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Crustal Deformation and
M
Mountain
t i Building
B ildi ((orogeny))
Mountains reflect geologic processes of uplift, deformation, and
metamorphism
eta o p s at work;
o ; tthey
ey a
are
e vivid
de
evidence
de ce o
of tecto
tectonic
c act
activity.
ty
Mt. Cook, New Zealand
http://eps.mcgill.ca/~courses/c201_winter/
Crustal Deformation and
M
Mountain
t i Building
B ildi ((orogeny))
Mountain
ou ta building
bu d g is
s a process
p ocess called
ca ed o
orogenesis.
oge es s
With the exception of the large volcanoes formed over hot spots, mountains do not
occur in isolation, but rather as part of linear ranges called mountain belts or orogens.
The Alps formed by collision of a
microplate (including Italy) with the
Eurasian plate
Crustal Deformation and
M
Mountain
t i Building
B ildi ((orogeny))
Mountain building
g involves uplift,
p , deformation,,
jointing, faulting, folding, foliation, metamorphism,
igneous activity, and sedimentation.
Sedimentary
y rocks are originally
g
y
deposited in horizontal layers
((typically
yp
y with no folds or bends))
Deformation changes the character of the rocks and is often easy to see.
Undeformed (unstrained) rocks, such
as sedimentary rocks, display
horizontal beds, with no folds or faults.
Deformed (strained) rocks
show tilted beds, metamorphic
alteration, folding,
g and faulting.
g
Deformation and Mountain Building
(Brittle - ductile; stress and strain; faults and folds)
What forces are responsible? How can it be?
How do these deformations fit into a historical perspective?
Deformed sedimentary strata, Rocky Mountains, British Columbia
From: Lutgens and Tarbuck (1992) Essentials of Geology
Deformation and Mountain Building
(Brittle - ductile; stress and strain; faults and folds)
Wh t forces
What
f
are responsible?
ibl ? H
How can it b
be?
?
South Georgia Island, Antarctica
From: Murck, Skinner & Mackenzie (2008) Visualizing Geology
Elastic deformation
Reversible or non-permanent deformation of a solid when it is
stretched, bent or squeezed and the force is then removed.
Stress – force applied per unit surface area
Strain – change in shape or volume
From: Skinner and Porter (2000)
The Dynamic Earth
Strain is proportional to stress.
Elastic deformation of rock occurs when stress
is applied slowly and under low pressure.
Ductile or plastic deformation
A solid exhibiting ductile deformation behaves elastically under low stress,
but, beyond a certain point, elastic properties cease and ductile flow occurs.
Folding occurs mostly at
high temperatures and
pressures - more common
in metamorphic rocks
Ductile or plastic deformation
A solid exhibiting ductile deformation behaves elastically under low stress, but a
point is reached where elastic properties cease and ductile flow occurs.
From: Skinner and Porter (2000) The Dynamic Earth
Brittle deformation and fracture
When the limits of elastic deformation are exceeded, a solid may fracture.
Brittle Versus Ductile Deformation
Fractures, like ductile deformation, produce permanent and
irreversible deformation.
Large scale fractures in rocks are called joints or faults.
faults
From: Skinner and Porter (2000) The Dynamic Earth
J
Joints
A crack along which there is little or no movement
Stress, strain and fracture
Development of a fault
A fracture with relative movement of the rocks on both sides
The
h behavior
b h
of
f a rock
k depends
d
d on:
¾ Temperature: the higher the temperature, the
weaker and less brittle a solid becomes.
¾ Confining pressure:
press re an increase in confining
pressure inhibits the formation of fractures.
¾ Time - Deformation/strain rate: the faster the
strain rate, the more susceptible a rock is to fracture.
¾ Composition:
p
the mineralogical
g
composition
p
and
water content of a rock determines its strength. (e.g.,
quartz is strong, carbonates and sheet silicates are
f i l weak).
fairly
k)
Brittle deformation and fracture
Low temperatures
temperatures, low pressures
pressures, and high strain rates
rates, conditions typical of the
upper crust, enhance brittle fracture. High temperature, high pressure, and low
strain rates, which are characteristic of the deeper crust and mantle, reduce the
likelihood of brittle fracture and enhance the ductile properties of rocks.
From: Skinner and Porter (2000) The Dynamic Earth
The
h behavior
b h
of
f a rock
k depends
d
d on:
¾ Temperature:
the higher the temperature, the
weaker and less brittle a solid becomes.
¾ Confining pressure: an increase in confining
pressure inhibits the formation of fractures.
¾ Time - Deformation/strain rate: the faster the
strain rate, the more susceptible a rock is to fracture.
¾ Composition: The mineralogical composition and
water content of a rock determines its strength. (e.g.,
quartz is strong, carbonates and sheet silicates are
f i l weak).
fairly
k)
Forces responsible for the
deformation of rocks
Most large scale crustal deformations
occur very slowly, at very low strain
rates, and often cannot be measured
over a lifetime, whereas ...
¾Compressive forces: squeeze and
shorten a body of rock (e.g., convergent
margins
i off plates
l t or where
h
plates
l t collide
llid
– subduction, orogenesis)
¾Tensional forces: stretch a bodyy of rock
and pulls it apart (e.g., divergent margins
or loading of a basin with sediments)
¾Shearing forces: push two sides of a
body of rock in opposite directions (e.g.,
transform fault – San Andreas fault)
Joints
- a crack along which there is little or no movement –
fractures along planes of weakness: bedding, foliation, mineral cleavage
Unloading and breakage along
planes of weakness ((e.g.,
p
g , bedding)
g)
Contraction of igneous body,
expansion due to thermal shock
Columnar joints
Joints occur in parallel sets and often control
the weathering of the rock in which they occur.
Classification of faults
When failure of the rock occurs by the sudden
dislocation of rocks along a plane - a fault results.
Dip slip faults
Dip-slip faults are characterised by vertical
displacement of rocks on either side of the fault.
Compression
Shortening
Extension
Stretching
Normal Dip-Slip Fault
When you stand in a tunnel excavated along
the fault, your head is near the hanging-wall
block and your feet rest on the footwall block.
The hanging
Th
h
i wallll moves d
down relative
l ti tto th
the ffootwall.
t ll
They accommodate crustal extension (pulling apart).
The fault below shows displacement and drag folding.
Dip and Strike
Lake water on a dipping bed
of strata defines a strike line.
line
Strike – direction of the fault plane at the surface
Dip – angle in degrees between the horizontal
plane, measured down from a plane perpendicular
to the strike
Horst and Graben
horsts
Reverse fault
(Compression)
Dip slip faults
Dip-slip
Normal fault
(extension)
Thrust fault
(Compression)
Thrust faults, Detachment fault
and Mountain Building
Movement on reverse faults pushes older rocks over younger
ones, thereby shortening and thickening the crust.
Thrust faults, Detachment fault
and Mountain Building
Th t faults
Thrust
f lt actt to
t shorten
h t and
d thicken
thi k mountain
t i belts.
b lt
Thrusts can transport sheets of rock hundreds of
kilometers and are common features at the
leading edge of orogenic deformation.
Thrust fault
Thrust faults are low angle reverse faults with dips less than 45o. They are
mostly found associated with great mountain chains where the hanging wall
block may have been moved many kilometers over the footwall block.
Strike-slip faults
Strike-slip faults are characterised by horizontal displacement of rocks on either
side of the fault. The forces (shear) involved are parallel to the strike line.
Transform Fault
The San Andreas Fault
Sudden movements along faults are
the dominant cause of earthquakes.
Oblique-slip
q
p faults
Oblique-slip faults are characterised by both horizontal and vertical
displacement of rocks on either side of the fault. Forces involves are a
combination
bi ti off th
those causing
i strike-slip
t ik li and
d di
dip-slip
li faulting.
f lti
Compression shortening
+ Shear
Extension stretching
+ Shear
Folding
Gradual movement, involving ductile
deformation under compressive forces,
leads to bending
g or folding
g of the crust.
The term “fold” implies that a structure that originally was planar, like a
sedimentary bed, has been bent. The deformation may be produced by
either horizontal, vertical, or shear forces in the crust.
Stress (force/area) and strain
The type of force (stress) will be reflected in the style of deformation (strain).
Types of stress
Extension
Compression
Stretching
Types of strain
Shortening
Long dimension perpendicular to
the direction of shortening
Long dimension parallel to
the direction of stretching
Shear stress and strain
Shear stress develops when one side of a rock body
moves sideways past the other side.
90
90°
< 90°
Three types of Stress
From: Murck, Skinner & Mackenzie (2008) Visualizing Geology
Folding is the most common form of deformation of layered rocks, its most
typical manifestation is in mountain belts. Folding, as during mountain building,
shortens and thickens the crust.
The complexity of folds is proportional to the degree of deformation
F lds
Folds
Broad, open folds form in the stable interiors of
continents, where the rocks are only mildly warped.
Complex folds develop in mountain belts where
deformation is intense.
Folding
g
Folding shortens and thickens the crust
When sedimentary rocks are buried deep within the crust, where confining pressures are
great, strata can be deformed into very tight folds stacked one atop of each other.
Fold Geometry
A hinge is a line along which curvature is greatest.
Limbs are the less curved “sides” of a fold.
Th axial
The
i l plane
l
connects
t hinges
hi
off successive
i llayers.
Fold g
geometry:
y Monocline
A monocline is a fold that looks like a
carpet draped over a stair step.
Monoclines are generated by blind basement
faults that do not cut the surface but which
still wrinkle the overlying sedimentary cover.
Fold geometry: Anticline
An anticline (an upfold) is a fold
that looks like an arch. The limbs
of the fold dip away from the axis
or crest of a fold.
The limbs dip out and
away from the hinge.
Fold geometry: Syncline
A syncline (downfold) is a fold that
opens upward like a trough.
The limbs dip inward
and toward the hinge.
Anticline, syncline and monocline
Limbs of the fold dip away from
the axis or crest of the fold
Horizontal or gently dipping
beds are modified by simple
step-like bends.
Syncline
Geologists represent the hinge of a fold by strike and dip symbols
on the outer layers of the fold (long lines parallel to the hinge with
a shorter line pointing in the direction of the younger layers).
Folding
g
Anticline
Anticlines and Synclines
Layered rock may be deformed into complex
folds by tectonic compression. Folds occur in
a variety of shapes, sizes, and geometries.
Anticlines and synclines are usually paired.
As
Asymmetrical
t i lA
Anticline
ti li
(tilted beyond the vertical)
A ti li
Anticline
Plunging and non-plunging folds
Plunge
Hinge: portion of the fold where
the curvature is greatest.
Plunge:
g angle
g between the fold
axis (or hinge) and the horizontal.
Folds do not continue forever,
their ends die out much like the
wrinkles on a piece of cloth.
Truncated folds and map patterns
After erosion,
Aft
i
folds
f ld b
become ttruncated.
t d
They are recognized by the pattern of
rock layers on the ground surface.
Layers in a non-plunging fold (anticline or syncline) of
sedimentaryy layers
y
appear
pp
as p
parallel stripes
p on the g
ground.
Layers in a plunging fold
( ti li ) h
(anticline)
have a U
Ushape on the ground.
Truncated folds and map patterns
Geologists represent the hinge of a fold by a heavy line bordered by outward-pointing
outward pointing
arrows for an anticline and inward-pointing arrows for a syncline or by strike and dip
symbols on the outer layers of the fold (short lines parallel to the hinge with a shorter line
pointing in the direction of the younger layers).
Sandstone 1
Sandstone 2
28
28
29
29
Sandstone 1
Sandstone 2
Differential resistance to erosion of adjacent strata in folded
structures leads to distinctive topographic features by which folds
can be recognized at the surface
surface.
Plunging Anticline
Plunging Anticline
Plunging Syncline
Dome and basin formation
A fold with an overturned bowl is called a dome,
whereas a fold shaped like a right-side-up bowl is called a basin.
Anticlinal structure
Synclinal structure
(
(can
make
k good
d oilil reservoirs
i if …))
(
(typically
associated with old mountain belt))
Domes and basins
Dome
Basin
Dome
Isostasy and Mountain building
Whereas typical continental crust has a
thickness of about 35-40 km (measured to
the surface of the Moho),
Moho) the crust beneath
some mountain belts may reach a thickness
of 50-70 km (or about double its normal
thickness).
Crustal roots are important because,
without their buoyancy, mountain ranges
would not sit so high on the surface of the
Earth Æ Archimedes’ Principle.
The soft but solid asthenosphere
p
can
slowly flow out of the way as the base of
the lithosphere sinks under its own
weight. When the buoyancy force pushing
the lithosphere up equals the gravitational
force pulling the lithosphere down, we
have isostacy.
An iceberg and its root
Archimedes’ Principle –
the mass of water displaced is equal to the
mass of the floating iceberg.
Convergence, Subduction and Orogeny
The Plate Tectonics theory has taught us that mountains form in response
to convergent boundary deformation, continental collisions, and rifting.
If plate movements push the continent tightly against the subduction zone,
compression on the continent side of the volcanic arc generates a fold-thrust belt.
Mountain Building
Numerous thrust faults and associated folds develop, accommodating
significant horizontal shortening and thickening of the crust.
Andes: Foldbelt Mountains
Convergence along the western coast of South America has
generated a fold-thrust belt on the eastern side of the Andes.
Foldbelt mountains consist of two basic components:
a complex core of igneous
igneous, metamorphic and volcanic rocks and
a sequence of folded and faulted sedimentary rocks.
From: Renton (1994) Physical Geology
A foldbelt also formed on the eastern
side of the North American Cordillera
during the Mesozoic and Cenozoic (over
)
the last 250 Ma).
Mount Kidd, the end of the Lewis Thrust Fault
From: Murck, Skinner & Mackenzie (2008) Visualizing Geology
Convergence, Subduction and Orogeny
A foldbelt also formed on the eastern side of the North American
Cordillera during the Mesozoic and Cenozoic (over the last 250 Ma).
During convergent
convergent-margin
margin orogeny,
orogeny offshore islands
islands, volcanic arcs and small
fragments of continental crust may drift into the convergent margin. These blocks –
exotic terrane - are too buoyant to subduct and accrete to the continent.
Exotic terranes
make up the western
half of the North
American Cordillera
During convergent-margin
orogeny, offshore islands,
volcanic arcs and small fragments
of continental crust may drift into
the convergent margin. These
blocks are too buoyant to subduct
and
d accrete
t to
t the
th continent.
ti
t
Continent-continent collision
and orogeny
During collision, intense compression generates fold-thrust
fold thrust
belts on the margins of the orogen.
Regional metamorphism in
an orogenic belt
In the interior of the orogen, where one continent overrides the edge of the
other, high-grade metamorphism occurs, accompanied by the formation of
flow folds and foliation. The crust below the orogen thickens to as much as
twice its original thickness.
Unmetamorphosed
shale
Hornfels
Migmatite
Gneiss
Schist
Slate
The collision of India with Asia
Rocks
R
k att d
depth
th iin th
the orogen heat
h t up and
db
become so weak
k th
thatt th
the mountain
t i belt
b lt may
collapse and spread out sideways. The Tibetan Plateau may have formed when crust,
thickened during the collision of India with Asia, spread to the northeast.
Ganges
Plain
Himalaya
Mountains
10
24
38
55
Suture between
Indian and
Asian Plates
Indian Plate
71
Asian plate
Mt. Everest
Thrust
faults
Normal
fault
Tibet
plateau
Multiple continental collisions
The Appalachian Mountains were first formed during the continental collision that
) Since their formation, the Appalachians have been
created Rodinia ((1 Ga).
reduced by erosion and re-uplifted (creation of Gondwana/Pangaea) by orogenic
forces that affected the entire eastern portion of North America.
From: Renton (1994) Physical Geology
Tectonic evolution of the
Appalachian Mountains
1000-1300 Mya
Rodinia
Volcanic island arc?
600 Mya
Exotic terrain, piece of crust?
450-470 Mya
420-370 Mya
300-350 Mya
250 220 M
250-220
Mya
Pangaea
Relief map of the Valley and Ridge
P i
Province,
Pennsylvania
P
l
i tto Virginia
Vi i i
Divergence
g
and Rift Mountains
When rifts form, uplift occurs because, as the lithosphere thins, hot
asthenosphere rises, making the remaining lithosphere warmer and less
dense As rifting continues
dense.
continues, stretching causes normal faulting in the brittle
crust above. This produces sediment-filled basins separated by narrow,
elongate mountain ranges Æ fault-block mountains (e.g., African Rift).
Basin and Range Mountain Belt:
southwestern U.S.A.
U S A (Utah,
(Utah Nevada & Arizona)
Fault-block mountains
From: Renton (1994) Physical Geology
Digital elevation map of North America
A craton consists of crust that has not been affected by orogeny for a long
time, typically 1 billion years. Hence, they are cool, strong and stable.
Shield: where
Precambrian (>500 Ma)
metamorphic and
igneous rocks crop out at
the surface (e.g.
Greenville).
Platform: where a
relatively thin layer of
Phanerozoic (<500 Ma)
sediment cover the
Precambrian rocks.
The Canadian Shield
The Canadian Shield, which occupies twothirds of Canada, contains traces of
Himalaya-like collision zones, Andean-like
convergent boundaries
boundaries, and East African
African-like
like
rifts, all formed more than 1 billion years ago.
These orogens are so old that erosion has
worn away the original topography, exposing
deep crustal rocks at the Earth’s surface.