Wrangellia Terrane Article

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The Wrangellia Terrane (named for the Wrangell Mountains, Alaska) is a terrane extending from the south-central part of Alaska through southwestern Yukon and along the Coast of British Columbia in Canada. Some workers 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: the Wrangell(ia) Terrane alone; a composite terrane (CT) consisting of the Wrangell Terrane, Peninsular Terrane, and other rock units that were not originally part of the North American craton; and a composite terrane which also includes the Alexander Terrane. Earlier workers sometimes used the term, " Talkeetna Superterrane," to describe Wrangellia. [2]

Origin

There are two conflicting hypotheses whether the Wrangellia Superterrane originated at polar or equatorial latitudes. One hypothesis proposes that Wrangellia accreted at a northerly latitude near it current location (when North America, or Laurentia, was located farther east as part of Pangaea), the other that it originated c. 3,000 km (1,900 mi) south of its current location, approximately where Baja California is now located. The former hypothesis is favoured in most plate tectonic reconstructions since the latter introduces rapid, implausible displacements of Wrangellia across the Panthalassic Ocean. [3]

Southern hypothesis

Rocks of Wrangellia (the individual terrane, not the CT) were originally created in the Pennsylvanian to the Jurassic somewhere, but probably near the equator, in the Panthalassic Ocean off of the west coast of the North American craton as island arcs, oceanic plateaus, and rock assemblages of the associated tectonic settings. Although composed of many different rocks types, of various composition, age, and tectonic affinity, it is the Late Triassic flood basalts that 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. [4]

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 CT. A subduction zone existed on the west side of Wrangellia. Seafloor rocks too light to be subducted were instead compressed against the western 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 CT 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. [5]

See also

References

Notes

  1. ^ Sarewitz 1983, Abstract
  2. ^ Wallace, Hanks & Rogers 1989, Abstract
  3. ^ Nokleberg et al. 1998, Paleomagnetic Dilemma: Loci of Accretion of Wrangellia Superterrane, pp. 9–10
  4. ^ Rogers & Schmidt 2002, Abstract; Greene et al. 2004, Introduction, p. 10
  5. ^ Nokleberg, Jones & Silberling 1985, Abstract; Trop et al. 2002, Abstract; Israel 2009, Abstract; Greene, Scoates & Weis 2005, p. 211

Sources


WRANGELLIA Latitude and Longitude:

59°59′02″N 140°35′17″W / 59.984°N 140.588°W / 59.984; -140.588