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Transcript
Chapter 5: Mountain Belts and Continental Crust
At this point in the course, you should appreciate that mountain building generally occurs at
convergent plate boundaries (subduction and collision zones), where compressional stress
predominates.
As with most topics in geology; however, the details can get somewhat complicated. In this
chapter, we’ll focus on various processes that occur in conjunction with mountain building.
Your text describes a mountain belt as a chain thousands of kilometers long composed of
numerous individual mountain ranges. Mountain belts are characterized by diverse rocks that
show evidence of intense deformation (folding, faulting, and metamorphism), vertical uplift, and
often a history of igneous activity.
Figure 5.2 in your text shows Earth’s major mountain belts. Here’s a map (below) showing the
world’s mountain belts in more detail. The light blue areas represent active mountain belts
(orogens):
Mountain Belts (Orogens) of the World (shown in light blue).
(image courtesy of Wikipedia; http://en.wikipedia.org/wiki/Orogeny)
You might notice that the Earth’s major mountain belts generally correspond with convergent
plate boundaries, both active an inactive. This is just what we’d expect, since plate
convergence is what causes most mountain building.
Isostasy
So far, we’ve mainly focused on horizontal motions of the Earth’s lithosphere, and we’ve seen
that such motions can be understood in terms of plate tectonics. In order to understand how
1
mountains are built, we also need to introduce an important process that accounts for much of
the vertical uplift that characterizes mountain belts: isostasy.
As defined in the glossary of your text, isostasy refers to the balance or equilibrium between
adjacent blocks of crust resting on a plastic mantle.
I prefer to think of isostasy as the buoyant equilibrium that exists between the lithosphere and
asthenosphere, such that lithospheric plates float within asthenospheric mantle at an elevation
that depends on both their thickness and density.
Isostasy is such an important concept that it’s appropriate to delve into it in more detail. In the
late 1800s, two different models of isostasy were developed, the so-called Pratt and Airy
models.
These two models are illustrated below:
elevation
3.0*
2.7
2.6
2.7
3.0
*density in g/cm3
sea level
2.7*
2.7
2.7
2.7
2.7
Base of crust
Airy Isostasy Model.
Pratt Isostasy Model.
In both the Pratt and Airy isostasy models (named after the folks who developed them), the
elevation of the Earth’s surface is explained in terms of crustal blocks that float atop a plastic
mantle.
In the Pratt model, different surface elevations are understood in terms of crustal blocks of
differing density. In this case, the blocks with lower densities float higher than those with higher
densities, in the same way that a block of styrofoam floats higher in a swimming pool than a
block of wood. In this model, the bottom of each block occurs at the same depth.
In the Airy model, different surface elevations are understood to be the result of the different
thicknesses of crustal blocks, with thicker crustal blocks sticking up higher and projecting
downward more deeply than thinner ones. In this model, the depth to the bottom of each block
differs from block to block.
So…which model is correct? Both models, it turns out, are accurate in some respects but not in
others.
For example, the Pratt model explains in a very general way why the continents stick up higher
above sea level than the ocean basins. Since continental crust is less dense than oceanic
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crust, we’d expect it to ride higher atop the mantle than oceanic crust. This is just what the Pratt
model of isostasy predicts. In other words, the continents are high and dry not just because
they’re above sea level. Rather, the continents are elevated because they’re composed of rock
types (e.g., granitic and metamorphic rock) that are more buoyant (less dense) than oceanic
crust, which is mainly basaltic.
But wait…in the Pratt model, notice that the base of the crust is the same everywhere. Seismic
investigations later revealed that this just isn’t the case. In general, the base of the Earth’s crust
beneath the continents (i.e., the Moho) occurs at a much greater depth than beneath the ocean
basins.
In the Airy model of isostasy, the depth to the crust-mantle boundary (Moho) varies, however,
this model assumes that the densities of crustal rocks are everywhere about the same, which
just isn’t the case. The Airy model predicts that regions of thickened continental crust should be
associated with higher elevations, which generally is the case. The really important prediction of
the Airy model is that Earth’s mountain belts should possess crustal roots (i.e., thicker crust that
projects downward into the mantle like the root of a tooth). Seismic data confirm that the world’s
major mountain belts such as the Himalaya are indeed underlain by thicker crust.
By the way, an easy way to remember the basic difference between the Pratt and Airy models is
the phrase, “Pratt is flat but Airy varies.”
Currently, the preferred model of isostasy applies to the lithosphere and asthenosphere and
combines aspects of both Pratt and Airy. In essence, we now recognize that continental
lithosphere rides higher atop the mantle than oceanic lithosphere because it’s both thicker and
less dense (Pratt); also, the depth to the base of the crust and lithosphere varies from place to
place, and many of Earth’s mountain belts are underlain by lithospheric roots (Airy).
An historical note:
You might remember from the chapter on plate tectonics that Alfred Wegener’s theory of continental drift
wasn’t widely accepted by his contemporaries, particularly geologists in North America. One reason for
this may have been that North American geologists in the 1920s and 1930s were reluctant to abandon
their oversimplified understanding of isostasy.
As U.C. San Diego scholar Naomi Oreskes discusses in her book, The Rejection of Continental Drift:
Theory and Method in American Earth Science (Oreskes 1999), the Pratt model of isostasy was
generally preferred by geologists in North America over the Airy model because it was easier to work
with. As mentioned above, a prediction of the Pratt model is that the depth to the base of Earth’s crust
should everywhere be the same; however, Wegener argued that the leading edges of the continents
were thickened as they drifted across the globe. Wegener’s critics in turn argued (quite correctly) that
thickened continental edges would imply a variable depth to the base of the crust—a conclusion clearly
at odds with Pratt isostasy. The result is that Wegener’s critics rejected his ideas in part because they
were incompatible with Pratt isostasy. In effect, they rejected Wegner’s ideas rather than rethink their
commitment to Pratt isostasy.
In the end, Wegener’s prediction that drifting continents become thickened at their leading edges has
been upheld (although for different reasons that Wegener suggested), whereas the Pratt model of
isostasy has been falsified. Score one for our hero, Alfred Wegener .
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Large vertical motions of the lithosphere occur when the lithosphere is out of isostatic balance
with the underlying asthenosphere.
In general, mountain belts develop in response to lithospheric thickening. Such thickened
lithosphere creates an isostatic imbalance. In response, the lithosphere within a mountain belt
typically rises to counteract this imbalance (Figure 5.15).
The lithosphere can also move vertically downward if loaded from above. For example, when
thousands of meters of ice accumulate on a continent during an ice age, the lithosphere of the
continent sinks in response. When the ice age ends and the glacial ice melts, the lithosphere
rises in response. Here’s a simple animation of Isostasy—Figure 5.14.
Although it may seem counterintuitive, erosion also leads to isostatic rise. As a mountain belt
erodes, the lithosphere rises in response due to unloading, although usually not up to its former
elevation, as shown here:
Erosion
(triggers isostatic rise)
Deposition
(triggers isostatic subsidence)
Deposition
(triggers isostatic subsidence)
LITHOSPHERE
ASTHENOSPHERE
Erosion, Deposition, and Isostasy.
Let’s consider some specific plate tectonic settings where mountain belts develop:
Ocean-Continent Convergence (plate subduction)—marginal, “Andean-type” mountain
belts
Figure 5.6 shows a geologic cross section through a typical “Andean-type” mountain belt.
Here’s a simplified sketch (next page):
4
Isostatic rise due to
magmatic
underplating + erosion
Accretionary
Wedge
Trench
Sea Surface
Volcanic/Magmatic
Arc
Fold / Thrust Belt
Forearc Basin
Rising magma
Crust and lithosphere
thickened underneath via
magmatic underplating
Isostatic subsidence
due to sediment
loading within forearc
basin
Simplified cross section through an Andean-type mountain belt.
Next to the trench, we find a complexly folded and faulted region called the accretionary wedge,
where seafloor rocks from the subducting plate are being scrapped off and plastered onto the
non-subducting plate.
Thrust and reverse faulting along with intense shearing characterize accretionary wedge rocks.
As you might imagine, the rocks within an accretionary wedge are highly deformed and mixed
up—they’ve been through a lot! In fact, a rock called “mélange” is commonly found in
accretionary wedges. Mélange consists of a complex mixture of various rock fragments jumbled
together in a matrix of sheared, fine-grained mudrock.
Here’s a photo of mélange from an ancient subduction zone in the Appalachian Mountains.
Notice the large rock fragments surrounded by fine-grained mudrock. Notice, too, how sheared
and broken up this stuff is. Think of it as tectonic gobbledygook!
Here’s another photo of some mélange from the Franciscan subduction complex, near San
Francisco, CA:
5
Melange with Chert Clasts, Marin Headlands, CA.
Image courtesy of Bruce Molina, United States Geological Survey
Image source: Earth Science World Image Bank
(http://www.earthscienceworld.org/images)
Moving landward from the accretionary wedge, we encounter an elevated region called the
volcanic / magmatic arc. This is where igneous activity is most intense. Intrusive igneous rocks
are being emplaced at depth, whereas volcanic rocks of generally intermediate composition are
erupting at the surface, creating large stratovolcanoes.
The lithosphere is thickest beneath the volcanic / magmatic arc due to addition of vast amounts
of hot, buoyant magmatic material to the underside of the arc as this material rises from near the
subducting plate, in effect, thickening the lithosphere from below. Your textbook authors refer to
this process as magmatic underplating.
Isostatic rise associated with the thickened lithosphere of the magmatic arc creates tall
mountains (Figure 5.6). Here’s a photo (below) of the mighty Sierra Nevada, a northwest
trending mountain range in eastern California. Most of the rocks in the Sierra are granitic rocks
that formed within a Mesozoic-age volcanic / magmatic arc east of an ancient subduction zone
that once existed just offshore of central and southern California:
Sierra Nevada Mountains.
(photo courtesy of Wikipedia; http://en.wikipedia.org/wiki/Sierra_Nevada_(U.S.)
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In between the accretionary wedge and the magmatic arc is the forearc basin, which receives
vast amounts of sediment eroded from the magmatic arc and sometimes also the accretionary
wedge.
Sedimentary rocks within the forearc basin are much less deformed than those in the
accretionary wedge.
As more and more sediments are added to the forearc basin, isostatic subsidence occurs,
making room for still more sediments (Figure 5.6). Thus, the forearc basin sinks isostatically
(due to loading from above), whereas the magmatic arc rises (due to thickening from
underneath).
So, to summarize, Andean-style mountain belts are characterized by a deep trench, a complexly
folded and faulted accretionary wedge adjacent to the trench, a forearc basin inboard from the
trench, and a volcanic / magmatic arc inboard from the forearc basin.
In some cases, a fold and thrust belt develops landward of the arc, where layered rocks are
horizontally shortened, stacked on top of each another like shingles, and thrust toward the
continent in response to compressional stress associated with ocean-continent plate
convergence (Figure 5.6).
A good example of a subduction-related fold and thrust belt is the northern Rocky Mountains,
where gigantic stacks of sedimentary rock have been thrust eastward where the Earth’s crust
has been crunched together like a bunch of shingles or dominoes.
The origin of the Rocky Mountains has been a perplexing problem for geologists because the
Rockies are so far inland from the Andean-type subduction zone that persisted along the west
coast of North America throughout Mesozoic and into Cenozoic time. In a typical subduction
zone, the subducting plate slides beneath the non-subducting plate at a fairly steep angle;
however, in late Mesozoic time, the angle of the subducting plate may have been significantly
flattened, shifting the focus of mountain building processes (e.g., magmatism, compressional
faulting) far inland.
The geology of central California fits the Andean-style subduction model quite well.
Along the central California coast, the Coast Ranges, including the Diablo Range east of San
Francisco and the Santa Cruz Mountains, consist of an intensely deformed mixture of various
rock types of mostly oceanic affinity known collectively as the Franciscan Complex.
With the development of plate tectonic theory, geologists recognized that Franciscan rocks
correspond to Mesozoic-age accretionary wedge deposits.
Moving east, California’s Central Valley region is underlain by thick sequences of relatively
undeformed marine and non-marine sedimentary rocks that accumulated within a Mesozoic
forearc basin.
Finally, the Sierra Nevada mountains east of the Central Valley represent the remains of a
Mesozoic volcanic / magmatic arc. Known collectively as the Sierra Nevada batholith, Sierran
7
granitic rocks are the cooled, crystallized remains of ancient magma chambers that once fed an
active volcanic arc.
Here’s a sketch to help you visualize these relationships:
Volcanic / Magmatic Arc
(Sierra Nevada Batholith)
Accretionary Wedge
(Franciscan Complex)
Forearc Basin
(Great Valley Sequence)
California’s geomorphic provinces.
Image courtesy of California Geologic Survey
http://www.conservation.ca.gov/cgs/geotour/Pages/Index.aspx
Before plate tectonic theory came along, geologists had a hard time making sense of
Franciscan rocks in particular. How could such a complex, intensely deformed group of rocks
have developed? With the advent of the plate tectonic model, the complexity of Franciscan
rocks began to make sense. In effect, Franciscan rocks represent material scraped off an
ancient, now-subducted oceanic plate and plastered onto the edge of the North American plate.
8
Arc-Continent Convergence (terrane accretion)
Imagine an old island arc, embedded in an oceanic plate, moving toward a subduction zone
(Figure 5.11A).
When the island arc reaches the subduction zone, it can’t subduct due to its high buoyancy, and
it eventually clogs the subduction zone.
With continued compression, a new subduction zone may be initiated seaward of the arc, and
the arc eventually becomes plastered onto the edge of the continent (Figures 5.11B and C).
When this happens, the island arc is effectively welded onto the edge of the non-subducting
plate, causing the plate to grow laterally—a process called terrane accretion.
Here’s an interesting animation of the accretion process.
A terrane is a fault-bounded package of rocks that have a different geologic history than the
rocks in adjoining regions. Notice the variant spelling. A terrane is an accreted, tectonic sliver,
whereas the word, terrain, simply refers to the character of the land (e.g., hilly vs. flat terrain).
Island arcs can become terranes. So can sea floor rocks, accretionary wedge rocks, and even
entire microcontinents. If a terrane originates from far away, it’s often referred to as exotic by
geologists to remind us that the rocks we’re looking at came from someplace else.
Much of the land in North America west of the Rocky Mountains, including most of Alaska, is
thought to be composed of exotic terranes that have been accreted to the North American plate
since Mesozoic time.
Both ocean-continent convergence and arc-continent convergence (terrane accretion) create
mountain belts near plate edges.
Speaking of accreted terranes, here’s a photo of North America’s tallest peak, Mt. McKinley. I
shot this from a small plane on a visit to Alaska several summers ago:
20,320 ft
Mt. McKinley (Denali).
9
Mt. McKinley (or Denali, as the Athabascans call it) is part of the Alaska Range and consists of
56 million year-old granitic rock that was intruded into older, exotic island arc terranes during
their accretion onto the North American plate.
Denali is still rising at about 1 mm/year, pushed up by compressional tectonics and isostatic rise
associated with nearby subduction of the Pacific plate beneath Alaska.
Continent-Continent Convergence (plate collision)—interior, “Himalayan-type” mountain
belts
Mountain belts found within plate interiors as opposed to plate edges are thought to be the
result of continent-continent convergence, where two continental plates collided and welded
together to become one.
The Alps, Urals, Appalachians, and Himalayas are all examples of mountain belts formed by
plate collision. In such settings, compressional stress and associated horizontal shortening can
be intense, as shown in Figure 5.13, a cross section through the Alps. Like subduction zones,
the lithosphere at collision zones becomes thickened, but in a different way than at subduction
zones. Instead of becoming thickened from underneath by the addition of hot, buoyant magma,
lithospheric thickening at collision zones occurs primarily by horizontal shortening, folding, and
stacking of thrust sheets, one atop another (Figure 5.13). As the lithosphere thickens, its
isostatic balance is upset, and isostatic uplift occurs as a result.
The lithosphere beneath the Himalayas is approximately 200 km thick, about as thick as
anywhere on Earth, where the Indo-Australian plate is attempting (unsuccessfully) to subduct
beneath the southern portion of the Eurasian plate, effectively doubling up lithospheric thickness
in this region.
Here’s a simple sketch of a typical collision zone (“Himalayan-type” mountain belt):
Suture Zone (plate boundary)
Horizontal shortening and
compressional faulting
Young mountain belt
(e.g., Himalayas, Alps)
Continental lithosphere
Continental lithosphere
Cross section of a collision zone.
(continent-continent plate convergence)
10
In the above diagram, the lithosphere is thickened beneath the mountain belt as a result of
accordion-style, horizontal shortening. At shallow depths within the crust, where the rocks are
cool and brittle, thrust and reverse faulting predominate. At greater depths, where the rocks are
warmer and more plastic, regional metamorphism and folding occur.
Post-Orogenic Uplift and Block-Faulting
You can just skim this section of Chapter 5. I won’t hold you responsible for this material.
Continental Growth
This last section of Chapter 5 discusses how continents evolve. As your text states, “Continents
grow bigger as mountain belts evolve along their margins.”
Hopefully by now you can appreciate how amazing this simple statement really is. Not only can
plate tectonic processes like subduction, terrane accretion, and collision explain the growth and
development of mountain belts, such processes can explain how the continents themselves
grow!
Nowhere is this better illustrated than along western North America, where dozens of large and
small, fault-bounded slivers of continental and oceanic crust (terranes) have been mapped in
recent years. In effect, much of North America can be thought of as a collage of accreted, exotic
terranes.
San Diego itself is built atop a 100-million year old exotic island arc terrane. The exposed
granitic rocks in the greater San Diego area formed in an ancient island arc that once lay
perhaps thousands of kilometers offshore. Eventually, these rocks were accreted onto the
leading edge of the North American plate. There’s a very famous fault zone (famous among
geologists, anyway ), the Cuyamaca-Laguna Mountain Shear Zone, just east of I-8 near
Sunrise Highway, that marks the eastern boundary of this terrane.
What is true of western North America is probably also true, to some extent, of most other
continents as well: the continents grow larger by terrane accretion as geologic time progresses.
Eventually, they collide, creating supercontinents like Pangea. Following this, the
supercontinents break up to repeat the cycle all over again.
Here’s a link to a fascinating New York Times article where you can scroll through various plate
configurations from 200 million years ago to 250 million years in the future, when the next
supercontinent, dubbed Pangea Ultima, may form…Wow!
References Cited:
Oreskes, Naomi. 1999. The Rejection of Continental Drift: Theory and Method in American Earth
Science. Oxford University Press.
11