Alexander Terrane

{{more footnotes|date=January 2026}} {{short description|Geological area in northwestern North America}} {{Infobox landform | name = Alexander Terrane | type = physiographic section | location = Alaska; British Columbia; Yukon | length_km = 1000 | width_km = 250 | coordinates = | part_of = Wrangellia Terrane | range = | pushpin_map = | map_caption =  | age = | orogeny = | formed_by = | geology = }}

The '''Alexander Terrane''' (named for the Alexander Archipelago in Alaska) is a Neoproterozoic continental fragment that originated far from its current location, before being transported and accreted to western North America in the Middle Jurassic.<ref name="Gehrels">{{cite journal | last1 = Gehrels | first1 = George E. | last2 = Saleeby | first2 = Jason B. | year = 1987 | title = Geologic framework, tectonic evolution, and displacement history of the Alexander Terrane | journal = Tectonics | volume = 6 | issue = 2 | pages = 151–173 | doi = 10.1029/TC006i002p00151 | url = https://doi.org/10.1029/TC006i002p00151 }}</ref> This terrane also forms part of the composite Wrangellia Terrane.<ref name="Isreal">{{cite journal | last1 = Israel | first1 = Steve | last2 = Bernanek | first2 = Luke P. | last3 = Friedman | first3 = Richard M. | last4 = Crowley | first4 = James L. | year = 2014 | title = New ties between the Alexander terrane and Wrangellia and implications for North America Cordilleran evolution | journal = Lithosphere | volume = 6 | issue = 4 | pages = 270–276 | doi = 10.1130/L364.1 | url = https://doi.org/10.1130/L364.1 }}</ref>

Rocks assigned to this terrane underlie much of Southeast Alaska, the north coast of British Columbia, and the Yukon southwest.<ref name="Gehrels"/> The Alexander Terrane is intruded by Late Jurassic and Early Cretaceous plutons and is overlain to the east by Upper Jurassic through mid-Cretaceous clastic strata and mafic volcanic rocks.<ref>{{cite journal | last = Gehrels | first = G. E. | year = 2001 | title = Geology of the Chatham Sound region, southeast Alaska and coastal British Columbia | journal = Canadian Journal of Earth Sciences | volume = 38 | issue = 11 | pages = 1579–1599 | doi = 10.1139/e01-040 }}</ref>

In British Columbia, the terrane crops out primarily in the Coast Mountains, including the Kitimat Ranges, Chatham Sound, and Haida Gwaii, and continues to Klemtu.<ref name="Gehrels"/>

==Origin== The original location of the Alexander Terrane remains uncertain. Some evidence, including detrital zircon geochronology, suggests it may have formed along the margins of ancient Baltica or Laurentia,<ref>{{cite journal | last1 = Beranek | first1 = Luke P. | last2 = van Staal | first2 = Cees R. | last3 = McClelland | first3 = William C. | last4 = Israel | first4 = Steve | last5 = Mihalynuk | first5 = Mitch G. | year = 2013 | title = Baltican crustal provenance for Cambrian–Ordovician sandstones of the Alexander Terrane, North American Cordillera: evidence from detrital zircon U–Pb geochronology and Hf isotope geochemistry | journal = Journal of the Geological Society | volume = 170 | issue = 1 | pages = 7–18 | doi = 10.1144/jgs2012-070 | url = https://doi.org/10.1144/jgs2012-070 }}</ref> however, paleomagnetic, isotopic, and fossil data indicate a possible origin near the margins of the ancient Siberian or Arctic continents.<ref name="Isreal"/>

==Development== The terrane developed through three distinct phases:

; Late Cambrian through Early Devonian : During the initial phase, the terrane probably evolved along a convergent plate margin.<ref name="Gehrels"/> ; Middle Devonian through Lower Permian : The second phase is marked by strata accumulated in tectonically stable marine environments.<ref name="Gehrels"/> ; Triassic : The third phase is marked by volcanic and sedimentary rocks which are interpreted to have formed in a rift environment.<ref name="Gehrels"/>

==References== {{Reflist}}

Category:Terranes Category:Coast of British Columbia Category:Triassic Alaska Category:Triassic British Columbia Category:Triassic Yukon Category:Geology of Canada