Plate Tectonics: Part IIFolding and Faulting

CGC 1D1

Horizontal movements of the

Earth’s CrustTakes place from compression and tensional forces

Tension – stress tending to pull rock masses apart

Compression – Stress tending to squeeze rock masses together

Responsible for formation of fold mountains.

As a result of compressional forces acting in 2 opposite directions, the crustal rocks bend and form a series of arches & troughs.

This is known as folding of crustal rocks that usually occur in a zone where the crust is weak.

The lines of fracture are known as faults

Both Processes are result in mountain building

What is a Fold?

A fold is a bend in rock strata resulting from compression of an area of the Earth’s crust .

The severity of compression is indicated by the tightness of the folds.

Since the compressional force is acting towards the centre, the intervening land rises up in a series of folds

Types of Folds: Anticline

An anticline is a convex up fold in rock that resembles an arch like structure with the rock beds (or limbs) dipping way from the center of the structure

Note how the rock layers dip away from the center of the fold are roughly symmetrical.

Types of Folds: Syncline

A syncline is a fold where the rock layers are warped downward

Both anticlines and synclines are the result of compressional stress

Note how the rock layers dip toward the center of the fold and are roughly symmetrical

Synclinal folds in bedrock, near Saint-Godard-de-Lejeune, Canada

More Complex types of FoldsMore complex fold types can develop in situations where lateral pressures become greater. The greater pressure results in anticlines and synclines that are inclined and asymmetrical

Recumbent FoldA recumbent fold develops if the center of the fold moves from being once vertical to a horizontal position

commonly found in the core of mountain ranges and indicate that compression and/or shear forces were stronger in one direction

Overthrust FaultExtreme stress and pressure can sometimes cause the rocks to shear along a plane of weakness creating a fault. We call the combination of a fault and a fold in a rock an overthrust fault

Fold MountainsThe compressional force exerted on the crust is a result of movement of earth’s plates.

When the plates converge, the weak rocks and sediments lying between the 2 plates get squeezed and get folded.

Parallel folds give rise to long chains of parallel mountain ranges with high peaks.

The up-folds form fold mountains while the down folds form longitudinal valleys.

Mountains of Canada

The Appalachian Mountains

Mountains of Canada

Innuitian Mountains

Mountains of Canada

The Rocky Mountains

The Valley of Ten Peaks, Banff National Park

Understanding Canada’s Mountain Ranges

Read “making connections” textbook pg. 139-142

Answer questions 1-5 on pg. 143

FaultingFaults are large scale internal movements causing large scale physical features especially mountains and valleys

If the force is moderate only cracks may appear in the rocks.

The displacement that occurs results in formation of faults.

Such fractures are divided into Joints & Faults.

A joint is a fracture where a rock seems to be joined together without any movement

it may become a fault when displacement takes place

Location of some of the major faults on the Earth.

Where do you notice that a majority of the faults in the world occur?

Normal FaultNormal faults occur when tensional forces act in opposite directions and cause one slab of the rock to be displaced up and the other slab down

Reverse FaultReverse faults develop when compressional forces exist Compression causes one block to be pushed up and over the other block.

Graben FaultA graben fault is produced when tensional stresses result in the subsidence of a block of rock. On a large scale these features are known as Rift Valleys

Rift Valleys (AKA Graben)Major relief feature of Faulting Activity is Rift Valley.

A Rift Valley may be formed in two ways :

(1) When the middle portion between two normal faults is dropped downward the sides remain stable.

(2) When the middle portion remains stable and the two sides are raised upward

These valleys have flat bottoms and steep sides

Example: Great Rift Valley, Africa

Why is this important?

Because the rapidly eroding highlands have filled the valley with sediments, a favorable environment for the preservation of remains has been created. The bones of several hominids ancestors of modern humans have been found there, including those of "Lucy“

http://georgengondo.hubpages.com/hub/About-The-Great-Rift-Valley-in-Africa

Ottawa-Bonnechere GrabenThe Ottawa-Bonnechere Graben (also known as the Ottawa Graben), with its branch the Timiskaming Graben, is an ancient rift valley in the Canadian Shield of Northeastern Ontario and Quebec, Canada

This rift valley was formed when the Earth's crust moved downward about a kilometre between two major fault zones known as the Mattawa and Petawawa faults

These ancient faults are still active and occasionally release stress in the form of earthquakes, such as the 1935 Timiskaming earthquake and the 2000 Kipawa earthquake.

The length of the graben is about 700 km (435 mi)

Runs from the Montreal area on the east to near Sudbury and Lake Nipissing on the west

Barron Canyon in Algonquin Park is along a fault associated with the Ottawa Bonnechere graben.

Horst Fault

A horst fault is the development of two reverse faults causing a block of rock to be pushed up- In German Horst = 'heap'

Fault-Block MountainsBlock mountains are another important feature of Faulting caused by the Tensional features in the Earth’s crust

The mechanism of formation of Block mountains is same as the Rift valley

The Block mountains are often found on the sides of the Rift valley

It is because the land between two parallel faults is either subsides to form a Rift valley or rises to form Block mountains

Strike-Slip (transform) FaultThe final major type of fault is the strike-slip or transform fault. These faults are vertical in nature and are produced where the stresses are exerted parallel to each other

A well-known example of this type of fault is the San Andreas fault in California.

http://www.youtube.com/watch?NR=1&v=tIuk2blBzHs

Challenge

With the Fault Models can you demonstrate:

A normal fault?

A reverse fault?

A graben fault?

A horst fault?