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Folding, Faulting, and Mountains

GLY 2010 – Summer 2013 - Lecture 13

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Stress

• Stress is a force that is capable of greatly deforming rocks, and may result in folding or faulting of rock, and even to the building of mountains

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Types of Stress

• There are three types of stress Compression Tension Shear

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Compression• Opposing forces directed inward along

a single line

• Compression shortens an object along the axis of compression, and thickens it in the directions perpendicular to the stress direction

Before After

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Tension• Tension is the result of divergence,

pulling an object in opposite directions along a common axis

• Tension lengthens an object along the axis of tension, and thins it in the perpendicular directions

BeforeAfter

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Tensional Cracking

• Tension can produce cracks in the direction perpendicular to the axis of tension

Cracking develops

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Tension Crack Pictures

• Nisqually Earthquake, 2/28/01, in Washington caused tension cracking.

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Shear

• Opposing stress is created by two plates moving in opposite directions

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Responses to Stress

• There are three responses to stress Elastic Plastic Rupture

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Elastic Substances

• Behave elastically, stretch without breaking

• Snap back to their original position, when stress is removed

• Elastic limit: a limit beyond which substances cannot be stretched without breaking

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Plastic Substances

• Slow deformation without breaking

• Plastically deformed substances do not return to their original shape when the stress is removed

• Rate of deformation is important - Stress applied quickly will cause rupture

Silly George, by Vern HartTime-lapse slumping of

silly putty. Notice movement in upper left corner.

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Plastic Deformation

• The rate of plastic deformation makes a difference

• Silly putty breaks if pulled rapidly, stretches if pulled slowly

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Rupture

• Elastic substances stretched beyond the elastic limit, or plastic substances deformed quickly, will rupture

• Rupture is called brittle failure

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Deformation

• Rocks subjected to stress may: Deform by folding Rupture, with subsequent movement

along the plane of rupture - this is called faulting

Fold, then rupture

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Folding

• Folds may be described in terms of two parameters: Axial Plane Limbs

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Anticline

• If the fold is convex upward, it is called an anticline

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Anticlinal Fold

• Rainbow Gap, Virginia• Photo: Henry Johnson

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Atlas Mountains Anticline

• One of the best exposures of a complexly folded mountain belt anywhere occurs in the Atlas Mountain system of northwest Africa

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Domes

• Domes are anticlines that curve in three dimensions, like an upside down bowl

• Figure shows the Black Hills, South Dakota

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Eroded Dome, Sinclair, Wyoming

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Syncline

• If the fold is convex downward, it is called a syncline

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Syncline Photo

• Photo: Duncan Heron• Synclinal fold exposed by roadcut

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Anticline-Syncline Pair

• Anticline-Syncline pair in Devonian Old Red Sandstone. SW Wales, UK

• Note the different fold shapes

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Basins

• Basins are syncline that curve in three dimensions, like a bowl

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Overturned Folds

• Overturned fold in lower center of picture

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Recumbent Folds

• Recumbent folds are defined as folds with horizontal (<10° dip) axial surfaces

• Photo: Ron Perkins

Fold Diagram

• Diagram shows the major types of folds

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Plunging Folds

• The axes of the folds may be tilted, creating a series of plunging folds

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Folding Animation

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Joints

• Three joint sets (left photo)• Joints and dikes, Acadia National Park video

(right)

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Faults

• A fault is a fracture along which definite movement has occurred

(Click picture to remove block)

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Strike and Dip

The strike-dip symbol, used on geologic maps, is shown - the long direction points in the horizontal direction, and the shorter side

shows dip direction

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Strike-slip Direction

• Strike-slip faults are further described as "right-lateral" or "left-lateral" depending if the block opposite the viewer moved to the right or left, respectively

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Strike Slip Fault

Photo: Arthur G. Sylvester. San Jacinto fault, Anza, Southern California

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Right-Lateral Strike Slip

• Block is displaced to the right, looking across the fault

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Strike Slip FaultsRight Lateral

•Near Coos Bay, Oregon

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Left-Lateral Strike Slip

• Block is displaced to the left, looking across the fault

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Strike Slip Faults - Left Lateral

Near Lillooet, British Columbia

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Dip-slip Faults

• Dip direction is always perpendicular to the strike line

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Fault Terminology

• Foot Wall and Hanging Wall are borrowed from mining terminology

• Ore veins are often deposited along faults

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Normal Fault

• Normal faulting results from tensional forces • Hanging wall moves down relative to the footwall

(here, to the right)•Places younger rocks on top of older

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Sevier Normal Fault

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Death Valley Normal Faults

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Reverse Fault

• Reverse faulting results from compressional forces• Hanging wall moves up relative to the footwall (here,

to the left)• Places older rocks on top of younger

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Reverse Fault

• Reverse faults and associated fold

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Thrust Fault

• Thrust faults are low angle reverse faults

• They sometimes move large distances (tens of kilometers)

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Lewis Overthrust

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Explanation of Lewis Overthrust

• Chief Mountain was moved about forty kilometers and isolated by erosion

• Chief Mountain is much older (Precambrian) than the rock upon which it rests (Cretaceous)

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Chief Mountain

Older rock above younger, typical of thrust faultsGlacier National Park, Montana

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Oblique Slip

• Oblique-slip is a combination of vertical and horizontal movement

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Horst and Graben

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San Andreas/Garlock

Faults From Space

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San Andreas Fault

• Pacific plate, left

• North America, right

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San Andreas Offsetting Fence

Fault Animations

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Fault Diagram Summary

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Orogenesis

• Tectonic forces often create mountains, a process called orogenesis

• There are several types of mountains Folded Faulted Upwarped Volcanic

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Orogenesis by Folding

• Plate collisions involving continental plates can produce high mountains Examples: Himalayas (India, Tibet, China) Alps (Europe) Urals (Europe/Asia boundary) Appalachians

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Himalayan Mountains

Mt. Everest High peaks in the Himalayas

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Owens Valley and the Sierra Nevada Range

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Orogenesis by Upwarping

• Formed when a large region of the earth’s crust is bent into a broad, regional uplift with little apparent deformation of the rocks

• Upwarping may be due to local vertical motion, rather than plate tectonic forces - often far from plate boundaries

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Custer State Park,Black Hills, South Dakota

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Volcanic Mountains

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Faults

• A fault is a fracture along which definite movement has occurred

(Click picture to restore block)