Folding, Faulting, and Mountains
GLY 2010 – Summer 2012 - Lecture 13
1
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
2
Types of Stress
• There are three types of stress
 Compression
 Tension
 Shear
3
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
4
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
6
Tension Crack Pictures
• Nisqually Earthquake, 2/28/01, in Washington
caused tension cracking.
7
Shear
• Opposing stress is created by two
plates moving in opposite directions
8
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
10
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
11
Plastic Deformation
• The rate of plastic
deformation makes a
difference
• Silly putty breaks if
pulled rapidly, stretches
if pulled slowly
12
Rupture
• Elastic substances stretched beyond the
elastic limit, or plastic substances
deformed quickly, will rupture
• Rupture is called brittle failure
13
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
14
Folding
• Folds may be described in terms of
two parameters:
 Axial Plane
 Limbs
15
Anticline
• If the fold is convex upward, it is
called an anticline
16
Anticlinal Fold
• Rainbow Gap, Virginia
• Photo: Henry Johnson
17
Atlas Mountains Anticline
• One of the best
exposures of a
complexly folded
mountain belt
anywhere occurs in
the Atlas Mountain
system of northwest
Africa
18
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
21
Syncline Photo
• Photo: Duncan Heron
• Synclinal fold exposed by roadcut
22
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
25
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
29
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
32
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
35
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
39
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
42
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
44
Reverse Fault
• Reverse faults and associated fold
45
Thrust Fault
• Thrust faults are
low angle reverse
faults
• They sometimes
move large
distances (tens of
kilometers)
46
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)
48
Chief Mountain
Older rock above younger, typical of thrust faults
49
Glacier National Park, Montana
Oblique Slip
• Oblique-slip is a combination of vertical and
horizontal movement
50
Horst and Graben
51
San
Andreas/Garlock
Faults From Space
52
San Andreas Fault
• Pacific plate, left
• North America, right
53
San Andreas Offsetting Fence
54
Fault Animations
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Fault Diagram Summary
56
Orogenesis
• Tectonic forces often create mountains, a
process called orogenesis
• There are several types of mountains
 Folded
 Faulted
 Upwarped
 Volcanic
57
Orogenesis by Folding
• Plate collisions involving continental plates
can produce high mountains
 Examples:
 Himalayas (India, Tibet, China)
 Alps (Europe)
 Urals (Europe/Asia boundary)
 Appalachians
58
Himalayan Mountains
Mt. Everest
High peaks in the
Himalayas
59
Owens Valley and the
Sierra Nevada Range
60
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
61
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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