Download Introduction: Tracking Past Plate Motions (2)

Chapter 20: The Earth Through Time
Introduction: Tracking Past Plate
Motions (2)
„
Evidence includes:
y
y
y
„
1
Introduction: Tracking Past Plate
Motions (1)
The evidence in support of plate motions comes
from measurements made with the Global
Positioning System (GPS).
„ However, evidence of past plate motions is not
obtained through GPS.
„
2
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Great arc-shaped belts of metamorphic rocks formed
by continental collisions.
The eroded remains of island-arc volcanic rocks.
Traces of Earth’s past magnetic field preserved in old
lava flows.
Plates have been moving and changing Earth’s
surface for at least 2 billion years.
3
Former Ideas About Continents (1)
In the sixteenth century, it became apparent that
the coasts were approximately parallel.
„ During the nineteenth century, the favored idea
was that Earth was originally a molten mass that
is cooling and contracting, with the crust being
gradually compressed.
„
4
Former Ideas About Continents (2)
„
Scientists discovered at the beginning of the
twentieth century that the Earth’s interior is kept
hot by radioactive decay.
y
y
y
The Earth might not be cooling but heating up (and
therefore expanding).
Heating would cause the Earth that had been covered
largely by continental crust to expand and then the
continental crust would crack into fragments.
As expansion continued, the cracks would grow into
ocean basins and basaltic magma from the mantle would
rise through the cracks to form the oceanic crust.
The New Idea: Plate Tectonics (2)
The energy needed to move plates comes from
the Earth’s internal heat energy, which causes
convective flows in the mantle.
„ Plate tectonics is the only theory ever developed
that explains all of the Earth’s major features.
„
5
7
The greatest shortcoming
of continental drift was
the lack of an adequate
mechanism for moving
the continents. Wegener
proposed that the Earth's
spin caused the
continents to move,
plowing through the
oceanic plate and
producing mountains on
their leading edges.
Geologists at that time
understood enough about
the strength of rocks to
know that this was highly
unlikely.
The New Idea: Plate Tectonics (1)
By the middle of the twentieth century, a totally
new approach was developed: plate tectonics.
„ When the lithosphere (岩石圈), called plate,
slides sideways across the asthenosphere(軟流圈):
„
y
y
y
Some parts of the lithosphere can be in compression.
Others can be in tension (that is, being pulled apart).
When a plate split in two, the broken edges of
continental crust match perfectly.
from Cox and Hart, 1986
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300 My BP
Pangaea (1)
Alfred Wegener’s theory of continental drift
originated when he attempted to explain the
match of the shorelines on the two sides of the
Atlantic, especially along Africa and South
America.
„ He hypothesized an ancient land mass called
Pangaea.
„ The northern half of Pangaea is called Laurasia,
the southern half Gondwanaland.
„
Figure 20.1 A
white area: covered by ice
solid lines: contours of 2000-m below sea level, along which the
continental crust meets oceanic crust
black area: overlapped region; red area: gaps
9
11
Present
Pangaea (2)
„
Laurasia includes:
y
y
„
Eurasia.
North America.
Gondwanaland includes:
y
y
y
y
y
India.
Africa.
Antarctica.
Australia.
South America.
Figure 20.1B
white area: old ice sheets
arrows: direction of movement of the old ice
dashed lines: connecting the glaciated regions if the continents
were in the present positions at 300 My BP.
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Pangaea (3)
„
During the late Carboniferous Period, about
300 million years ago, a continental ice sheet
covered parts of South America, southern
Africa, India, and southern Australia.
y
„
Apparent Polar Wandering (1)
This is explained by continental drift: 300 million
years ago, the regions covered by ice lay in high, cold
latitudes surrounding the south pole, while north
America and Eurasia were close to the equator
„
From the mid-1950s to the mid-1960s,
geophysicists discovered paleomagnetism (古地
磁).
y
Certain igneous and sedimentary rocks can preserve a
fossil record of the Earth’s magnetic field at the time
and place the rocks formed.
It remained unconvinced that how the solid
rock of a continent could possibly overcome
friction and slide across the oceanic crust.
13
15
Reunite of Pangaea
backward in time
Breakup of Pangaea from
200 My BP to the present
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Figure 20.2
Apparent Polar Wandering (2)
„
Three essential bits of information are contained
in that fossil magnetic record:
y
y
y
The Earth’s polarity (the magnetic field was normal
or reversed at the time of rock’s formation).
The location of the magnetic poles at the time the rock
formed.
The magnetic inclination (磁傾角, I) (indicating how
far from the point of rock formation the magnetic
poles lay).
blue lines: horizontal surface
red diamonds: magnetic inclination
17
The earth’s magnetic field is nearly a dipole field, with the
dipole axis tilted about 11° away from the earth’s rotation axis.
19
θm: colatitude
φm: latitude = 90°－θm
Apparent Polar Wandering (3)
The paleomagnetic inclination (I) is a record of
the place between the pole and the equator (that
is, the magnetic latitude (φm) where the rock was
formed (use the dipole formula: tanI=2tan φm).
„ In the 1950s, geophysicists determined that the
strange plots of paleopole positions indicated
apparent polar wandering.
„
Inclination (I) :
the angle between the horizontal and the magnetic field direction
measured positive downward (-90°~90°)
Declination (D) : the angle between geographic north and the field direction measures
positive clockwise (0°~360°)
G
Assume the earth is a sphere with radius of a, the best dipole magnetic field B :
G
µm
B = 0 3 (1 + 3sin 2 ϕ m )1/ 2
4π a
tan I = 2 tan ϕm
μ0: permeability of free space(=4π×10-7tesla/A)
m: dipole moment (Am2)
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20
If the continents were not moved in the past, the paleopole of the
earth’s magnetic fields would wander along the track denoted by
solid lines.
Seafloor Spreading (2)
Hess postulated that magma rose from Earth’s
interior and formed new oceanic crust along the
midocean ridges.
„ Three geophysicists (Frederick Vine, Drummond
Matthews, and Lawrence Morley) proposed that
lava extruded at any midocean ridge becomes
magnetized and acquires the magnetic polarity
that exists at the time the lava cools.
„
Figure 20.3
21
23
Seafloor Spreading (1)
Seafloor Spreading (3)
In 1962, Harry Hess of Princeton University
hypothesized that the topography of the seafloor
could be explained if the seafloor moves sideways,
away from the oceanic ridges.
„ His hypothesis came to be called the theory of
“seafloor spreading,” and was soon proven to be
correct.
„
„
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The oceanic crust contains a continuous record
of the Earth’s changing magnetic polarity.
„ Successive strips of oceanic crust are magnetized
with normal and reversed polarity.
„ The magnetic striping allows the age of any place
on the seafloor to be determined.
„ Magnetic striping also provides a means of
estimating the speed with which the seafloor had
moved.
24
Relative velocity of
plate motion v can be
determined by
distance to the ridge d
divided by age of the
seafloor t, i.e.,
v = d / t.
Magnetic Record And Plate Velocities
(1)
The most recent magnetic reversal occurred
730,000 years ago.
„ The oldest reversals so far found date back to the
middle Jurassic, about 175 million years ago.
„ From the symmetrical spacing of magnetic time
lines it appears that both plates move away from
a spreading center at equal rates.
„
Figure 20.4
25
27
Figure 20.5
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Magnetic Record And Plate Velocities
(2)
Magnetic Record And Plate Velocities
(4)
All that can be deduced from magnetic time lines
is the relative velocity of two plates.
„ Absolute velocities requires information from
GPS measurements (for present-day plate
motions), or hot spot tracks, or past plate
motions;
y
„
y
y
One consequence of different plates velocities is that
the width of new oceanic crust bordering a spreading
center increases with the distance from the rotation
pole.
Each transform fault lies on a line analogous to a line
of latitude around the rotation pole.
Plates with only oceanic lithosphere tend to have high
relative velocities (Pacific and Nazca plates).
29
31
Magnetic Record And Plate Velocities
(3)
y
y
Plates with a great deal of thick continental
lithosphere, such as the African, North American,
and Eurasian plates, have low relative velocities.
Plate velocities vary with the geometry of motion of a
sphere.
Plates
of lithosphere are pieces of a shell on a
spherical Earth.
The motion of a rigid plate on the surface of a sphere
can be described by a rotation pole (Euler pole) and a
constant angular velocity rotating around the pole.
Figure20.6
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Relict Plate Boundaries In The
Geologic Record (2)
The paleomagnetism of continental rock can be
used to follow plate motion further back in time,
but without the breadth and continuity of
seafloor data.
„ Today’s continents were assembled from many
distinct plates or plate fragments.
„ Small fragments of continental crust that have
drifted as a single unit in the Earth history are
called terranes.
„
Figure 20.7
Figure 20.8
33
Relict Plate Boundaries In The
Geologic Record (1)
Seafloor magnetic strips help us reconstruct
plate motion only as far back in time as the
Jurassic, some 175 million years ago.
„ All large expanses of older oceanic crust have
been subducted back into the mantle at
convergent plate boundaries.
„
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35
Ophiolites (1) (蛇綠岩系)
The igneous rock formed at a spreading center,
known as midocean ridge basalt (MORB), has a
distinctive chemistry;
„ When MORB is found on land, it usually lies
within a body of rock that appears to be a
fragment of oceanic crust caught up in a
continental collision.
„
36
Ophiolites (4)
Ophiolites (2)
The minerals that characterize basalt, if buried
deep within the collision zone, transform into a
assemblage dominated by a distinctive green
fibrous mineral called serpentine (蛇紋石).
„ These serpentine-dominated fragments of
oceanic crust found on continents are called
ophiolites, from the Greek word for serpent,
ophis.
„
37
Subduction Mélange (混雜(成)岩) And
Blueschists (1)
Ophiolites (3)
„
Their structure matches well the crustal
structure expected at a midocean ridge:
y
y
y
y
y
39
At the top is a thin layer of sediment that was
deposited on the ocean floor.
Beneath the sediment is a layer of pillowed basalt.
Still deeper are sills of gabbro (輝長岩), the plutonic
equivalent of basalt.
Beneath the gabbro sills there is often a layer of
peridotite.
The contact between gabbro and peridotite (橄欖岩) is
interpreted to be the former Moho at the base of what
was formerly oceanic crust.
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Along convergent margins, a distinctive feature
is the development of a mélange: a chaotic
mixture of broken, jumbled, and thrust-faulted
rock.
„ A sinking plate drags the sedimentary rock
formed from accumulated sediment downward
beneath the overriding plate.
„
40
Subduction Mélange And Blueschists (2)
Back-arc Basins And Plate Extension (1)
Caught between the overriding plate and the
sinking plate, the sediment becomes shattered,
crushed, sheared, and thrust-faulted, forming a
mélange.
„ As the mélange thickens, it undergoes
metamorphism, common in many mélange zones,
to form low-temperature metamorphism
blueschists (藍片岩)and eclogites (榴輝岩).
„
„
When the sinking of a subducting plate is faster
than the forward motion of the overriding plate,
the margin of the overriding plate can be
subjected to tensional (pulling) stress.
„ If the overriding plate is oceanic or if the
extension of a continental margin has progressed
to an extreme state, an arc-shaped basin forms
behind and parallel to the magmatic arc of the
subduction zone.
41
Figure 20.10
43
Figure 20.11
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44
Supercontinents And Vanished Oceans
(1)
The average composition of continental
lithosphere is quartz-rich, compared to olivine
and pyroxene-rich oceanic lithosphere.
„ Continental lithosphere is always less dense than
oceanic lithosphere and is not subducted.
„ Seismic evidence suggests that a root of mantle
rock is attached to the base of old, cool
continental crust.
„
Figure 20.12
45
Back-arc Basins And Plate Extension (2)
„
Basaltic magma may rise into such a back-arc
basin at a newly formed spreading center, and
new oceanic crust may form.
47
Supercontinents And Vanished Oceans
(2)
Continental buoyancy can be enhanced by the
detachment and loss of this dense mantle root
during the final stages of a continental collision.
„ When plate movement bunches continental
fragments together, the heat of the mantle
beneath still must escape Earth’s interior.
„
y
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A supercontinent impedes heat flow from the deep
mantle to the surface, effectively forming a layer of
insulation.
48
Supercontinents And Vanished Oceans
(3)
„
Convective motion within Earth’s mantle:
y
y
Will accumulate heat and cause thermal buoyancy at
the base of the continental lithosphere.
Warms and softens the lithosphere, which begins to
rift.
Red strip marks
the location of an
orogen that once
formed a common
suture of several
cratons.
The opening of the Atlantic ocean was heralded by the
eruption of large basalt flows;
In some cases, these eruptions persisted and became hot spots
(example: the Parana flood basalt in Brazil).
Figure 20.13
49
Supercontinents And Vanished Oceans
(4)
„
Two supercontinents existed in the past:
Pangaea, formed roughly 350 million years ago,
y Rodinia, an earlier Proterozoic* (原古界) continent
formed roughly 1100 million years ago and split apart
roughly 750 million years ago.
*see geological time scale in Figure 11.10 of page 286.
51
Supercontinents And Vanished Oceans
(5)
„
y
„
The sutures between continental fragments can be
found in the many mountain ranges and belts of
ophiolites that formed in the late Paleozoic period,
roughly between 450 and 350 million years ago.
Examples include:
y
y
y
y
„
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The Appalachian mountains in eastern North America.
The Atlas mountains in Morocco.
The Ural mountains in Russia.
The Hercynian mountains in Europe.
Each of these mountain ranges is heavily eroded now.
52
Supercontinents And Vanished Oceans
(6)
„
As Pangaea formed, sediment sequences on
continental shelves around the world record
evidence that global sea level fell, draining
shallow continental seas and exposing large areas
of continental shelves.
Supercontinents And Vanished Oceans
(8)
Pangaea lasted as a supercontinent for at least
150 million years.
„ New oceans formed when Pangaea separated:
„
53
Supercontinents And Vanished Oceans
(7)
„
Geologists interpret the sea level drop as
evidence for:
y
y
„
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The Atlantic.
y
Southern Oceans.
55
Ice Ages in Earth History (1)
„
A general slowdown in plate movement.
A slowdown in the rate of formation of young,
buoyant oceanic lithosphere at midocean ridges.
The oceanic lithosphere at the time of Pangaea
was, on average, older, colder, and denser,
leading to a deeper world ocean.
y
Large continental ice sheets have existed several
times during Earth’s history:
y
y
„
In the Carboniferous period, 286 to 360 My ago.
In the Ordovician and Silurian periods, roughly 450 to
425 My ago.
Both of the glacial periods occurred at times
when large continental landmasses lays over
Earth’s south pole, providing a platform for the
accumulation of vast quantities of snow and ice
at high latitudes.
56
Figure 20.14
Phanerozoic (顯生宙(元)) Glaciations (1)
„
Three major periods of high-latitude glaciation
have been recognized in the Phanerozoic:
y
y
y
„
Rapid carbon burial and reduced volcanic
activity because of slow plate motion and low
spreading-ridge volcanism lower concentrations
of CO2 in the atmosphere, reducing the
greenhouse effect and encouraging glaciation.
57
Ice Ages in Earth History (2)
„
Factors leading to an ice sheet:
y
y
y
y
The amount of carbon dioxide and other greenhouse
gases in the atmosphere.
The tilt of the Earth’s axis.
Changes in the Earth’s orbit around the sun.
The height of continents and their positions in relation
to the north and south poles.
59
Phanerozoic Glaciations (2)
Carboniferous coal deposits occur within
repetitive layered sedimentary sequences of
marine and nonmarine sediments called
cyclothems.
„ Cyclothems indicate repeated transgressions and
regressions by the sea.
„
y
y
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Pleistocene (0 to 5 million years ago).
Carboniferous (roughly 300 million years ago).
Ordovician (roughly 455 million years ago).
Timing connected to the longer Milankovitch orbital
cycles of 100,000 years.
glacial and interglacial climate may have alternated in
a regular pattern.
60
Phanerozoic Glaciations (3)
„
New continental glaciers:
y
y
y
„
Would have stolen water from the oceans, causing sea
level to drop.
Drained Coastal coal swamps as the sea level fell.
Decreased carbon consumption because of reducing
photosynthesis.
Snowball Earth Hypothesis (I)
„
Atmospheric CO2 levels could return to normal
levels.
y
An increase in greenhouse warming would melt the
glaciers and raise the sea level, rejuvenating the coal
swamps and repeating the glacial cycle.
61
Phanerozoic Glaciations (4)
„
There is evidence for continental glaciation near
the end of the Proterozoic Eon, between 600 and
800 million years ago.
y
„
The extensive low-latitude glaciation occurred during
a climatic extreme in which the whole Earth must
have experienced glaciation.
Extensive low-latitude glaciation did not occur at
any time during the Phanerozoic.
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A hypothesis attempts to explain a number of extreme
phenomena in the geological record by proposing that the
earth’s ocean was covered by ice for long periods 580-750 My
ago. The extensive continental glaciation existed at low
latitudes, possibly even at the equator near the end of
Proterozoic Eon. Each glacial period lasted for millions of
years and ended violently under extreme greenhouse
conditions. These climate shocks triggered the evolution of
multicellular animal life, and challenge long-held
assumptions regarding the limits of global change.
63
Snowball Earth Hypothesis (II)
„
Paleomagnetic evidence suggests that the continents were
clustered around the Equator around 750 Myr ago, known
as supercontinent of Rodinia. If CO2 in the atmosphere were
to slowly drop over millions of years due to a slow reduction in
volcanic activity, global temperatures would drop and glaciers
would cover the high-latitude regions. The ice sheets prevent
chemical weathering, the process that consumes CO2 to become
carbonate. This stabilizes the levels of CO2 in the atmosphere.
When all the continents were in the tropics, the consumption of
CO2 would not work, as the continents would remain ice-free
even as the Earth grew colder, approaching the critical threshold
for a snowball.
64
Regional Structures Of Continents (1)
„
Because the most ancient oceanic crust known to
exist in the ocean dates only from the midJurassic Period (175 My ago), the only direct
evidence concerning geologic events more
ancient than the mid-Jurassic comes from the
continental crust.
Regional Structures Of Continents (3)
An assemblage of cratons and ancient orogens
that has reached isostatic equilibrium is called a
continental shield.
„ Because it is a stable platform, a continental
shield is covered by a thin layer of littledeformed sediments.
„
65
Regional Structures Of Continents (2)
„
Two kinds of structural units can be
distinguished within the continental crust:
y
y
The craton is a core of very ancient rock that has
attained tectonic and isostatic stability.
Orogens are elongate regions of crust that have been
intensely folded and faulted during continental
collision.
Only the youngest orogens are mountainous today.
Ancient orogens reveal their history through the kinds of
rock they contain and the kind of deformation present.
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67
(Green)
Four young
orogens:
NE: the
Caledonide
E: Appalachian
W: Cordilleran
N: Innuitian
Each is younger
than 600 My.
Canadian Shield
consists of
ancient orogens
(brown) and
cratons older
than 2 billions
years (orange).
Figure 20.15
68
Regional Structures Of Continents (4)
„
The fragmentation, drift, and welding together
of pieces of continental are responsible for the
five types of continental margins we know of
today:
y
y
y
y
y
The rifting of
continental crust
formed a ocean
basin bounded by
passive continental
margin.
Passive.
Convergent.
Collision.
Transform fault.
Accreted terrane.
Figure 20.16
69
71
Passive Continental Margins (1)
„
A passive continental margin occurs in the stable
interior of a plate.
y
„
The eastern coast of North America, for example, is in
the stable interior of the North American Plate.
Passive continental margins develop when a new
ocean basin forms by the rifting of continental
crust (for example, the Red Sea).
Figure 20.17
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Passive Continental Margins (2)
Passive Continental Margins (4)
Passive continental margins are places where a
great thickness of sediment accumulates.
„ This accumulation apparently occurs in the
following manner:
„
„
y
y
Basaltic magma, associated with formation of the new
spreading edge, splits the continent.
Heating expands the lithosphere so that a plateau
forms as the lithosphere expands, with an elevation of
as much as 2.5 km above sea level.
The formation of three-armed rifts with one of
the arms not developing into an ocean is a
characteristic feature of passive continental
margins.
„ The Gulf of Aden, the Red Sea, and the
northern end of the African Rift Valley meet at
angles of 1200.
y
Such a meeting point formed by three spreading
edges is called a plate triple junction.
73
75
Pangaea
Passive Continental Margins (3)
y
y
y
y
y
Tensional forces cause normal faults and form a rift with
pronounced topographic relief between the plateau and rift.
Before the rift floor sinks low enough for sea water to enter,
clastic nonmarine sediments shed from the steep valley
walls accumulate in the rift.
Associated with these sediments are basaltic lavas, dikes,
and sills that form by magma rising up the normal faults.
As the rift widens, a point is reached where seawater enters.
The high rate of evaporation results in the deposition of a
strata of evaporate salts laid down on top of the clastic
nonmarine sediments
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SW Indian Ridge
Triple junction
Spreading is very slow
Figure 20.18
Figure 20.17
76
Continental Convergent Margins (1)
Continental Convergent Margins (3)
At a continental convergent margin, the edge of
a continent coincides with a convergent plate
margin in which oceanic lithosphere is being
subducted beneath continental lithosphere.
„ Subduction produces intense deformation of a
continental margin.
„
„
Sediment subjected to deformation in such a
setting forms a mélange (see slide 42 or Figure
20.10).
y
y
Sediments subjected to high-pressure and lowtemperature metamorphism.
Adjacent and parallel to the belt of mélange, the crust
beneath the continental volcanic arc is
metamorphosed, but here the metamorphism is
regional. One distinctive feature of a continental
convergent margin, therefore, is a pair of parallel
metamorphic belts.
77
Continental Convergent Margins (2)
„
The Andean coast of South America is an
example.
y
y
The Nazca Plate (capped by oceanic crust) is being
subducted beneath the South American Plate.
Wet partial melting in the mantle activated by water
released by the subducted Nazca Plate produced the
andesitic magma that formed the Andes ( a
continental volcanic arc).
79
An example of the
continental volcanic arc is
the Cascade Range. Each
volcano has been active
during the last 2 Myr.
Magma to form the volcano
comes from wet partial
melting above the oceanic
crust of the Juan de Fuca
Plate subducting beneath the
North American Plate.
Where the volcanoes have
been eroded, granitic
batholiths, the remnants of
the magma chambers that
once fed the vocalnoes can
be oberved
Figure 20.19
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80
Continental Collision Margins (1)
„
The edges of two continents, each on a different
plate, come into collision at a continental
collision margin.
y
„
Metamorphism and igneous activity are always
present.
y
This results in intense folding and thrust faulting.
„
Continental Collision Margins (2)
The Himalayan Mountain chain.
Within a fold-and-thrust mountain system,
strata are compressed, faulted, folded, and
crumpled, commonly in an exceedingly complex
manner.
„
The Appalachians and the Urals are older,
Paleozoic-aged, examples.
81
Collision between the
Australian-Indian Plate and
the Eurasian Plate is an
example of continental
collision margins. The
downgoing plate of
lithosphere capped by
oceanic crust breaks off and
continues to sink, but the
edge of the remaining
segment of plate capped by
buoyant continental crust is
partly thrust under the edge
of the overriding plate,
causing further elevation of
the collision zone.
The Alps, the Himalayas, and the Carpathians are all
young fold-and thrust mountain systems formed
during the Mesozoic and Cenozoic eras.
83
Continental Collision Margins (3) -Fold-And Thrust Mountain System
„
These systems develop from piles of sedimentary
strata commonly 15,000 m or more in thickness.
y
Sediments are predominantly marine, accumulated
along passive continental margins.
The American geologist J.D. Dana coined the
term geosyncline to describe a great trough that
received thick deposits of sediment during slow
subsidence through long geologic periods.
„ One distinctive feature of mountain systems
formed by collision is that an ocean disappears
and a new mountain system lies in the interior of
a major landmass.
„
Figure 20.20
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Transform Fault Margins
„
A transform fault continental margin occurs
when the margin of a continent coincides with a
transform fault boundary of a plate.
y
„
The San Andreas Fault.
The San Andreas Fault apparently arose when
the westward-moving North American continent
overrode part of the East Pacific Rise.
y
The San Andreas is the transform fault that connects
the two remaining segments of the old spreading
center.
Accreted Terrane Margins (1)
An accreted terrane continental margin is a
former convergent or transform fault margin
that has been further modified by the addition of
rafted-in, exotic fragments of crust such as island
arcs.
„ The northwestern margin of North America,
from central California to Alaska, is an example
of an accreted terrane margin.
„
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San Andreas Fault (SAF) is an example of the transform fault continental
margins. 20 My ago, the edge of N. America overrode the Farallon Plate
creating two small plates, the Cocos and Juan de Fuca Plate. The SAF is
the transform fault connects the two segments of East Pacific Rise.
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Accreted Terrane Margins (2)
Eventually, any fragment not consumed by
subduction is added (accreted) to a larger
continental mass.
„ In the western Pacific Ocean, there are several
examples:
„
y
y
y
Taiwan.
The Philippine Islands.
The many islands of Indonesia.
Figure 20.21
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Accreted Terrane Margins (3)
Mountain Building
Each fragment, called a terrane, is a geologic
entity characterized by a distinctive stratigraphic
sequence and structural history.
„ An accreted terrane is always fault-bounded and
differs in its geologic features from adjacent
terranes (also called suspect terranes).
„ Some terranes have moved 5000 km or more.
„
„
Today’s fold-and-thrust ranges are the orogens
that formed during the last few hundred million
years.
„ Examples include:
y
y
y
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The western margin
of N. America is a
jumble of terranes
accreted during the
Mesozoic and
Cenozoic eras.
The Appalachians.
The Alps.
The Canadian Rockies.
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The Appalachians (1)
The Appalachians are a Paleozoic fold-andthrust mountain system 2500 km long, that
borders the eastern and southeastern coasts of
North America.
„ They contain mud cracks, ripple marks, fossils of
shallow-water organisms, and, in places, fresh
water materials such as coal.
„ The sedimentary strata thicken from west to east.
„
Figure 20.22
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The Appalachians (2)
Many of Pennsylvania’s oil pools were found in
these gently folded strata.
„ In the region known as the Valley Ridge
Province, the strata have been bent into broad
anticlines and synclines.
„ In the west, the strata are nearly flat lying, but as
one moves further east the strata dip more
steeply to the east.
„
Figure 20.23
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Figure 20.25
Figure 20.24
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The Appalachians (3)
„
The surface along which movement occurred is
known as a detachment surface, and the slice
that moved is commonly referred to by its
French name, decollement.
y
y
The style of deformation above the detachment
surface is usually different from that below.
The brittle sedimentary strata above the detachement
surface has been fractured, moved along thrust faults,
and stacked like a series of thin cards, while the older
rocks below tend to have resisted faulting and largescale translation and to have been deformed by
ductile deformation.
The Alps (1)
The Alps and associated mountain ranges in
southern Europe were formed during the
Mesozoic and Cenozoic eras, as a consequence of
a collision between the European and African
plates.
„ The Jura Mountains, which mark the
northwestern edge of the Alps, have the same
folded form and origin as the Valley and Ridge
Province in North America.
„
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A sequence of subduction,
collision, thrusting, and
accretion produced the
present-day structure.
Iapetus ocean disappeared
~350 Myr ago when Africa
collided with N. America.
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The Alps (2)
The Jura Mountains were formed from shallowwater sediments deposited on an ancient
continental shelf.
„ In the high Alps, thrusting appears to have
developed on a much grander scale than in the
Appalachians.
„ The high Alps are composed of deeper-water
marine strata.
„
Figure 20.26
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Figure 20.27
Figure 20.28
Revisiting Plate Tectonics And The
Earth System (1)
For more than 2000 years, Arab traders sailed to
India during the hot summer months, because
summer winds blow from the west. They sailed
back while winter winds blow from the east.
„ The Arabic name for this seasonal reversal of
wind and weather is mausim, from which we
derive our word monsoon.
„
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The Canadian Rockies
„
„
„
In the Canadian Rockies, a central zone has been intensely
metamorphosed.
The thrust sheets in the Canadian Rockies moved eastward
away from the core zone.
Each sedimentary unit becomes thinner from west to east.
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Revisiting Plate Tectonics And The
Earth System (2)
The monsoon winds of India and Southeast Asia
are a consequence of the collision between the
Eurasian Plate and the Australian-Indian Plate.
„ The high mountains and plateau divert the
normal flow of westerly winds.
„
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Revisiting Plate Tectonics And The
Earth System (4)
Figure 20.31
Sediment cores from the northern Indian Ocean show
rates of sedimentation until about 10 million years ago.
„
y
Two peaks in sediment supply (9 to 6 and 4 to 2 million years
ago) are evidence of major intervals of Himalayan uplift.
Some evidence suggests that monsoon winds started only
during the second major phase of uplift.
„
y
In winter, the snow-covered plateau reflects Summer heating of high plateau
solar radiation and cools down, creating a
causes warm air to rise and form a low
high pressure region from which dry cold air pressure region draws moist air inland
flows outward and downward.
About 8 Myr ago, fine dust eroded by strong winds from desert
basins north of the Tibetan Plateau began accumulating in
central China. An increase in the rate of dust accumulation
occurred during 3.5 to 2.5 Myr ago. The onset of this vigorous
wind erosion coincided with the beginning of the East Asian
monsoon climate system.
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Revisiting Plate Tectonics And The
Earth System (3)
„
Chinese geologists have discovered that these
high lands are rather recent topographic
features.
y
They base their conclusion in part on evidence of
plant fossils of the Pliocene Epoch (5.3 to 1.6 million
years ago), collected at altitudes of 4000 to 6000 m,
that include many subtropical forms that today exist
only at altitudes below 2000 m;
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Summary (1)
„
„
„
„
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How was paleomagnetic evidence used to prove seafloor
spreading? How do we use it to measure the moving velocity of a
plate?
y magnetic strips, magnetization of extruded lava that form
oceanic crust at a mid-ocean ridge
How to quantitatively describe the movement a plate in a sphere?
Why does velocity differ from place to place on a tectonic plate?
y rotation pole, angular rotation rate
Explain how the apparent wandering of magnetic poles throughout
geological history can be used to help prove continental drift.
y Dipole magnetic field, magnetic latitude
What is a mélange and what kind of metamorphism is associated
with mélange?
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Summary (2)
„
What evidence can be used to identify former convergent plate boundaries?
y Ophiolites, relics of island arcs, mountain range
„
Why did the Carboniferous ice sheets wax and wane?
y Greenhouse effect, CO2 cycle
What evidence leads to the hypothesis of a Snowball Earth? When
did the Snowball Earth most likely occur in the geological history ?
What are the main structural units of a continent?
y Craton, orogens
What are the types of continental margins we know today?
Describe the characteristics and the place on Earth for each type of
the margines.
y Passive, convergent, collision, transform fault, and accreted
terrane
„
„
„
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