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Abstract

The eastern Alaska Range, between 141CW (International Boundary) and 145°W long, in south-central Alaska, provides clues to the tectonic development of northwestern North America.

The Denali fault system, a major structural feature extending in an arcuate path from the Bering Sea to the Gulf of Alaska, transects the eastern Alaska Range and separates extremely diverse geologic terranes. North of the Denali fault lies a widespread ferrane of highly deformed, metamorphosed sedimentary and minor igneous rocks of Precambrian to Devonian age. South of the Denali fault system these rocks are absent, and the oldest rocks exposed are a heterogeneous series of Pennsylvanian(?) or Permian volcanic and volcaniclastic rocks derived from a late Paleozoic volcanic island arc probably built directly on oceanic crust. These rocks are overlain by a succession of Permian marine clastic beds and limestones; Triassic carbonaceous shales, subaerial tholeiitic basalt flows, and marine limestones; and Jurassic-Cretaceous argillite, graywacke, and conglomerate. The cumulative thickness of the succession locally exceeds 10,000 ft (3,050 m). Sedimentation culminated in middle(?) Cretaceous time with a short-lived and restricted episode of andesitic volcanism. Relatively un-deformed continental sedimentary rocks of Cretaceous age, or younger, and late Cenozoic terrestrial volcanic flows overlie the older rocks with marked angular unconformity.

Linear bodies of serpentinized ultramafic rocks are present with the Permian rocks to the west in the central Alaska Range and to the east in Canada. In the eastern Alaska Range, ultramafic rocks have not been observed south of the Denali fault, but they do occur locally along the fault zone and in the older ferrane just north of the fault.

All pre-Late Cretaceous rocks south of the Denali fault system have been cut by high-angle normal faults and by numerous reverse and thrust faults that dip north toward the Denali fault. The Jurassic-Cretaceous marine sedimentary rocks also exhibit complex folding, locally isoclinal, and fold axes plunge at low angles generally toward the northwest.

The geologic data suggest that the oceanic ferrane south of the Denali fault collapsed against, and was added to, the continental American plate, probably in Early Triassic time. Since then, this terrane has undergone multiple deformation as later oceanic plates impinged against the continental margin. The Denali fault, which represents an ancient subduction zone, was reactivated as a ridge-arc dexfral transform fault—probably during the early Pliocene—in response to a change in the direction of spreading in the North Pacific oceanic plate. The Totschunda fault system, which diverges from the Denali structure near 144°W long, and trends southeasterly toward the Fairweather fault in the Gulf of Alaska, is another major right-lateral strike-slip fault that may have developed as recently as the middle Pleistocene. At present, the Denali fault system apparently is inactive southeast of the Denali-Tofschunda junction.

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