Transcript Ductile Deformation

CHAPTER 11:
MOUNTAIN
BUILDING
What kind of
plate
boundary
produced
mountains
like these?
Mt. Assiniboine
(back right) in
the Rockies of
British Columbia
Rocks in the crust are deformed (bent,
stretched, and broken) by directed stresses
Types of Differential Stress
• Shear
• Tensional
• Compressive
Strain and Stress
• Strain is the change in shape and or volume of a rock caused by Stress
• In other words, stress is the applied force and strain is the resulting change in
the rock
• Strain takes place in three stages:
– elastic deformation,
– ductile deformation,
– and fracture
Three Stages of Strain
Steven Earle
• “stress is proportional to strain” (Hooke’s Law)
Fracture
non-permanent
strain
Elastic limit
Stress
permanent strain
Strain
Dan Gibson
Both ductile deformation and fracture are evident in these rocks.
Identify examples of each.
Deformations
Elastic Deformation
(fully reversible – stretch the
spring a little and it springs
back)
Ductile Deformation
(irreversible – stretch the spring too much
and it won’t go back to its original shape)
Fracture
(stretch the spring so much that it breaks)
The type of strain is dependent on …
•
•
•
•
Temperature
Confining Pressure
Rate of Strain
Amount and duration
of stress
• Presence of Water
• Composition of the
Rock
Is the deformation in this photo a result of
compressional or tensional stress?
Strain in the crust produces joints, faults, and
folds
Faulting
• Faulting takes place when stress leads to displacement across the
fracture surface
Joints, folds, and faults
Is there likely to be any rock displacement in the case of the
joints in the photo on the left?
Maps
• Maps are two-dimensional
representations of three-dimensional
surfaces
• Geologic Structures are mapped to
interpret stress history
• Strike-and-Dip Symbols (among other
geologic symbols and colors) are
used to indicate, age, lithology, and
structure
Strike and Dip
• The Strike of a body of rock is a line representing the intersection of that
feature with the plane of the horizon (always measured perpendicular to the
Dip).
• Dip is the angle below the horizontal of a geologic feature.
Sip-Slip and Strike-Slip Faults
• Dip-slip and strike-slip faults are the most common types of faults
Which type of fault is depicted here?
Major Fault Types
• Dip-slip Faults (slip in the dip-direction, either up or down)
– Normal (tensional stress)
– Reverse (compressive stress)
– Thrust (low-angle reverse fault, <150, compressive stress)
• Strike-slip Faults (shear stress, relative motion on the fault plane is horizontal
– slip along the strike direction)
– Left-lateral (a.k.a., sinistral)
– Right-lateral (a.k.a., dextral)
Major Fault Types
Strike-Slip Faults
• To someone looking across the
fault line (from either side),
the rock on the far side has
moved to the left.
• To someone looking across the
fault line (from either side),
the rock on the far side has
moved to the right.
Imagine standing on the ground
looking across these fault lines.
Visualize how the rocks have moved.
Some of the World’s Most Dangerous Fault
Systems are Strike-Slip
Izmit, Turkey, 1999 (M7.4)
Loma Prieta (San Francisco), 1989 (M6.9)
Port-au-Prince, Haiti, 2010 (M7.0)
These were not huge earthquakes. Why was the damage so great?
Rock Folds
• Rock folds are the result of ductile deformation caused by compressive stresses
acting over long periods.
Fold Terminology
Simple Folds
Syncline
• Syncline – concave upward arch with younger rocks in the center of the fold
• The apparent offsets in this fold are actually benches in the rock cut.
Anticline
•
Anticline – convex upward arch with older rocks in the center of the fold
Use your hand to show the axis of the anticline, and those of two synclines on either
side.
Faulted Fold
• Faulted Fold – severe compression
Is this a normal or reverse fault? Is that consistent with compression?
What is the direction of plunge?
Basins and Domes
What might be the
geological origin of a
dome like this?
Folding
Animation
Outcrop Patterns
• Outcrop patterns reveal the structure of the crust exposed by erosion
• Mapped and studied by structural geologists
Representing Layers
• A layer of tilted rock can be represented with
a plane.
• The orientation of that plane in space is
defined with strike-and-dip notation.
• The water will flow straight down the sloping
bed. This represents the dip direction. The
strike is perpendicular to the dip direction.
The Strike
• The strike is the compass
orientation of a horizontal line
on a sloping surface
a
b
c
Which of these lines represents the strike?
Geologic map showing topography, lithology,
and age of rock units in “Map View”
An outcrop pattern of “V”s indicates plunging
folds
Steven Earle
Mountain building may be caused by
volcanism
Mt. Garibaldi in the background, Mt. Price middle right, The Table middle left – all
were formed by volcanism.
Garibaldi Lake (foreground) was dammed up by lava flows from Mt. Price.
Volcanic Mountains
What is the plate tectonic setting portrayed here?
Volcanic Mountain Formation
• Volcanic mountains are formed of
volcanic products, not by deformation
Cascade Mountains
• Located in the US Pacific Northwest and
southwestern British Columbia
• Includes several active volcanoes
• Many volcanic centers were active during
the late Tertiary and Quaternary
• Volcanic arc developed above a subduction
zone
• Explorer, Juan de Fuca and Gorda plates
subducted beneath the North American
Plate
Creating Mountains with Igneous Rocks
• Intrusive igneous rocks that formed below volcanoes can be uplifted to create
mountains
The Stawamus Chief is a
granitic mountain that is part
of the Coast Range of BC
Mountains Formed By Faulting
FAULT-BLOCK MOUNTAINS
Fault Block Mountains
Mountains Formed By Folding
FOLD-AND-THRUST MOUNTAINS
Strain build-up at the Cascadia subduction
zone
How does this situation lead to fault-related mountains?
Isostasy
• Newly-built mountains are weathered and
eroded.
• As their mass is reduced, they “float”
higher in the mantle.
Crustal extension formed mountains in the Basin
and Range Province of the southwestern US
Basin And Range Province
• Located in the Idaho, Nevada and
Utah area.
• Decompression melting and high
heat developed above a
subducted rift zone.
• Former margin of Farallon and
Pacific plates.
• Thickening, uplift ,and tensional
stress caused normal faults.
• Horst and Graben structures
developed.
Steven Earle
Rocky Mountains – Mt. Yamnuska
Cambrian
Cretaceous
• Thrusting is also an important part of the formation of the Canadian Rockies.
Here at Mt. Yamnuska Cambrian rocks have been thrust over the top of
Cretaceous rocks that are hundreds of millions of years younger.
What important geological principle is used to show that there
must be a fault between the Cretaceous and Cambrian rocks here?
Fold and Thrust Belts
• Large compressive stresses are generated in the crust at convergent margins
when continental crust collides.
• Rocks located in the collision zone blocks are folded, faulted, and thrust
faulted.
• Crustal thickening pushes peaks upward and builds deep roots, forming foldand-thrust mountains.
• Fold-and-thrust belts form the highest and most
structurally complex mountain belts
Appalachian Fold Belt
• Proterozoic Eon - Iapetus Ocean
opened during rifting of Rodinia.
• Paleozoic Era – Iapetus began to
close, "squeezing" island arcs and
continental fragments between North
America and Africa.
• Tconic Orogeny
• Acadian Orogeny
• Alleghenian Orogeny
• Mountain building ended when
Pangaea rifted.
Mt. Everest
What kind of tectonic event led to the formation of the Himalayan Range?
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