The Wrangellia Terrane (named for the
Wrangell Mountains
The Wrangell Mountains are a high mountain range of eastern Alaska in the United States. Much of the range is included in Wrangell-Saint Elias National Park and Preserve. The Wrangell Mountains are almost entirely volcanic in origin, and they i ...
, Alaska) is a crustal fragment (
terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
) extending from the south-central part of
Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U. ...
and along the
Coast of British Columbia
, settlement_type = Region of British Columbia
, image_skyline =
, nickname = "The Coast"
, subdivision_type = Country
, subdivision_name = Canada
, subdivision_type1 = Province
, subdivision_name1 = British ...
in
Canada
Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world ...
. Some geologists contend that Wrangellia extends southward to
Oregon
Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...
, although this is not generally accepted.
Extent and terminology
The term Wrangellia is confusingly applied to all of:
* The Wrangell(ia) Terrane alone;
* A composite
terrane
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
(CT) consisting of the
Wrangell Terrane,
Peninsular Terrane, and other rock units that were not originally part of the
North American craton
North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography.
Etymology
The word ''north'' i ...
;
* A composite terrane which also includes the
Alexander Terrane.
Earlier geologists sometimes used the term "
Talkeetna Superterrane" to describe Wrangellia.
Origin
There are two conflicting hypotheses about whether the Wrangellia Superterrane originated at polar or equatorial latitudes:
# That Wrangellia accreted at a northerly latitude near its current location (when North America, or
Laurentia
Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, although ...
, was farther east as part of
Pangaea
Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
).
# That Wrangellia originated south of its current location, approximately where
Baja California
Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Me ...
is now. This hypothesis is not favoured in most plate tectonic reconstructions, since it introduces rapid implausible displacements of Wrangellia across the
Panthalassic Ocean.
Northern hypothesis
Geological evidences indicate that the Caledonide closure of the
Iapetus
In Greek mythology, Iapetus (; ; grc, Ἰαπετός, Iapetós), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other ...
and
Rheic Ocean
The Rheic Ocean was an ocean which separated two major palaeocontinents, Gondwana and Laurussia (Laurentia-Baltica-Avalonia). One of the principal oceans of the Palaeozoic, its sutures today stretch from Mexico to Turkey and its closure resulted ...
s along the Laurentian westcoast (modern coordinates) also opened an ocean between the northern margin of Laurentia and
Baltica
Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains.
The thick core of Baltica, the East European Craton, is mo ...
on one side and
Siberia
Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part o ...
on the other. This effectively dispersed continental fragments—the Alexander, Eastern Klamath, Northern Sierra and Okanagan terranes—westward along the shores of this ocean in a
back-arc process similar to that of the present-day
Scotia Plate
The Scotia Plate () is a tectonic plate on the edge of the South Atlantic and Southern oceans. Thought to have formed during the early Eocene with the opening of the Drake Passage that separates South America from Antarctica, it is a minor plat ...
between the
South America Plate and the
Antarctic Plate. During the
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonif ...
, the Alexander terrane migrated westward into the northern Panthalassic Ocean where it merged with Wrangellia in the late Carboniferous—the two continental fragments remained isolated in the open ocean until they were accreted to Laurentia in the Middle
Jurassic.
Southern hypothesis
Rocks of Wrangellia (the individual terrane, not the composite terrane) were originally created in the
Pennsylvanian to the
Jurassic somewhere, but probably near the equator, in the
Panthalassic Ocean off the west coast of the Laurentia (North American craton) as
island arc
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alo ...
s,
oceanic plateau
An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.
There are 184 oceanic plateaus in the world, covering an area of or about 5.11% of the ...
s, and rock assemblages of the associated tectonic settings. It is composed of many rocks types, of various composition, age, and tectonic affinity, but the Late
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
flood basalt
A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot reach ...
s are the defining unit of Wrangellia. These
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low- viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More tha ...
s, extruded onto land over 5 million years about 230 million years ago, on top of an extinct Pennsylvanian and
Permian
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
island arc, constitute a
large igneous province
A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive (sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The formation ...
, currently exposed in a long belt.
Wrangellia collided and amalgamated with the
Alexander Terrane by Pennsylvanian time. By the end of the Triassic, the Peninsular Terrane had also joined the Wrangellia composite terrane. A
subduction zone
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...
existed on the west side of Wrangellia. Seafloor rocks too light to be subducted were compressed against the west edge of Wrangellia; these rocks are now known as the
Chugach Terrane. A complex fault system, known as the
Border Ranges Fault, is the modern expression of the suture zone between Wrangellia and Chugach Terranes. Over time,
plate tectonics
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
moved this amalgamation of crust generally northeastward into contact with the North American continental margin. The Wrangellia composite terrane collided with and docked to North America by
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
time.
Strike-slip
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tecton ...
displacement, with Wrangellia travelling northward, continued after docking, although the amount of post-
accretion
Accretion may refer to:
Science
* Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity
* Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...
displacement is controversial.
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See also
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References
Citations
General sources
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External links
Google search for maps of WrangelliaLarge Igneous Provinces Commission December 2008 LIP of the Month: The Accreted Late Triassic Wrangellia Oceanic Plateau in Alaska, Yukon, and British Columbia
Terranes
Lava plateaus
Plateaus of the Pacific Ocean
Flood basalts
Triassic volcanism
Volcanism of Alaska
Volcanism of Yukon
Volcanism of British Columbia
Coast of British Columbia
Large igneous provinces