The Wrangellia Terrane (named for the
Wrangell Mountains, Alaska) is a crustal fragment (
terrane) extending from the south-central part of
Alaska and along the
Coast of British Columbia in
Canada. Some geologists contend that Wrangellia extends southward to
Oregon,[1] although this is not generally accepted.
Extent and terminology
The term Wrangellia is confusingly applied to all of:
Earlier geologists sometimes used the term "
Talkeetna Superterrane" to describe Wrangellia.[2]
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, was farther east as part of
Pangaea).
That Wrangellia originated
c. 3,000 km (1,900 mi) south of its current location, approximately where
Baja California is now. This hypothesis is not favoured in most plate tectonic reconstructions, since it introduces rapid implausible displacements of Wrangellia across the
Panthalassic Ocean.[3]
Northern hypothesis
Geological evidences indicate that the Caledonide closure of the
Iapetus and
Rheic Oceans along the Laurentian westcoast (modern coordinates) also opened an ocean between the northern margin of Laurentia and
Baltica on one side and
Siberia 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 between the
South America Plate and the
Antarctic Plate. During the
Carboniferous, 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.[4]
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 arcs,
oceanic plateaus, 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
Triassicflood basalts are the defining unit of Wrangellia. These
basalts, extruded onto land over 5 million years about 230 million years ago, on top of an extinct Pennsylvanian and
Permian island arc, constitute a
large igneous province, currently exposed in a 2,500 km (1,553 mi) long belt.[5]
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 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 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 time.
Strike-slip displacement, with Wrangellia travelling northward, continued after docking, although the amount of post-
accretion displacement is controversial.[6]
Nokleberg, W. J.; Jones, D. L.; Silberling, N. J. (1985). "Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska". Geological Society of America Bulletin. 96 (10): 1251–1270.
Bibcode:
1985GSAB...96.1251N.
doi:
10.1130/0016-7606(1985)96<1251:OATEOT>2.0.CO;2.
Nokleberg, W. J.; Parfenov, L. M.; Monger, J. W. H.; Norton, I. O.; Khanchuk, A. I.; Stone, D. B.; Scholl, D. W.; Fujita, K. (1998).
Phanerozoic tectonic evolution of the Circum-North Pacific(PDF) (Report). USGS Open-file report 98-754. Retrieved 30 April 2018.
Trop, J. M.; Ridgway, K. D.; Manuszak, J. D.; Layer, P. (2002). "Mesozoic sedimentary-basin development on the allochthonous Wrangellia composite terrane, Wrangell Mountains basin, Alaska: A long-term record of terrane migration and arc construction". Geological Society of America Bulletin. 114 (6): 693–717.
Bibcode:
2002GSAB..114..693T.
doi:
10.1130/0016-7606(2002)114<0693:MSBDOT>2.0.CO;2.
Large Igneous Provinces Commission: December 2008 LIP of the Month: The Accreted Late Triassic Wrangellia Oceanic Plateau in Alaska, Yukon, and British Columbia