Download Mountain Building Structural Geology Chapter 10

Mountain Building
Structural Geology
Chapter 10
Learning Standard: I will analyze the
theories associated with geologic events
that change the earth’s physical
environment.
Learning Target: I will compare different
types of mt-building processes and the
resulting structures of each process.
Deformation
• Types of deformation:
– 1. Elastic – change that can be recoverable, like a
rubber band. Stress is gradually applied but when
released, earthquakes are generated (elastic rebound)
– 2. Brittle – rock fractures,
– temps and pressure are low
– Happens near the Earth’s surface
– 3. Ductile – rock changes in size and shape without
fracturing
– Temps and pressure are high
– Happens deep under the Earth’s surface
Deformation
Deformation is a general term that
refers to all changes in the original form
and/or size of a rock body
Factors that influence the strength of a
rock
• Temperature and confining pressure
• Rock type
• Time
Folds
Rocks bent into a series of waves
Most folds result from compressional forces
which shorten and thicken the crust
Types of folds
• Anticline – linear upfolded, or arched, rock layers
• Syncline – linear downfolded rock layers
• Monocline – large step-like folds in
horizontal layers
• Brittle and Ductile deal with mountain building
processes and occur due to temperatures and
confining pressures.
Folds
A series of anticlines
and synclines
Anticlines and synclines can be
• Symmetrical - limbs are mirror images
• Asymmetrical - limbs are not mirror images
• Overturned - one limb is tilted beyond the
vertical
• Where folds die out they are said to be
plunging
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Plunging folds
Outcrop patterns of
plunging folds
Folds
Other types of folds
• Dome
• Circular, or slightly elongated
• Upwarped displacement of rocks
• Oldest rocks in core
• Younger rocks on the outer ends
• Basin
• Circular, or slightly elongated
• Downwarped displacement of rocks
• Youngest rocks in core
• Oldest rocks on the outer ends
Geologic Maps
The bedrock geology of
the Michigan Basin
• A map that shows the types, ages,
distribution, and orientation of rocks on the
surface of the Earth
• By examining the map, one can interpret
the nature of the rocks below the surface
and find what forces caused any
deformation
2
Ozarks
Illinois
Basin
The bedrock geology of
the Michigan Basin
The Black
Hills of
South
Dakota are a
large dome
Strike and Dip symbol
Orientation of Structures: Strike and Dip
• Strike and dip are used to define the orientation
of a rock layer or fault.
• Strike is the trend or horizontal plane of the rock
layer or fault exposed at the surface
Strike
– Generally expressed as a direction
– Ex. North-South or East-West, etc.
• Dip is the inclination of the surface of a rock unit
or fault from the horizontal plane (strike)
– Dip is expressed as an angle of inclination and a
direction which the rock is inclined.
– Dip direction is always perpendicular (90°) to the
strike
– Example: 45°SW
30°
Dip
N
Strike is SW to NE
Dip is 30°S
3
Strike and Dip
Geologic Map w/ Strike and Dip
Symbols
Faults
Concept of hanging wall and
footwall along a fault
Faults are fractures (breaks) in rocks
along which appreciable displacement
(movement) has taken place
Types of faults (Dip-slip and Strike-slip)
• Dip-slip fault
• Movement along the inclination (dip) of fault
plane
• Parts of a dip-slip fault
• Hanging wall – the rock above the fault
surface
• Footwall – the rock below the fault surface
Faults
A normal fault
2 Types of dip-slip faults, Normal and
Reverse
• Normal fault
• Hanging wall block moves down relative to
the footwall
• Prevalent at spreading centers
• Caused by tensional forces
• Create horsts and grabens
• Series of uplifted blocks and downward
valleys
4
Fault block mountains
produced by normal faulting
Faults
Second type of dip-slip fault
Reverse and thrust faults
• Hanging wall block moves up relative to the
footwall
• Caused by strong compressional stresses
• Reverse fault - dips greater than 45º
• Thrust fault - dips less than 45º
A reverse fault
Faults
A thrust fault
A strike-slip fault
Strike-slip faults
• Dominant displacement is horizontal and
parallel to the trend, or strike
• Due to shear stresses/forces
• Transform fault
• Large strike-slip fault that cuts through the
lithosphere
• Often associated with plate boundaries
5
Other Fractures in the crust
Mountain belts
• Orogenesis refers to processes that
collectively produce a mountain belt
– These events are called orogenies
Joints
• Fractures along which no appreciable
displacement has occurred
• Most are formed when the pressure on rocks
are released.
• The rock rebounds upward and cracks
• Types of Mountains that form through
orogenesis:
•
•
•
•
1. Volcanic Mountains
2. Folded Mountains
3. Fault-Block Mountains
4. Upwarped/Dome Mountains
• Other Structures
• 1. Plateaus
• 2. Eroded/dissected Plateaus
• website
Mountain belts
Mountain belts
• Folded Mountains
• Volcanic Mountains
• Mountain ranges made from
igneous/volcanic activity
• Examples:
•
•
•
•
Cascades
Western Andes
Japan
Philippines
Formation of the Himalayas
• Occur when rock strata is twisted and
folded
• Regional, high-grade metamorphism
• Compressional stresses with reverse and
thrust faulting
• Anticlines and synclines are present
• Examples: Himalayas, Appalachians, N.
Rockies
Mountain belts
• Fault-block Mountains
– Tensional stresses pulling crust apart
– Associated with normal faulting
– Formed by the displacement of rock along
a single fault
– Horsts and grabens
• Examples: Grand Tetons (WY), Sierra
Nevada (CA), Basin and Range (UT,
NM, AZ, CA)
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Fault block mountains
produced by normal faulting
Mountain belts
• Upwarped/Dome Mountains
– Associated with doming episodes
• Upwelling of magma underneath forces rock layers
upward
– Associated with reverse faulting and
compressional stresses
– Faults on two or more sides of the mt. range
• Examples: Southern/Colorado Rockies,
Black Hills, Adirondacks, St. Francois Mts.
Rocky Mountains
• Southern Rockies
– Colorado Rockies, Sangre de Cristo Range in New
Mexico, and Big Horns in Wyoming
• Upwarped Mountains
– Different from the Northern Rockies
• N. Rockies are Folded
– Formed during the Laramide Orogeny (60 million
years ago)
• One of the last Mt. building episodes of western N. America
(Cordilleran)
Mountain belts
• Dissected Plateau
– A highly eroded plateau
– Erosion by streams and rivers cutting through
the uplifted area
– Examples
• Ozark Plateau
• Cumberland Plateau
Mountain belts
• Other Structures
– Plateaus: uplift of flat land with little
deformation.
– Generally created behind mt ranges
– Features of plateaus:
• Mesas and Buttes
• Example: Colorado Plateau
Ozark nonMountains
• Ozark Dissected Plateau
• A highland dome
– Encompasses most of Missouri
– Oldest rocks start in Southeastern Missouri with the
St. Francois Mts.
• Igneous rocks
• (544 million years and older)
– The surrounding rocks are primarily limestone (a sed.
rock)
• Range from 505 million years to 286 million years
• Deposited by a vast sea in the Paleozoic era (540 m.y to 250
m.y)
– Once uplifted and sea regressed, rivers and streams
cut through the rock making the hills and knobs
mistakenly referred to as the Ozark Mountains
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Ozark nonMountains
Ozark Plateau/Dome
• Separated into three sections
– Salem Plateau
• Central to SE Missouri
– Springfield Plateau
• SW Missouri
– Boston Mountains
• Northern Arkansas
Mountain belts
Buoyancy and the principle of isostasy
• Evidence for crustal uplift includes wavecut platforms high above sea level
• West coast of North America
• Reasons for crustal uplift
• Not so easy to determine
• Isostasy
• Concept of a floating crust in gravitational
balance
• When weight is removed from the crust,
crustal uplifting occurs
• Process is called isostatic adjustment
Isostasy
The principle of
isostasy
• Because of the principle of isostasy:
– The thickest part of the crust occurs in
mountain ranges
– The thinnest part of the crust occurs in the
oceans
• As a mountain range erodes away, the
crust will rise and rebound upwards until a
gravitational equilibrium is reached
– Known as isostatic adjustment
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Erosion and resulting
isostatic adjustment of the
crust
Erosion and resulting
isostatic adjustment of the
crust
Erosion and resulting
isostatic adjustment of the
crust
9