NATURE OF PERMIAN FAUNAS IN WESTERN NORTH AMERICA: A KEY TO THE UNDERSTANDING OF THE HISTORY OF ALLOCHTHONOUS TERRANES

Along the western margin of the North American craton, Permian fusulinid and colonial rugose coral faunas vary somewhat latitudinally, but there are many marked similarities. West of this margin, there are three sets of terranes bearing different faunas that were accreted to the craton after Permian time. The first set, located mostly immediately west of the craton and commonly referred to as the McCloud Belt, bears somewhat different faunas from those of cratonal North America. The original distance between these terranes and North America during the Permian, based on faunal differences, suggest 2000-3000 km of original separation. Farther west there are terranes and blocks in melanges that bear typical Tethyan faunas interpreted to have originated much farther to the west. Today the dispersed Wsrangellia terrane, which bears faunas somewhat related to those of the McCloud Belt, lies west of the other terranes.

Data from the fusulinid and colonial coral faunas, with consideration of the available paleomagnetic data, suggest that many of these terranes had complicated histories, including changes in positions relative to the North American craton during the Permian. In the Early Permian these terranes were mostly at somewhat different latitudes relative to cratonal North America than they are today. The Wrangellia terrane in Alaska and the Stikine terrane probably lay somewhat farther south than today and the Quesnellia and Eastern Klamath terranes lay farther north. The Tethyan faunas occupied shelves around islands in the tropics.

During the Middle Permian the terranes of the McCloud Belt and the Wrangellia terrane migrated southward, probably almost 2000 km relative to cratonal North America, and all of the terranes, including the Tethyan terranes, may have begun to converge on the North American craton. In post-Permian time all of these terranes were accreted to the craton and smeared out thousands of kilometers along strike-slip faults at its margin.