Download GEOL100 4-5-10 Historical Geology of North America

Introduction to Physical Geology Syllabus
GEOL100
4-5-10
Historical Geology of North America
Geologic provinces of North America
This section links extensively to the USGS Geologic Provinces of the United States web site. You should visit this
site and study it directly.
But first, two important definitions:
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Active margin: A continental margin that is adjacent to the nearest plate boundary. Active margins are
usually the scene of tectonic activity like peripheral orogenies or trnasform boundaries. E.G.: the North
American Pacific Northwest.
Passive margin: A continental margin that is far from nearest plate boundary. Passive margins are usually
tectonically quiet. E.G.: the North American Atlantic coast.
Atlantic Plain
province:
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Image
During the Jurassic,
the Atlantic ocean
began to form.
Initially, the crust near
the new divergent
boundary was hot, so it
stood out in high relief
like the shores of the
Red Sea today. As it
cooled, the continental
crust shrank and sank.
Sediments began to be
deposited on it. Today,
its bedrock is covered
with an thick apron of
nearly horizontal
sediments from Jurassic to recent in age. Topographic relief is low - the classic passive margin.
● Appalachian
highlands:
Image
During the
Carboniferous,
continental collisions
resulted in the creation
of Pangea. One major
suture zone was
marked by the
Himalaya-sized
TriassicTransPangean Mountains.
During the Jurassic
Pangea rifted apart
along the zone of
weakness that these
mountains represent.
Their western foothills
became the Appalachians. Their eastern foothills, the Atlas Mountains of northwestern Africa.
Ouachita-Ozark
Interior Highlands:
Same origin as the
Appalachian highlands
but farther south. In
this case, the
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continental collision
was with the part of
Gondwana that would
become South America.
Interior Plains:
Rocks of the ancient
craton of North
America, formed by
Pre-Phanerozoic
continental collisions
underlie this region.
These rocks were
mostly unaffected by
the Phanerozoic
collisions that formed
Pangea. From the
Paleozoic onward, this
region has received
sediment from the
various mountains
closer to the
continent's periphery
(Appalachians,
Ouachitas, and
Rockies) and has periodically been covered by shallow seas. The most recent of these occurred during the
Cretaceous.
● Laurentian Upland
Province: sometimes
called the Canadian
Shield.
Image
The oldest rocks of
North American craton
and absolute core of
the continent. These,
however, have been
widely exposed by the
action of continental
glaciers during the
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Quaternary.
Rocky Mountains:
Image
The Rocky Mountains
took shape during a
period of intense plate
tectonic activity that
formed much of the
rugged landscape of
the western United
States, starting with the
Cretaceous - Paleogene
Laramide orogeny,
about 70-40 million
years ago is
responsible for raising
the Rocky Mountains.
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Colorado plateau:
Image
Ancient PrePhanerozoic rocks,
exposed only in the
deepest canyons, make
up the basement.
Throughout the
Paleozoic Era, the
Colorado Plateau
region was periodically
inundated by tropical
seas. Beginning in the
Triassic (about 250
million years ago),
deposits of marine
sediment waned and
terrestrial deposits
dominated. These
include great accumulations of dune sand hardened to form sweeping arcs in cross-bedded sandstone. The
Colorado Plateau is remarkably stabile. Relatively little rock deformation has affected this high, thick crustal block
within the last 600 million years or so. In contrast, the plateau is surrounded by provinces that have suffered severe
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deformation.
Basin and Range
(aka Great Basin):
Image
Here, the Earth's crust
and upper mantle has
been stretched up to
100% of its original
width. Beginning in
the Paleogene, the
entire region was
subjected to extension
that thinned and
cracked the crust as it
was pulled apart,
creating long parallel
normal faults. Along
these roughly northsouth-trending faults,
fault blocks alternately
stood out or sunk down. Upstanding fault blocks were then eroded into parallel ranges of low mountains.
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Columbia Plateau:
Image
The Columbia Plateau
province is enveloped
by one of the worlds
largest flood basalts.
Over 500,000 square
kilometers. These
erupted between 17-6
million years ago
during the Neogene
Period. Most of the
lava flooded out in the
first 1.5 million years.
An extraordinarily
short time for such an
outpouring of molten
rock.
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Pacific Mountain
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System:
Image
The most geologically
young and tectonically
active region in North
America. The
generally rugged,
mountainous landscape
of this province
provides evidence of
ongoing mountainbuilding. To a large
extent, this province
consists of poorly
sutured microplate
terrains, each with its
own geologic history.
How did North America come to have these provinces? To answer this, we must make an excursion into the
discipline of historical geology. This lecture offers a glimpse of in through a synopsis of the history of North
America. In it, I will link extensively to two sets of paleogeographic maps:
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The Paleomaps Project by Chris Scotese
Paleogeography Through Geologic Time by Ron Blakely of Northern Arizona University
Starting in the Cambrian, the central craton of North America appears as the ancient continent of Laurentia. As we
move forward in history, pay attention to two themes:
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The evolution of the Appalachians through subduction, continental collisions, and rifting
The evolution of the Western Cordillera through subduction and the accretion of microplate terrains.
In the previous lecture, we introduced the topics of Rodinia and Pannotia, the supercontinents that preceded
Pangea. We therefore take up the story after the breakup of Pannotia in the latest Proterozoic. (650 m.a.)
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Late Proterozoic
In the latest Proterozoic, the central craton of North America, called Laurentia, was fully assembled. At this point
we don't call it "North America" because, strictly speaking, North American was formed by the rifting of Pangea,
which occurred later. The rocks of Laurentia, however, were destined eventually to become part of North America.
Cambrian - Passive margins
As Laurentia rifted away from Pannotia it assumed a positon near the equator. By the Cambrian Period, it was
surrounded on all sides by passive margins - continental margins that are far from any plate boundary.
Additionally, sea levels were historically high, so much of the continent was submerged as shallow sea.
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Ordovician - Taconic Orogeny
During the Middle Ordovician , however, Laurentia was passively carried toward an oceanic-oceanic convergent
boundary. Being a large continent, it couldn't be subducted. Instead, the subduction zone was shut down and its
associated volcanic arc was welded onto Laurentia's (modern) east coast in the Taconic Orogeny, the first of three
collisions that would create the Appalachians. For the next 250 million years, what's now the east coast would be
an active margins.
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Silurian - Devonian - Acadian Orogeny in the East, Antlers Orogeny in the West
By the middle Paleozoic, the east coast was an active margin, fringed by a new active subduction zone. During the
Silurian , a small continent called Avalonia rode the oceanic lithosphere into this subduction zone and in the early
Devonian , was welded to Laurentia as a microplate terrain. Farther north, the craton of Europe, called Baltica,
collided with Greenland (which was part of Laurentia.) Collectively, this constitutes the Acadian Orogeny, the
second in the Appalachian-building series. Thick deposits of sediment eroded from these mountains were
deposited to the west, in in low-lying flood plains and shallow seas where the Appalachians now stand.
Slightly later, in the Late Devonian, Laurentia-Baltica was carried into a subduction zone along its modern western
border and collided with the associated volcanic arc. Now the west coast was also an active margin. This event,
the Antler Orogeny, was like a western version of the Taconic Orogeny, and signaled the beginning of the slow
accretions of many microplate terraines to Laurentia's western margin.
Carboniferous - Alleghenian Orogeny
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During the Carboniferous, Laurentia-Baltica collided with Gondwana (a giant continent containing the makings of
Africa, South America, Antarctica, India, Australia, and New Zealand.) The portion of Gondwana destined to
become Africa struck Laurentia's eastern margin, while the portion destined to become South America struck the
southeast margin. Collectively, this is called the Alleghenian Orogeny. The huge mountain range created is called
the Trans-Pangean Mountains. The sediments that had eroded from Taconic and Acadian mountains and
deposited to the west were caught up in this new orogeny and extensively folded.
While this was happening, other continents, including Kazakhstan and Siberia, were colliding with Baltica and
with one another. The result was Pangea - the last supercontinent, which contained every major landmass except
the two continents destined to become North and South China. Pangea was hour-glass shaped with two major
regions:
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Laurasia in the north
Gondwana in the south
Early Mesozoic - Accretion and rifting
By the Late Triassic, Pangea was beginning to rift apart along the suture of the Trans-Pangean Mountains.
Remains of rift valley lakes are preserved in the sandstones of the Newark Supergroup. By the Early Jurassic
(right), actual sea-floor crust was forming between Laurasia and Gondwana. This rifting marks the birth of North
America, containing the former Laurentia at its center; and of the Atlantic Ocean. Ironically, the high mountains
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of the central Trans-Pangean Mountains (the region closest to the new ocean basin) now eroded away, leaving the
former foothills as sad remnants. These are the Appalachians, the Ouachitas of Oklahoma and Arkansas, and the
Atlas of Morocco and Mauretania. Florida, formerly a chunk of Gondwana, remained stuck to eastern North
America. From this time onward, eastern North America has been a passive margin.
In the west, an active margin was present, and Laurentia experienced a series of collisions with volcanic arcs and
small continents, as microplate terrains continued to accrete.
Late Mesozoic - Sea level highstand and the Laramide Orogeny
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Plate tectonic processes were running at full speed as the Atlantic continued to lengthen and widen. During periods
of vigorous plate tectonic activity, the ocean floors, being warmer than usual, actually stand up somewhat higher
topographically. This, in turn, pushes sea water onto the continents. The Cretaceous was such a time. In addition,
superplumes, unusually large and abundant mantle plumes, caused widespread volcanic eruptions in the West
Pacific. Finally, the Cretaceous was an ice free world. The net effect was that sea level stood at its highest since
the early Paleozoic. At this time, North America was bisected by a shallow Central Cretaceous Seaway. The
Proterozoic bedrocks of much of the continent today are hidden by thick limestones accumulated during this
interval.
Elsewhere in the world, Africa and India rifted away from Gondwana. Still, North America remained firmly
attached to Laurasia in the east and west.
Laramide Orogeny: In the west, two interesting processes were occurring:
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The west coast remained an active margin, with a subducting plate - the Farallon Plate - feeding a volcanicmagmatic range - the Sierran arc. (Today's Sierras are the uplifted roots of this Mesozoic range.)
Compression caused by this subduction resulted in the first uplift of the Rocky Mountains. This is weird,
because the rockies were a considerable distance inland from the western margin. Apparantly, the Farallon
plate subducted at an unusually shallow angle. The building of the Rockies is known as the Laramide
Orogeny.
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Cenozoic - Basin and Range extension
Finally, during the early Cenozoic, eastern North America rifted from what we now call Eurasia, opening the
gateway between the Altantic and Arctic oceans. In the west, the two continents remain connected.
Starting around 40 million years ago, the western edge of North America gradually encountered and overrode the
divergent boundary separating the Pacific Plate from the Farallon Plate. As this happened, its western margin
changed from a convergent to a transform boundary. Today's Juan de Fuca and Cocos Plates are last sad remnants
of the Farallon Plate. As a result of this transformation, large regions of western North America, which had been
compressed for over 100 m.y. by the shallowly subducting Farallon slab were suddenly released from compression
and "relaxed," widening to nearly twice their original width. This region is called the Basin and Range and the
widening event is the "Basin and Range extension." Its widening is marked by numerous parallel normal faults,
forming a series of parallel mountain ranges and valleys.
Note: The normal
fault-bound blocks
created in this way
are known as:
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Horsts: The
upthrown
blocks,
which are
weathered
into
mountain
ranges.
Grabens:
Downthrown
blocks
buried
beneath
valley
sediments.
In map view, the Basin and Range looks like a caterpillar drill team, with numerous parallel ranges oriented toward
the north northeast.
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Latest Neogene - Isthmus of Panama
A final finishing touch during the late Neogene was the evolution of the Isthmus of Panama, an island arc
coalescing to connect North and South America within the last five million years. This even had momentous
implications:
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Ocean circulation was drastically altered when tropical waters of the Atlantic and Pacific were separated.
The exchange of indigenous North and South American land animals, the Great American Interchange,
resulted in the extinction of many South American forms.
Key concepts and vocabulary:
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Continental margins
Passive margin
Active margin
Geologic provinces of North America
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Atlantic Plain province
Appalachian highlands
Ouachita-Ozark Interior highlands
Interior plains
Laurentian Upland province
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Rocky Mountains
Colorado Plateau
Basin and Range
Columbia Plateau
Pacific Mountain System
Historical Geology
Pannotia
Laurentia
Cambrian - time of passive margins for Laurentia
Ordovician - Taconic Orogeny in (future) east
Silurian - Devonian
❍ Acadian Orogeny in (future) east
❍ Avalonia
❍ Baltica
❍ Devonian - Antler Orogeny in (future) west - Active margins all around Laurentia
Carboniferous
❍ Alleghenian Orogeny in (future) east
❍ Pangea's hourglass shape - Laurasia in north, Gondwana in south
❍ Trans-pangean mountains
Early Mesozoic Era (Triassic and Early Jurassic)
Accretion of microplate terrains to future) wes
❍ Rifting of Pangea in east to form early North Atlantic
Late Mesozoic Era (Cretaceous)
Sea-level highstands, Central CretaceousSeaway of North America
❍ Laramide Orogeny yields Rocky Mts.
❍ Subduction of Farallon Plate
Early Cenozoic Era (Paleogene)
❍ North America rifts form Eurasia in east
❍ Basin and Range Extension
❍ Horsts and grabens
Late Cenozoic Era (Neogene)
❍ Isthmus of Panama
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