Paleomagnetism Of The Wrangellia And Alexander Terranes And The Tectonic History Of Southern Alaska

Abstract

Wrangellia was the first Alaskan tectonostratigraphic terrane to be widely accepted as allochthonous with respect to North America. There is, however, considerable disagreement as to the age of emplacement of this terrane as well as the hemisphere in which it originated. Some 800 paleomagnetic samples were collected from 24 localities in southern Alaska to elucidate the paleolatitude translation history of Wrangellia and other associated terranes. Data of known polarity from the Skolai Group (Pennsylvanian/Permian) strongly suggest that Wrangellia originated at 10-15 degrees North latitude. The Permian Pybus Dolomite yields a 9 degree S paleolatitude and suggests that the Alexander terrane moved southward in late Paleozoic and Triassic time. Evaluation of geologic data indicates that the Wrangellia and Alexander terranes amalgamated in an oceanic setting in mid to Late Jurassic time. Paleomagnetism of the Brothers Volcanics (Alexander terrane) and MacColl Ridge Formation (Wrangellia) documents a low latitude for both terranes during the Cretaceous, thereby precluding a pre-Tertiary age of emplacement for the amalgamated superterrane. Speculative apparent polar wander paths for Wrangellia and the Alexander terranes, in addition to geologic and biogeographic constraints, allow development of the following hypothetical tectonic model. Both the Alexander and Wrangellia terranes originated in the northern hemisphere adjacent to western North America in mid-paleozoic and late Paleozoic times, respectively. The Alexander terrane moved into the southern hemisphere during the Paleozoic and Wrangellia began moving southward in the Late Triassic or Early Jurassic. These two terranes amalgamated in mid to low southern paleolatitudes in later Jurassic time and formed part of a composite terrane, here termed the Southern Alaska superterrane. This superterrane began northward translation in Late Jurassic time, accreting to North America in Tertiary time.