Download Paleogeography, Southwestern US

Het ontstaan van Noord-Amerika
[Image: The present tectonic structure of North America, mapped by Ron Blakey, Professor
of Geology, Northern Arizona University].
While out in California last month, hiking through Death Valley – on a cloudy day it looked
like a painting by Caspar David Friedrich: an earth of snow, not salt flats – I read that
California is actually a welded-together mass of remnant archipelagos and former island arcs.
In other words, down there in the Californian gravel are the buried edges of old island chains
– and the whole state is still shivering with collision, making adjustments, popping loose and
sticking, always on the move... then stopping.
Much is made of the apparent poetry of driving over the San Andreas Fault, which divides
California into a left-half and a right-half, a coastal zone and a continental shield; but what of
the silent lines you walk across everyday, from one former island to the next, unaware that
those lands had even once been separated?
Midway through the trip, I picked up a copy of John McPhee's Assembling California, in
which he describes residual structures of ancient geology with "no known bottom" because
they cut so deep. He talks about "the metamorphosed remains of what had once been an island
arc," and how, through constant collision and restlessness, entire "Newfoundlands,
Madagascars, New Zealands, Sumatras, [and] Japans" have all jostled together, ramming one
into the other, year after year, piling up, forming "the outermost laminations of new
landscapes."
This is "the docking of arc and continent," a mismatched mess of rock "now consolidated as
California."
"California," then, is just the temporary shape taken by these lost islands and unknotted
seafloors.
Of course, then I found these unbelievable maps by Ron Blakey last week and I almost passed
out. Utterly ingenious, each map represents "the paleogeography of North America over the
last 550 million years of geologic history." You can actually watch as California comes home
to collide.
[Image: The southwestern coastal archipelagos of North America, 310 million years ago; map
by Ron Blakey].
But let's pull back and start 420 million years ago (bypassing some 130 million years' worth
of Blakey's maps).
This is North America, a tropical archipelago, covered in surreal vegetation, blowing seeds
across itself – Shelley's "thousand nameless plants sown by the wandering winds" – crosspollinating, hybridizing, surrounded by shallow seas, chugging northward over the equator.
75 million years later (below), what will eventually be North America has broken into pieces,
partially flooded, surrounded by clusters of islands. These scattered subcontinents are about to
collide with the north by northwestern edge of Africa – and unbelievable arcs, inlets, atolls,
and bays all stretch across the landscape.
What was it like to live in that geography? What sounds did the forests make? What did the
stars look like? What half-legged fish swam through those waters?
Another 50 million years pass, and the collision with Africa is well underway. Deserts are
forming in the American southwest. Global wind patterns shift due to the distribution of
landmasses.
Weird species that only live for a million years, and leave no fossil record, run unimpeded
across giant landscapes of exposed bedrock.
Then the Atlantic rift begins; the Appalachian Mountains, which passed through Morocco, are
now split onto three continents: North America, north Africa, and the embryonic British Isles,
which drift northward, cultivating Albionic energy in ancestral swells of warm sea. William
Blake will be born there – and Shelley, who'll write of a time "when St. Paul's and
Westminster Abbey shall stand, shapeless and nameless ruins, in the midst of an unpeopled
marsh."
150 million years ago. California is already starting to form: it's a distant horizon of islands in
the Pacific. Those mountains of rock above water are coming in toward the mainland, slowly,
lurching forward on broken faults, a conveyor belt moving masses east to raise hills in a ring
around Los Angeles, propelling the Sierra Nevadas upward, walling-in the great Arizonan
desert now temporarily beneath the sea.
Could you map today's California according to the island chains it used to be?
And look at Mexico: it's a weird spit of land, hooked and crooked through the oceans,
collecting islands onto itself. What would it have been like, walking through those coastal
mountains? And if you carved an entire island to look like the cathedral of Notre-Dame, what
would it look like, breaking waves, coming toward you over ten million years, eventually
colliding with the cliffs you stand on?
What's amazing in the above two images is that, in only 40 million years, an entire system of
archipelagos has rammed into the mainland, assembling what will eventually become known
as California, tipping the Rockies, torquing belts of metal into metamorphosed ribbons today
exposed by roadcuts.
What must that time period have been like? With thousands of small islands – a whole
Indonesia – off the coast, groaning at night with tectonic pressure, shattering from strain,
causing landslides, you could have boated from bay to bay, mapping species, collecting rocks
– knowing that beneath your feet is what will someday be Bakersfield, Santa Barbara, Death
Valley.
But now, in the map below, look at Mexico again: it's a broken ridge of almost-islands,
cooking in the Cretaceous sun. The Yucatan is an island. The whole Pacific coast is a
weather-beaten cliff of caves and pockets.
This was 65 million years ago.
Now the North American inland sea has sealed up, and a riverine bay several orders of
magnitude larger than the Mississippi or the Saint Lawrence flows southeast across Texas.
Then it, too, is gone (below), leaving behind the massive fossil reefs of the Guadalupe
Mountains National Park. There is no Florida yet; Alaska and Russia are one; the Caribbean is
a malarial bog of proto-islands, choked with seaweed, overrun with electric eels and tide
pools.
Somewhere humans are chipping flints together and hallucinating spaceships.
Till North America hits its Ice Age. Thousands of acres of frozen arches stand whistling in
frigid winds above the future site of Washington DC. Humans huddle in caves – then move
south, populating the New Mexican desert. Global sealevels drop, revealing caverns in the
coasts of every continent.
The magnetosphere howls in the freezing air.
Then it's today. New oceans continue to form. Major geological events continue to happen.
Someday the coast of California will drag along the edge of Alaska, depositing pieces of
Culver City in the front range.
Eventually the planet will melt, cities forming rivers of liquid stone.
It's interesting in this context to note that, if the Antarctic ice cap melts, Antarctica itself may
rise: "The continental shelf of the South Polar land lies four times lower than normal," New
Scientist reported, "suggesting that if the ice (more than a mile thick below sea level at some
points) were removed, the continental surface would rise." This is called post-glacial
rebound.
Fascinatingly, Antarctica's "present mountains would attain considerable heights and
introduce new frictional opposition to prevailing winds, so new weather patterns would be
created."
In any case, every one of those maps above suggests about six hundred novels or short stories
just waiting to be written; but the most exciting part of all of it is that we are already living in
that world, of altered weather and island arcs, abraded coasts and rivers. It's what we're doing
right now.
Beneath the house I write this in are seams of lost continents. Outside the place you read this
in are moving unmapped geographies yet to come.
(Note: All maps in this post are by Ron Blakey, Professor of Geology, Northern Arizona
University – perhaps, if enough people ask, we can get him to map North America as it will
someday be).
http://jan.ucc.nau.edu/~rcb7/RCB.html
Paleogeography of the Southwestern US
Geologic History of Western US: Text
To return to this page, click 'Return to TEXT' at bottom of each
paleogeographic page. You can also advance or go back in time by clicking the
appropriate link without returning to this page.
Precambrian
The interpretation of the earliest geologic history of Southwestern North
America is very speculative. At approximately 1.7 Ga (1700 Ma), the region
probably resembled modern SE Asia. As various tectonic blocks collided,
orogenic (mountain-building) events were generated. This view shows what
the region may have looked like during these collisional events.
About 1.1 Ga North America was part of the supercontinent Rodinia. The
Southwest possibly lay adjacent to Australia and/or Antarctica or possibly
Siberia. An arc complex lay off southern North America. Various sedimentary
rocks were formed near the margins of North America.
The western margin of North America (NAM) was originally formed during
the late Proterozoic (Precambrian), about 650 million years ago, during a
major rifting event in which North America separated from the
supercontinent Rodinia. The map shows the hypothetical pattern of rifting and
the resulting patterns of late Proterozoic siliciclastic sedimentation. The rifted,
attenuated (thinned and stretched) margin subsided and was the site of thick
sequences of sandstone, mudstone, and limestone. The Cordilleran passive
margin was born.
Cambrian
The hingeline marks the sharp, linear zone that separates thin sedimentary
rocks of the cratonic interior from westward-thickening sedimentary rocks
deposited on the more rapidly subsiding passive margin miogeocline. The
hypothetical rifted microcontinent fragments apparently lingered off SW
North America during much of the Paleozoic and were accreted to the
southwest margin to form parts of Mexico and California in the Mesozoic.
From the Late Cambrian through the Devonian, the region was dominated by
carbonate deposits across the craton and miogeocline. Slope-rise and adjacent
oceanic environments were the site of very fine-grained terrigenous mud and
carbonate ooze deposition.
Ordovician
Cambrian depositional and tectonic patterns continued .
Silurian
Along the margin of North America, the above patterns continued. However, a
volcanic arc approached the continent from the west.
Devonian
Sometime during the middle Paleozoic, a subduction zone developed off the
coast of the western US. Some debate exists as to the orientation of this
subduction; it is shown as initially dipping under the approaching Antler arc
in this interpretation. The distance between the arc and NAM decreased
through the Devonian. The hypothetical offshore blocks, if present, would
have been the first elements of North America to collide with the approaching
arc. During Late Devonian the encroaching arc began to effect and collide
with the passive margin of western NAM.
Mississippian
As the arc approached, and then collided with NAM, neither the arc nor the
continent could be subducted. The collision likely temporarily stopped the
subduction process. Most probably, a new subduction complex developed
offshore. In the interpretation shown here, the arc fragmented into several
pieces, some with east-dipping subduction zones (labeled as the reversal of arc
polarity on the west map) and others with west-dipping subduction zones (the
part of the arc that collided directly with North America).
The arc collision produced the Antler orogeny. The extent of this initial
orogenic event of western NAM is unknown because both the north and south
margins have been removed or obliterated by younger tectonic processes. The
orogeny developed in several phases:
 1. As the forearc of the arc and distal passive margin of NAM converged,
folding occurred.
 2. The forearc and parts of the distal passive margin were then thrust
eastward over the miogeocline. This produced the Roberts Mountain thrust.
 3. The thrust belt added weight to the already subsiding miogeocline causing
abrupt, rapid subsidence -- the Antler foreland basin. In response to this, a
broad, gentle forebulge (slight uplift) occurred east of the Four Corners region
on the craton.
 4. The foreland basin was filled with reworked chert gravels (from the
uplifted cherty sediments) at its western margin, but most of the basin was the
site of dark-colored, deep water mud deposition.
 5. During thrusting and initial collision and accretion with NAM, the arc
and associated thrust belt formed low highlands. However, when subduction
was turned off or reversed and compression ceased, the uplift became cooler
and subsided along with the general pattern of passive-margin subsidence.
Pennsylvanian
By Middle Pennsylvanian, the Antler orogenic belt was buried under a
widespread overlap assemblage. Subduction was renewed an unknown
distance off the west edge of the continent. This interpretation shows the
Stikine fragment being transported northwest along a transform fault. This
same fault may have been resposible for the late Paleozoic truncation of SW
NAM. Northward, the Quesnell arc began its general approach. Meanwhile,
the supercontinent Pangea was being assembled. Europe, Africa, and South
America collided with eastern and southern NAM to form the Allegheny
(Appalachian) orogeny. The southwest margin of that belt, the OuachitaMarathon mountains, lay just to the SE margin of the map. The extreme and
complicated collisions caused parts of cratonic North America to fail by
faulting and the Ancestral Rockies orogenic belt was formed.
Note the complex relations of sedimentation to tectonics. Sand and gravel were
deposited adjacent to the mountains while lime formed in clear, shallow seas.
Eolian dune deposits and evaporites confirm an arid climate.
Permian
Pennsylvanian tectonic and sedimentation patterns continue. The Quesnell arc
(Sonomia terrain) is closing on western NAM. Eastward-dipping subduction
developed along the Gondwana western margin (south of California).
Microcontinents accreted to this margin, especially during the ensuing
Mesozoic. Left-lateral shearing and truncation of the southwest craton may
have been active at this time. At the close of the Permian, various arc collisions
(there is debate as to how many were involved) formed the Sonoman orogeny.
The various accreted terrains are collectively known as Sonomia.
A second interpretaion is also shown. In this model, the McCloud arc is a
single arc, probably related to North America and possibly a rejuvenation of
the older Antler arc. Its collapse (by telescoping the back-arc basin between
the arc and NAM) created the Sonoman orogeny. The Stikine arc is well off
shore and is interpreted as exotic to NAM, possibly originating in the Tethys
Ocean near China.
Triassic
Note that the collision of Sonomia was very similar to that of the older Antler
event. The McCloud arc is part of the Sonomia terrain. It contains blocks and
fragments of late Paleozoic limestone that contains fossils more closely related
to Japan and China than NAM. This suggests that part or all of the arc
originated far from NAM. The two somewhat different interpretations shown
here explain this fact in different ways.
Sonomia (or McCloud arc) was fused to NAM. The back-arc basin was thrust
eastward over the western margin of NAM. This is the classic Sonoman
orogeny. The Cache Creek interarc basin was trapped between the two
elements of the McCloud arc. The trench-forearc along western NAM
developed into an accretionary wedge prism.
By Late Triassic, the new, west-facing Cordilleran arc was now well developed
along most of SW NAM. A backarc basin developed between the arc and the
craton over the site of the older Antler and Sonoman orogenies. Remnants of
the Ouachita systems fed northwest-flowing streams of the Upper Triassic
Chinle Formation.
The "exotic interpretation" requires only a single McCloud arc. The basic
mechanics of the Sonoman orogeny, at least along the margin of NAM, are
similar to the interpretation shown above; however, a new exotic arc, Stikinia,
approaches from the west. The dip of the subduction zone under the exotic arc
is westward. Therefore, an ocean basin between the Stikine and NAM
subduction zones is subducted under both; as this occurs, the ocean basin
shrinks and Stikinia moves towards NAM.
Jurassic
A major shift in plate motions resulted in highly oblique convergence between
the ancestral Pacific plate and NAM. The active arc trended across the entire
region and part was built on the craton in Arizona. The oblique intersection
resulted in complex compressional, tensional, and transtensional forces. A
huge dune field (Navajo Sandstone and related units) developed behind the
arc. In Arizona, California, and Nevada, dune deposits are interbedded with
arc volcanics.
Complex margins like the Mesozoic/Cenozoic Cordilleran margin of the
western US can have arcs build on a variety of geologic materials including
oceanic crust, continental crust, including the craton, and various accretionary
materials such as older arcs, oceanic plateaus, and accretionary wedges. All of
the above were sites of arc building at one time or the other along the
Cordilleran margin of the western US.
Areas of extension resulted in rifts. Some of these along SW NAM are related
to the opening of the Gulf of Mexico. Continued subduction caused broad,
plateau uplift in Nevada. A large oceanic plateau, Wrangellia (or Insular
superterrane) came into contact with SW NAM. Fossils and paleomagnetic
patterns suggest that it traveled from far to the southwest before encountering
NAM. Lateral offset displaced major pieces of previously accreted terrane
including parts of Wrangellia. Several pieces collided violently in the Pacific
northwest. Thrusting and uplift began in Nevada as the Nevadan orogeny
initiated.
Volcanic arc activity began to subside, perhaps in response to subduction
zones jumping westward past newly accreted terrane. Continuing offset of
extreme SW NAM formed the Mojave-Sonoran megashear. Rifting continued
inboard of the megashear.
By Late Jurassic, the Nevadan orogeny was fully active. Terrane suturing,
thrusting, lateral offset, and rifting occurred simultaneously in different
places. NAM abruptly changed to northwestward drift. The main arc was
temporarily shut down. Broad basins formed in the forearc region. Major
rivers drained northeastward across the Western Interior and deposited the
Morrison Formation.
During the Jurassic, the differences between the "exotoc" model and the one
presented above are very obvious. Northern California and Oregon contain a
number of different ophiolites. Ophiolites are fragments of ancient oceans
later trapped in continental crusts. Most ophiolites represent suture zones
between collided continental blocks or between collided arcs and continents.
The first model presented above mainly uses transform processes and interarc
basins to form the ophiolites. Although some exotic material is accreted, most
of the terrains were once part of NAM. Lateral transform movement
transposed the blocks and trapped ophiolites as slivers between them. Blocks
are doubled up and then collide trapping the ophiolites between them. The
modern Adaman Sea south of Burma may be an example; the sea is along a
transform margin created by oblique collision between the Indian Plate and
western SE Asia. No exotic continental block is involved in the tectonic
sequences.
The more exotic models create ophiolites in back arc basins, mostly associated
with exotic arcs that approach western NAM from far to the west. Back arc
basins are commonly formed on fast-moving oceanic plates when the arc
moves faster than the plate as a whole and "pulls away" creating a back arc
basin. The numerous arcs west, north, and east of New Guinea are modern
examples of this model. Many back arc and interarc basins dot the region.
When (and if) these back arc basins are trapped in collisions between New
Guinea-Australia and another continent, numerous ophiolites will undoubetly
be formed. Note the various back arc basins and collisional epidodes that
occur on the exotic model for the Middle and Late Jurassic along western
NAM. Each time an arc pulls away to form a back arc basin, another potential
ophiolite setting is created.
Both models consider Wrangellia exotic, although considerable debate exists
as to when and where Wrangellia first collided with western NAM. There is
also considerable debate as to whether the Wrangellia collision was related to
major orogeny. The collision has been related to the Nevadan (Late Jurassic),
Sevier (Cretaceous), and Laramide (Early Tertiary) orogenies; perhaps it's
not closely related to any of them.
Cretaceous
The NAM and paleo-Pacific plates collided nearly head-on. The arc was
rebuilt. Continued rapid subduction built a major arc complex. Thrusting and
uplift signify the beginning of the Sevier orogeny. At times, the Western
Interior seaway extended from the Gulf of Mexico to the Arctic region.
Streams spawned by the Sevier uplift deposited terrigenous sediment into the
seaway.
Extended periods of rapid subduction led to intense volcanism and intrusion of
major batholithic complexes. The arc also shifted somewhat eastward and the
paleo-Pacific plate moved northeastward, oblique to the movement of NAM.
These events produced the culmination of the Sevier orogeny. Right-lateral
slip inboard of the subduction zone resulted in major northward displacement
of previously accreted terranes and accretionary wedge complexes.
Tertiary
Although some Cretaceous trends continued into the Tertiary, major changes,
especially in arc tectonics and in locations of arc elements, occured across the
region. The following maps show only broad tectonic and sedimentary
patterns. The first two maps illustrate the Early Tertiary. The first (Eocene)
shows the highlights of the Laramide orogeny. The second (Oligocene)
illustrates the great expanse of Tertiary volcanism that followed the uplift of
the Rocky Mountain Region. Subduction ceased across much of SW NAM.
Two hypotheses have been evoked to explain these events: 1)The Baja British
Columbia hypothesis emphasizes extreme right lateral juxtaposition of
terranes; these terrains either jammed up the subduction zone or forced it to
migrate elsewhere. 2) The flat slab hypothesis relates Laramide (Rocky Mtn)
events to a very shallow subduction zone along western North America.
The Baja BC hypothesis suggests that much of modern British Columbia was
situated in the Baja region during late Mesozoic and was then transferred
north along major right lateral strike slip faults to present positions. As these
blocks moved northward between the previous arc (eg. Sierra Nevada) and the
westward subduction zone, the arc was built on the Baja BC blocks and the
inboard Mesozoic arc was shut down.
The development of the Rocky Mountains and related structures in western
North America is one of the most puzzling of tectonic events. Why were the
Rockies built so far inland? The shallow slab hypothesis suggests that a
shallowing of the angle of the Cordilleran subduction zone caused inboard
migration of arc activity into the Rocky Mountain region. Are both the Baja
BC and shallow subduction hypoyheses workable at the same time and place?
This question remains to be resolved.
By the Oligocene, the old elements of the Cordilleran arc remain inactive and
magmatism expanded in the Rocky Mtn region. The Farallon plate was
rapidly subducted and the spreading center (East Pacific Rise) closed on North
America. The Laramide orogeny waned and the Cordilleran region remained
high. Sediments shed eastward from both regions were spread over the Great
Plains. Magmatism slowly retreated back to the west into a more normal
position. A new arc formed along the Cordilleran margin and forearc deposits
formed along the Pacific margin. The Pacific spreading center neared North
America; a major transform fault, the Mendocino fracture zone, intercepted
the NAM continent. A triple junction resulted and the Farallon became two
separate plates.
During the Miocene, more of the Mendocino FZ intercepted western NAM
and a long transform margin was established. Subduction continued to the
north (Cascade Arc) and south. Along the transform margin, a strike slip
rather than a convergent plate boundary formed as both the Pacific and North
American plates had strong westward components of movement. The
landscape behind this zone became extended and the Basin and Range orogeny
developed. Volcanism enveloped large areas of western NAM.
North of the Mendocino triple junction normal subduction continued and the
Cascade arc remained active. Southward, extension spread northward and
waned to the south. Volcanism continued over wide areas. Most of western
NAM was high topographically but lower basins received locally thick
sediments. As the Sea of Cortez (Gulf of Cal.) opened, a large block of SW
NAM slid northwestward on the Pacific Plate along the San Andreas Fault.
Middle Proterozoic (~ 1.7 Ga) -- This highly speculative reconstruction is
based on recent work by geologists at Northern Arizona University and the
University of New Mexico
Mesoproterozoic (~1.1 Ga) -- North America was part of the supercontinent
Rodinia and was possibly adjacent to Antarctica and Australia
Neoproterozoic (~ 650 Ma) -- North America rifted away from Antarctica and
Australia as the Precambrian supercontinent broke up. Western North
America began a long history as a passive margin as the continent moved
away from a mid-ocean rift zone. Some blocks of rifted North America likely
lay off shore, similar to the island of Madigascar off east Africa today
Cambrian Paleogeography, Southwestern US
Cambrian Paleogeography, Southwestern US (~510 Ma). The passive margin
setting continued; fluvial and shallow marine sandstone and mudstone grade
westward to shelf carbonate rocks. The slope-rise at the continental margin
was the site of deep marine turbidity and slump deposits
Silurian Paleogeography, Southwestern US
Silurian Paleogeography, Southwestern US (430 Ma). Two views are shown,
one focuses on the Western Interior and the second focuses off the west coast.
They match across the zone of overlap. Shelf carbonates and deeper water
shelf margin deposits grade westward into continental rise deposits and
oceanic sediments in western Nevada and California
Mississippian Paleogeography, Southwestern US
Mississippian Paleogeography, Southwestern US (340 Ma). The colliding
Antler arc shoved strata deposited in deep water over the continental shelf to
form the Antler orogeny. The weight of the thickened crust caused eastern
Nevada and western Utah to subside and thick Upper Devonian and Lower
Mississippian strata formed there. Farther east a broad carbonate shelf
blanketed much of the Western Interior
View to west of above. The Antler arc has collided with the continental
margin. However some parts of the arc possibly collided with the off shore
continental blocks. Here subduction continued but the polarity of the
subduction (direction of dip of subduction zone) reversed so that subduction
now dipped eastward. One or more of these portions of the reversed Antler
arc may have been the precursor of the McCloud arc, a Late Paleozoic
tectonic element of western North America. Many of the interpretations of
events off the western margin of North America continue to be strongly
debated. As will be shown beginning with Permian paleogeography, some
geologists interpret the McCloud arc to be exotic to North America
Pennsylvanian Paleogeography, Southwestern US
Pennsylvanian Paleogeography, Southwestern US (310 Ma). Huge changes in
paleotectonics from earlier periods affected western North America. The
Ancestral Rockies rose across the southern Western Interior. The Antler
Mountains were already being worn down by erosion and even began
subsiding in some areas. A marine basin covered most of western Utah and
eastern Nevada.
View to west of above. In this interpretation, the Stikine fragment of the
southern McCloud arc migrated westward while the Quesnell fragment of the
Northern McCloud arc remained near western North America. The transform
fault between the two may have been the same transform that truncated SW
North America during the Late Pennsylvanian and Permian. Follow this
interpretation and watch for a contrasting one in the Permian and Mesozoic
Permian Paleogeography, Southwestern US
Permian Paleogeography, Southwestern US (280-270 Ma). Parts of the
Ancestral Rockies remain high and shed sediments into adjacent basins. The
Havallah back arc basin expands over the site of the previous Antler
Mountains
View to west of above. The Stikine fragment has moved NW to a position west
of the Quesnell fragment. In other words, the McCloud arc is a double arc; the
western portion was transported along a transform fault
Triassic Paleogeography, Southwestern US
Early Triassic Paleogeography, Southwestern US (240 Ma). A broad fluvial
and shallow marine depositional system (the Moenkopi Formation) covers
much of the southern Western Interior. To the west, the Stikine and Quesnell
elements of the McCloud arc collide and in turn collapse the Havallah back
arc basin as the Sonoman orogeny continues. The Cache Creek forearc
complex is trapped between the two arc elements
View to west of above. Oceanic plateau fragments continue to acrete to
western North America and the ocean plate subducts below the continent. As
subduction continued, the Cordilleran arc became established
Late Triassic Paleogeography, Southwestern US (215 Ma). The Chinle fluvial
system covered much of the Western Interior. Chinle rivers were sourced in
remnant highlands of the Appalachian Mountain system. As the Cordilleran
arc developed, a new back arc basin formed behind the McCloud arc.
View to west of above. Pacific subduction continues below North America
Jurassic Paleogeography, Southwestern US
Early Jurassic Paleogeography, Southwestern US (200 Ma). The great Navajo
sand sea has spread across much of the Wertern Interior. A continental arc
has developed across the Southwest and some Navajo dunes spill westward
into the topographically low arc. Farther north the arc is off shore separated
from the continent by a back arc basin. The Stikine and Quesnell elements are
separated by the Cache Creek interarc basin.
Middle Jurassic Paleogeography, Southwestern US (170 Ma). A long, narrow,
epicontinental seaway extends into the Western Interior from the north. Dunes
rim the southeast margin. The elements of the McCloud arc collapse against
the continent and thrusting in Nevada creates uplands. The large Wrangellia
oceanic plateau approaches from the south. Most interpretations consider
Wrangellia to be an exotic terrain. However, the details of the history of this
terrain and its collision with North America are greatly debated
Late Jurassic Paleogeography, Southwestern US (150 Ma). The uplift in
Nevada reverses stream direction across the region and the Morrison fluvial
system expands eastward across the Western Interior. Stresses related to the
opening of the Gulf of Mexico oceanic basin caused oblique, transtensional
basins to form across the southwest portion of North America. Thick
continental and later marine deposits filled these basins. The collapse of
Stikine arc against Quesnell and perhaps the initial collision of Wrangellia
trigger the first phase of the Nevadan orogeny.
Latest Jurassic Paleogeography, Southwestern US (145 Ma). A large saline
lake spread across the eastern Colorado Plateau. To the west, uplands sourced
streams of the Morrison Formation. At the conclusion of the Nevadan
orogeny, a continental arc extended along most of the western coast of North
America
Cretaceous Paleogeography, Southwestern US
Early Cretaceous Paleogeography, Southwestern US (130 Ma). The Western
Interior was the site of an encroaching epicontinental seaway from the north.
To the west lay uplands and a thrust belt in Nevada and western Utah.
Transtensional basins in southern Arizona and california were the sites of
thick marine and continental deposition. The Cordilleran arc was now a
classic continental (Andean-style) arc along all or most of its extent. The arc
consisted of a fore arc-trench system, fore arc basin, and Andean arc. The fore
arc-trench was the site of the famous Franciscan melange'. The Great Valley
sequence was deposited in the fore arc basin and the Sierra Nevada batholith
complex formed in the bowels of the arc
Middle Cretaceous Paleogeography, Southwestern US (90 Ma). The Western
Interior seaway has expanded to one of the greatest eoicontinental seas of all
times. It stretched from central Utah to the western Appalachians and from
the Arctic to the Gulf of Mexico. The Sevier orogenic belt bordered the sea to
the west. This great thrust belt resulted from compression of the Cordilleran
arc, perhaps further mitigated by collision with the huge Wrangellia oceanic
plateau. The nature of the collision, its exact location, and its magnatude are
anything but clear. Pieces of Wrangellia and Wrangellia-like plateaus are
presently strewn along the west coast of North America from Mexico to
Alaska
Late Cretaceous Paleogeography, Southwestern US (75 Ma). The Western
Interior seaway slowly retreated to the northeast. Vast alluvial plains marked
its past locations. Some of the World's greatest dinosaur remains are found in
these deposits. The Sevier orogeny was at its climax. Regional metamorphism
affected western Arizona and eastern California. Paleozoic and Mesozoic
sandstone, mudstone, and limestone were metamorphosed to quartzite, schist,
and marble. The west coast was marked by strongly oblique collision between
the oceanic and continental plates and right-lateral transform faults
transported pieces of Wrangellia and other terrains northward along the edge
of North America. The Franciscan forearc complex was a broad accretionary
prism that was constantly being deformed by complex stresses into a melange'
Latest Cretaceous-Early Tertiary Paleogeography, Southwestern US (65 Ma).
Huge changes marked western North America. The Western Interior seaway
had withdrawn and in its place were the embryonic Laramide uplifts of the
Rocky Mountains. Debate rages over the origin of the Rockies, but the most
popular theory states that the subduction of the Farallon oceanic plate under
western North America changed to a very shallow angle, perhaps less than 10
degrees. This drove subduction eastward into the Rocky Mountain region and
caused the uplifts and plutons there while the Cordilleran arc in California
was shut down. Other theories blame collision with Wrangellia and the
resulting compression for the Laramide orogeny. Perhaps large fragments of
Wrangellia were partly responsible for the shallow angle of subduction.
Tertiary Paleogeography, Southwestern US
Early Tertiary (Eocene) Paleogeography, Southwestern US (50 Ma). As the
Rocky Mountains were uplifted, basins formed and subsided between major
uplifts. Huge lakes filled the basins during the Eocene. Although the lakes
were in basins, paleobotanical data suggests that their absolute elevation was
near the current elevation of the deposits. This fact suggeste that much of the
Rocky Mountain region and the Colorado Plateau were uplifted in the Eocene.
Mountainous terrain existed in Nevada, western Utah, and central and
southern Arizona. Streams drained these uplands onto the Colorado Plateau
and probably into the lakes. The coastal region continued to have right-lateral
translation of terrains northward. Some of these terrains lie outboard of
marginal seas along the west coast and formed continental borderlands at the
edge of the continent. The most famous of these, Salinia, probably started out
in northern Mexico and was progressively translated northward; you can
follow its progression on the four Tertiary maps
Early Tertiary (Oligocene) Paleogeography, Southwestern US (35 Ma). Much
of the drainage of the Rocky Mountain and Colorado Plateau regions was
poorly developed and probably internal to the region. However, some streams
found outlets to lower elevations and carved canyons into the margins of the
Colorado Plateau. Volcanism was widespread and extreme across much of the
Western Interior. The first stages of the Rio Grande Rift developed as parts of
western North America evolved from compressional tectonics to extensional
tectonics. California borderlands continued their northward translation and a
series of complex basins formed between and east of them. The East Pacific
Rise, a Pacific spreading center, neared the coast of southwestern North
America. Its impending collision would have profound effects on the evolution
of the region
Middle Tertiary (Early Miocene) Paleogeography, Southwestern US (20 Ma).
Drainage systems of the Western Interior were still not well integrated. Note
the absence of a through going stream on the Colorado Plateau. Extensive
volcanism continued. On the west coast, normal subduction resumed, but only
north of where the East Pacific Rise had already collided with North America.
Since the Permian or Triassic, the Farallon and related plates were subducted
below North America. Once the East Pacific Rise collided with the continent,
the westerly plate, the Pacific Plate, came into contact with the continent.
Because both the Pacific Plate and North American Plate have westerly
components of motion, a new stress regime was established. Extension
dominated regions east of where North America contacted the Pacific Plate. A
large transform fault system, the San Andreas, was created. Blocks moving
northwest along the Pacific side of the fault were caught in a bend of the fault
system in southern California. These blocks were rotated in clockwise fashion
to form the Transverse Ranges
Latest Middle Tertiary (Late Miocene) Paleogeography, Southwestern US (10
Ma). The tectonic patterns of the earlier Miocene continued. Volcanism
continued widespread and streams continued to gnaw at the Colorado Plateau.
Extension spreads northward as the Mendocino triple junction, the point of
contact between the Farallon, North American, and Pacific plates moved
northward. Note that Salinia is now in central California. The Cordilleran arc
shuts down south of the triple junction as subduction ceases and transform
motion begins.