show both the surface and subsurfaceto show both the surface and subsurface

Seeing a block of the earth illustrated Seeing a block of the earth illustrated how it would look if you could cut a how it would look if you could cut a piece out like using a cookie cutter and piece out like using a cookie cutter and see the top and the sides – allow you to see the top and the sides – allow you to get an idea of the feature in a three get an idea of the feature in a three dimensional aspectdimensional aspect

Map View and Cross Map View and Cross Sectional View to a Block Sectional View to a Block DiagramDiagram

The diagrams above are all illustrating the same features just from in different views and or dimensions

The Anticline…The Anticline…

Anticline: Anticline: an arch like structure with older beds near its an arch like structure with older beds near its center that are flanked by younger rockcenter that are flanked by younger rock

Є OOS S

Cross-sectional View

Erosion

Map View

Click to add Strike And dip

Є OOS S

The SynclineThe Syncline

Syncline: Syncline: a basin like structure with younger beds a basin like structure with younger beds near its center that are flanked by older bedsnear its center that are flanked by older beds

Є OO S Є

Cross-sectional View

Erosion

Map View

Click to add Strike And dip

Є OO S Є

Plunging Folds (Map Plunging Folds (Map View)View) In map view, non plunging folds have straight contact In map view, non plunging folds have straight contact

lines while plunging folds contact lines curve around axial lines while plunging folds contact lines curve around axial trace. trace.

For a plunging anticline the bed contacts converge around For a plunging anticline the bed contacts converge around the fold axis in the direction the fold plunges and for a the fold axis in the direction the fold plunges and for a syncline the bed contacts diverge in the direction of dip syncline the bed contacts diverge in the direction of dip around the fold axisaround the fold axis

Three Types of FaultsThree Types of Faults

Foot Hanging Wall 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.

Foot Hanging Wall Wall

Hanging VS Foot WallHanging VS Foot Wall

If a tunnel was If a tunnel was placed along placed along the fault the fault plane, a plane, a persons feet persons feet would be on would be on the foot wall the foot wall and their head and their head by the by the hanging wall hanging wall for a normal or for a normal or reverse faultreverse fault

Foot Hanging Wall Wall

Fault RulesFault Rules

1.) The up thrown side of a normal or reverse fault 1.) The up thrown side of a normal or reverse fault will be eroded to the level of the down thrown side will be eroded to the level of the down thrown side of the fault in time.of the fault in time.

2.)Contact lines and associated rock formations (KEY 2.)Contact lines and associated rock formations (KEY BED) always shift in the direction of dip as the land BED) always shift in the direction of dip as the land surface is lowered by erosion.surface is lowered by erosion.

The Key Bed is any layer of rock that may be traced The Key Bed is any layer of rock that may be traced along the fault even though the faulting may have along the fault even though the faulting may have displaced the bed tens to hundreds of feet, pick a displaced the bed tens to hundreds of feet, pick a bed to be the key bed to determine which fault is bed to be the key bed to determine which fault is the up thrown blockthe up thrown block

Illustrating Rules 1 Illustrating Rules 1 and 2and 2

For a fault cutting inclined beds:For a fault cutting inclined beds:

Upthrown Erosion

Cross-sectional View of Fault

Map View of Fault

Fault Plane

Arrow showing dip of fault plane, points down dip and toward the hanging wall

Key Bed

O

S

D

M

Erosion O

S

D

M

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Direction of apparent bed

migrationo

S

D

O

S

D

M

O

S

D

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Additional Fault Additional Fault Exercises:Exercises:

1.) Place appropriate 1.) Place appropriate strike and dip strike and dip symbols on the symbols on the diagramdiagram

2.) What is the up 2.) What is the up thrown side?thrown side?

3.) What type of fault is 3.) What type of fault is illustrated?illustrated?NormalNormal

4.) What is the dip of 4.) What is the dip of the fault plane?the fault plane?EastEast

N

Up thrownSide (West)

Answer the following Answer the following for the diagram below:for the diagram below:

o

o

s

ss

s

D

D

D

D

1.) Place 1.) Place appropriate strike appropriate strike and dip symbols.and dip symbols.

2.) Draw in the fold 2.) Draw in the fold axis.axis.

3.) What type of fold 3.) What type of fold is shown?is shown?– AnticlineAnticline

4.) What type fault 4.) What type fault is cutting the fold?is cutting the fold?– NormalNormal

5.) What is the up 5.) What is the up thrown side?thrown side?

Southwest is up Thrown

Answer the following Answer the following for the diagram below:for the diagram below:

O S DЄ

O S D M

1.) Place appropriate 1.) Place appropriate strike and dip strike and dip symbols on the symbols on the diagramdiagram

2.) What is the up 2.) What is the up thrown side?thrown side?

3.) What type of fault is 3.) What type of fault is illustrated?illustrated?ReverseReverse

4.) What is the dip of 4.) What is the dip of the fault plane?the fault plane?NorthNorth

NNorth Side Up Thrown