Transcript Folding, Faulting, and Mountains

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
Before
After
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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
Silly George, by Vern Hart
Time-lapse slumping of
silly putty. Notice
movement in upper left corner.
• 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
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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
• AnticlineSyncline 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
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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 "leftlateral" 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 Faults
Right 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 faults
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Glacier National Park, Montana
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
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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)
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