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Introduction to Earth Sciences I
Topic 4
Mantle Dynamics and Plate Tectonics
Lecture 4.5: Subduction & Plate
Collision
In this lecture, we will cover four topics related to
subduction. First, we will briefly discuss subduction zone
earthquakes, following on the mention of arc volcanic
hazards in the previous lecture. Second, we will introduce
ideas about mountain building, resulting from unsuccessful
subduction – “collision” – of neighboring continental
plates. Third, we will introduce the recrystallization of
rocks due to pressure and temperature, known as
metamorphism. As it turns out, almost all metamorphic
rocks have been recrystallized under conditions unique to
subduction zones. Fourth, we will illustrate the concept
that the growth of continents has proceeded largely from
the amalgamation of arc crust and other small crustal
fragments via subduction and collision.
Subduction zone earthquakes
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We begin this lecture with another view of the Wadati-Benioff
zone, the zone of deepening earthquakes that define the top of
descending, cold, oceanic plates in subduction zones. Here, in an
image created by the Alaska Volcanological Observatory in
Fairbanks, you can see the subduction zone beneath the Aleutian
islands and south central Alaska deepening northward from the
trench (red line).
The Aleutian subduction zone has been the site of two of the three
largest earthquakes ever recorded. One was the 1964 earthquake
near Anchorage, in which more than 70 people died. Subduction
zone earthquakes in general are extremely hazardous. Recent
techniques for measuring motions of the Earth’s surface, related to
the Global Positioning System (GPS) have made it much easier to
understand and - in a sense - predict these earthquakes.
The following two diagrams - from the web pages of Jeff
Freymuller of the University of Alaska in Fairbanks, illustrate
ground motion measured via GPS for the Kenai Peninsula south of
Anchorage over a period of several years. The top diagram shows
cumulative vertical motion, while the lower diagram shows
horizontal motion in terms of velocities relative to the stable
interior of Alaska.
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Freymuller and his colleagues have shown that these motions are
completely consistent with elastic bending of the overthrust plate,
which is currently completely coupled to the subducting Pacific
plate. Elastic deformation means reversible deformation and, like
a steel leaf spring, the more the elastic upper crust is bent, the
higher the stresses which act to undo this deformation. Eventually,
these elastic stresses will be sufficient to overcome friction on the
subduction zone fault system, and another great earthquake will
shake the Anchorage area. Freymuller et al. estimate that the 1964
Anchorage quake completely released stored elastic stress built up
over hundreds of years, so - if the frictional properties of the
subduction faults haven’t changed too much - it might be a while
before the next one.
Not all of the Aleutian subduction system is locked like it is in the
Anchorage area. The following map, which is greatly reduced for
reproduction here, is available online from the Alaska Volcano
Observatory. Among other things, it shows the rupture areas of the
largest earthquakes in pink. These include most of the Aleutian
subduction zone, except for the notorious Shumagin gap. Since
there haven’t been any great earthquakes recorded in this gap, if
the subduction zone faults are locked, a big one may be immanent.
However, GPS measurements have shown that this part of the
subduction system is not locked. The overthrust plate is not
undergoing measurable elastic deformation, and instead seems to
be stably and quietly sliding over the subducting Pacific plate in
this region. The inference is that the Shumagin gap is not a high
hazard area ... it actually seems to be a low hazard area.
QuickTime™ and a
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Subduction zone earthquakes, like other earthquakes, are
generally interpreted in terms of elastic deformation
followed by frictional failure along brittle faults, as we
have done in the previous paragraphs. This is because
viscous deformation, at higher temperatures and greater
depths, is generally irreversible. Once deformed, the shape
of things is permanently changed and won’t “bounce back”.
The following diagram summarizes some important
differences between elastic and brittle behavior, on the one
hand, and viscous deformation on the other.
QuickTime™ and a
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As noted in the figure above, elastic/brittle deformation
generally occurs in relatively cold rocks, and/or as a result
of relatively rapid movement, while viscous deformation
occurs at higher temperatures and/or in response to slower
movements. Brittle deformation - the permanent
component of elastic/brittle behavior - is highly localized
along thin fractures, whereas conventionally it is supposed
that viscous deformation is much more broadly distributed.
There is a dramatic decrease in the number of earthquakes
along Wadati-Benioff zones below a depth of 50 to 70 km,
and this is interpreted in terms of a transition from brittle to
ductile deformation along the subduction fault system.
However, some observations don’t fit this explanation very
well. Again, we reproduce here the tomographic image of
the NE Japan subduction zone from Zhao et al., 1992.
Now we wish to focus on the extensive occurrence of
earthquakes at greater than 70 km depth. They may be rare
compared to shallower quakes, but they certainly do occur.
Also, in this data set, one can clearly see a double seismic
zone, with a dipping plane of earthquakes about 50 km
below the top of the subducting oceanic plate. The rocks
here started out quite a bit hotter than the top of the
subducting crust. A first-order estimate for the temperature
at 50 km in an old oceanic plate would be about 700°C, and
this can only be increased by conductive heating during
subduction into the hotter mantle. All the earthquakes in
this diagram below 70 km depth are usually called
“intermediate depth earthquakes” (because there are even
deeper ones).
Some scientists have proposed a role for subducted H2O in
causing intermediate depth earthquakes. Release of H2O
bound in minerals, during chemical reactions driven by
conductive heating of the subducting plate, could cause
“dehydration embrittlement”. Never mind what this is, in
detail. This might be an OK explanation for intermediate
depth earthquakes along the top of the subducting plate, but
how about the lower seismic zone? What evidence is there
for penetration of H2O to 50 km depth in oceanic plates?
An alternative may be that there are ductile mechanisms for
earthquakes. Recently, scientists have begun to revisit an
idea for periodic viscous heating along ductile shear zones,
first proposed by Jack Whitehead in the 1970’s. Simply
speaking, it might work as follows. Deformation is
confined to narrow shear zones, which are pre-existing
zones with very small crystals, because the strength of
rocks is strongly dependent on grain size. Maybe the zones
of small crystals originally form by crushing of crystals
along brittle faults during earthquakes. Anyway, we know
they exist, because we can find them in ophiolite mantle
sections and other exposures of rocks from great depth.
The strength, or viscosity of rocks depends on temperature
as well as grain size. As stresses build up along a plate
boundary, viscous deformation begins in the shear zone.
Frictional heating begins to lower the viscosity, so the same
stress leads to faster viscous deformation, which in turn
heats the rock still more, in a feedback loop which
ultimately results in release of all the stored stress in a
rapid, viscous earthquake. With stress released,
deformation and frictional heating stop, and the shear zone
may cool conductively, getting ready for another cycle of
instability.
This could explain intermediate depth earthquakes, without
a role for H2O or brittle failure.
Continental collision
We now turn to the topic of mountain building. The
world’s highest mountain ranges are the HimalayaKarakorum mountains in Asia, the Alps and Caucasus in
Europe, the Andes in South America, and the Alaska Range
– St. Elias Mountains in North America. By far the largest
of these is the Himalaya-Karakorum chain, including the
high plateau of Tibet. This is produced by northward
movement of the Indian continental plate into Eurasia.
The following link leads to an animation of the motion of
India relative to Eurasia and Africa, reconstructed on the
basis of oceanic and continental paleomagnetic data:
http://earth.leeds.ac.uk/dynamicearth/himalayas/india/anim
ation.htm
As you can see, the data show that India migrated
northward until it collided with Eurasia, while the
intervening ocean basin was consumed along a subduction
zone along the south margin of Eurasia. This second
animation shows a somewhat more speculative crosssectional history of the collision:
http://earth.leeds.ac.uk/dynamicearth/himalayas/deformatio
n/collide/index.htm
As noted before, the continental crust is composed of lowdensity rocks rich in silicon, aluminum, sodium and
potassium. Because it has such a low density, it is difficult
or impossible to subduct large proportions of continental
crust. Thus, scientists infer that the subducting oceanic
plate north of India broke off, and was consumed into the
mantle, while the continental crust in the Indian and
Eurasian plates simply pushed together – thickening and
rising as a result of compression. The resulting thickening
of the continental crust on both sides of the original line of
collision – called a “suture zone” – caused the mountains to
rise.
As noted earlier in the course, the elevation of a particular
plate is largely controlled by its density. Like a block of
wood in a pool of water, the crust floats on the viscously
deforming mantle, and the height with which it floats
depends on the density relative to the mantle and the
thickness of the crust.
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http://earth.leeds.ac.uk/dynamicearth/
For crust with different density and the same thickness, the
less dense plate floats higher. For crust with the same
density and different thickness, the thicker plate floats
higher.
http://earth.leeds.ac.uk/dynamicearth/
http://earth.leeds.ac.uk/dynamicearth/
QuickTime™ and a
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http://earth.leeds.ac.uk/dynamicearth/
The increased thickness of the crust due to the continental
collision between India and Eurasia led to the formation of
high mountains on both sides of the suture zone.
Compression in the lower crust, due to plate collision,
essentially folds and pushes low-density continental
material upward and outward. Thus, compression in the
deep interior of the Himalayas and other large mountains is
accompanied by extension and outward growth of the
mountain range near the surface.
The following picture illustrates some of the complex
features resulting from this process. Please don’t worry
about or try to memorize the many technical terms used in
this diagram.
As mountains grow, erosion of the highest ridges becomes
very rapid, due to the ordinary action of rainfall and wind,
but also to additional mechanisms that are rare in more
subdued terrain, such as glacial erosion, avalanches and
landslides. Here is another schematic cross-section
illustrating the effects of erosion as well as folding, uplift,
and lateral extension of the mountain range due to collision.
In this cross-section, all of the faults illustrated are still
reverse, or thrust faults. However, sometimes the extension
of the steep mountain range even produces extensional,
normal faults. This can best be visualized as the result of
piling up too much material in the middle of a mountain
range with steep sides. The steep sides undergo
“gravitational collapse” and the interior of the range
extends outward.
The previous three diagrams come from an excellent web
site on mountain building,
http://www.uwm.edu/Course/geosci697-tectonic/
Continental collisions don’t just cause compression and
thickening of the crust. They also cause lateral motions. A
famous paper by Tapponier & Molnar in J. Geophys. Res.
proposed that the Chinese crust is being propelled eastward
into the South China Sea (accommodated by subduction
further east) as a result of the collision of India and Eurasia:
Tapponier realized that the pattern of faults and crustal
thickness in east Asia resembled deformation experiments
in plastic materials, and went on to do a series of
experiments with wooden blocks and putty that produced
striking images of faults and deformation patterns similar to
that seen in east Asia:
This idea may have seemed pretty speculative to
geoscientists in the 1980’s when it was first proposed, but
has recently received dramatic confirmation from GPS
measurements. The following diagram comes from a paper
in J. Geophys. Res. this year:
In this diagram, the velocities of the crust measured with a
large number of GPS instruments are shown with arrows,
and the ellipses indicate the uncertainty of the
measurement. Colors indicate elevation, with green being
low and tan being the highest. Large arrows along the front
of the Himalayan mountains record continued collision of
India into Eurasia. Smaller arrows to the north and east
provide dramatic confirmation of Tapponier & Molnar’s
hypothesis that China is moving eastward, driven by the
plate collision to the south.
Another example of this type of lateral “extrusion” is in
northern Turkey, where a block is being pushed westward
by collision between the Arabian and Eurasian plates, with
much of the resulting, lateral motion between northern
Turkey and Eurasia accommodated on the North Anatolian
strike-slip fault. Unfortunately, this large fault is
seismically active and passes through areas with high
population density, leading to disasters such as the 1999
earthquake that killed more than 50,000 people.
http://earth.leeds.ac.uk/dynamicearth/
This concludes our section on mountain building via
continental collision, except to note that many older
mountain ranges formed in the same way. Thus, the Urals
in Asia and the Appalachians in North America are
erosional remnants of Himalayan-style collisions that
occurred hundreds of millions of years ago.
Metamorphism
Metamorphism occurs when changes in pressure and
temperature cause recrystallization of rocks. So for
example minerals which are stable – “happy” – at low
temperature and pressure, like clays, break down as they
are heated, giving rise to new minerals which are stable at
higher temperature, such as micas. In turn, micas break
down with the formation of garnet and feldspar and other
minerals. Most of these changes also involve release of
“volatiles”, like H2O and CO2, which are abundantly
bound in the molecular structure of the low temperature
minerals but are much less abundant in the high
temperature minerals.
We don’t have time to go into the details of this process. A
good web page that illustrates some general concepts is
http://www.ig.uit.no/~kaarek/geology_intro/metamorphism
.swf
What we do wish to emphasize here is that most
metamorphism recorded by rocks that are exposed on the
Earth’s surface occurred at depth in subduction zones and
arcs. Part of the reason for this is that the minerals in rocks
tend to record the highest temperature that the rock has
experienced. The reason for this is that, as alluded to
above, changes in minerals during heating evolve H2O and
CO2 as gases and fluids, which then leave the rock.
Because the H2O and CO2 are gone, it is hard for the low
temperature minerals to form again as the rock cools and
decompresses later in its history. So, the high temperature
mineral assemblages are often preserved. And, for most
crustal rocks, the highest temperature and pressure
conditions they can reach are attained beneath subductionrelated volcanic arcs.
To elaborate on this a little further, we need to illustrate
typical geotherms for different tectonic settings. We found
that most textbooks don’t do this very well, so we have
made our own summary diagram:
Continental interiors are old and cold. Nothing much has
happened to them for millions or even billions of years, and
meanwhile they have been losing heat by conduction to the
Earth’s surface. We can estimate the temperature at a
given depth beneath the continental interiors by looking at
mineral compositions in blocks of mantle material from
near the base of the plate (xenoliths) transported in
explosive volcanic eruptions. Because the mineral
compositions are sensitive to temperature and pressure, we
can measure their compositions and estimate the conditions
at which they formed.
Also cold are subduction zones, because cold oceanic crust
is being rapidly transported into the Earth’s interior, with
little time to be conductively heated by the surrounding,
hotter mantle. We’ve shown several figures – of thermal
models for subduction zones, and seismic data – illustrating
that this is true.
As noted in the previous lecture, there is ample evidence
that the crust beneath arcs is very hot at a given depth,
compared to most other tectonic settings. This is so,
apparently, because of upwelling of hot mantle material to
replace cold material pulled from the base of the overthrust
plate during subduction. Arcs are hotter than old oceanic
plates, which generally are about 100 km thick and have
undergone extensive cooling, losing heat to the oceans by
conduction and hydrothermal convection.
The hottest geotherms are for oceanic spreading centers,
where hot mantle material rises almost to the seafloor, to
replace colder material that is diverging due to plate
spreading. Sometimes, continental plates undergo partial
or complete rifting as a result of tectonic extension. This is
happening in the “Basin and Range” province of Arizona,
Nevada and Oregon now. Under these circumstances, the
geotherm may approach arc or even oceanic spreading
center temperatures. However, because old continental
rocks have in many cases already undergone
metamorphism, with loss of H2O and CO2, it is likely that
continental rifting does not cause important metamorphic
changes in the crust.
Why go into all this? In order to illustrate that
metamorphic rocks are mostly recrystallized in arcs. The
following figure shows a summary of the pressure and
temperature conditions recorded by metamorphic rocks
worldwide, from Prof. John Winter’s web site and
textbook.
The labeled blue and purple lines, and the grey fields, show
metamorphic conditions recorded by rocks from many
different places. On this diagram are superimposed the
geotherms we constructed and presented in the previous
figure. Here, we are only looking at the shallowest, coldest
parts of those curves. What you can see is that the great
majority of metamorphic rocks (purple lines, grey fields)
record such high temperatures at a given depth that they
must have been recrystallized beneath an arc.
In a few regions, there are belts of “high pressure, low
temperature” metamorphic rocks (HP, shown with blue
lines on the previous diagram), and even “ultra-high
pressure” rocks (UHP, lying at higher pressures than in the
previous diagram). These generally have very young
radiometric ages, and still contain many hydrous minerals,
so we know that they are not old, dehydrated rocks from
continental interiors. Instead, they are rocks that were
recrystallized during subduction, and that’s why they
record such low temperatures at high pressure.
If HP and UHP rocks began to subduct, how come they are
now exposed on the surface for geologists to sample and
study? It’s not certain, but scientists note that most of the
UHP rocks are low density (meta) sediments and blocks of
continental crust. Perhaps, they started to subduct,
presumably while attached to an old, cold oceanic plate.
But as they got hotter, their attachment to the old, dense
material grew weaker, and eventually they “floated”
buoyantly back to the surface. This scenario seems
particularly likely in continental collision zones, and indeed
UHP rocks are found close to the suture zones in the
Himalaya, as well as along more ancient suture zones in the
Alps and western Norway.
Thus, while metamorphic rocks record both high and low
temperatures at a given depth, they both were recrystallized
in subduction-related tectonic settings, arcs for the higher
temperature examples, and in the subducting plate itself for
the lower temperature rocks. This is reflected in the fact
that HP and UHP rocks are often found in “paired
metamorphic belts”, together with higher temperature
metamorphic rocks.
QuickTime™ and a
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The diagram above illustrates two famous examples of
paired metamorphic belts, from SW Japan associated with
ongoing subduction, and in California associated with
subduction that ended 50 million years ago. These paired
belts appear to reflect the juxtaposition of very low heat
flow and very high heat flow in the overthrust plate of
subduction zones, as illustrated in the previous lecture.
Continental accretion
Finally, we wish to briefly mention that the process of
collision and mountain building has probably played an
important role in forming the continental crust.
Collisional mountain belts are complicated, and often
include many small fragments of young arc crust and
metamorphosed, young sediments between larger, older
continental blocks.
Again, without going into detail, we use the diagram above
to illustrate this. In this schematic mountain range
(modeled on the southern Appalachians), there are several
“suture zones” representing ocean basins that were
consumed during collision. Between these sutures lie
small fragments of buoyant crust that did not subduct.
Some of these fragments are the remnants of arcs, others
are fragments of older continents, perhaps broken off by
rifting, and still others are metamorphosed piles of
sediments, similar to those that accumulate in deep sea
trenches today where rapid erosion and river transport leads
to landslides into the steep, deep trenches. The following
figure illustrates the tremendously complex geology of
western North America, in which each small block outlined
in black is a different fragment, whose geologic history
prior to collision with North America along subduction
zones was different from the history of all the surrounding
fragments.
Bit by bit, they are added to the margins of continents, and
eventually some become incorporated in the continental
interiors after big collision events.
The next two illustrations show how this kind of
accretionary tectonics has affected the eastern part of North
America.
In dark grey are fragments of the
African continent, plus
metamorphic and intrusive rocks
that formed during a collision
between Africa and North
America. These were left behind
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when Africa rifted away from
are needed to see this picture.
North America, about 150
million years ago in this region,
during formation of the modern
Atlantic Ocean. The light grey
rocks are sediments and arc rocks
that were added to the North
American margin during small
collision events, and then caught in the big collision
between Africa and North America. The resulting structure
is shown in this schematic cross-section:
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The pink rocks used to belong to the African plate. The
green rocks are ophiolites (remember them, fragments of
oceanic crust thrust onto continental margins?). The grey
rocks are metamorphosed rocks and sediments. And the
red, yellow and orange rocks are sediments that were
derived from the high mountains along the zone of
continental collision, and accumulated to the west on the
stable interior of the North American plate.
You have to go north to see the fragments of old African
crust, but the belt of metamorphosed arc rocks and
sediments continues through the New York area, and can
be seen as the Manhattan schist and Fordham gneiss in
outcrops throughout the area, most famously in Central
Park. If you go there – as some of you did during one of
the lab sessions in this course – you will see sediments
formed by submarine landslides, that were later
metamorphosed and folded during the collisional events.
This concludes the section on mantle convection and plate
tectonics. We started off with simple principles of
convection, and ended with the daunting complexity of
continental tectonics and growth. Some things are easy to
understand and well established, like the causes and
structures of thermal convection, the evidence for seafloor
spreading and subduction, the driving forces for plate
tectonics due to hot new oceanic crust at spreading centers
and cold, old subducting oceanic crust, the basic causes of
melting beneath mid-ocean ridges and arcs, the causes of
brittle subduction zone earthquakes, and the basic nature of
metamorphism.
Some things are less easy to understand, and are topics of
ongoing scientific research. Some of these include the
morphology of melt transport networks in the mantle, the
cause(s) of intermediate depth earthquakes, and the
fundamental principles that govern deformation during
continental tectonics. We hope some of you will contribute
to understanding these topics, or at least keep an educated
eye on the newspapers to understand future developments.