Structure and Stratigraphy of Eastern Alaska Range, Alaska: ABSTRACT

Abstract

The eastern Alaska Range, between 141°W (international boundary) and 145°W in south-central Alaska, provides clues to the tectonic development of northwest 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 terrane of highly deformed metamorphosed sedimentary and minor igneous rocks, Precambrian to Devonian in age. South of the Denali fault system these rocks are absent and the oldest rocks exposed are a heterogeneous series of Pennsylvanian(?) or Permian volcanics and volcaniclastics derived from a late Paleozoic volcanic island arc probably built directly on oceanic crust. These rocks are overlain by a succession of Permian marine sediments and limestones, Triassic carbonaceous shales, subaerial tholeiitic basalt flows and marine limest nes and Jurassic-Cretaceous argillite, graywacke, and conglomerate with a cumulative thickness locally exceeding 10,000 ft. Sedimentation culminated in middle(?) Cretaceous time with a short-lived and restricted episode of andesitic volcanism. Relatively undeformed 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 ultramafics occur with the Permian rocks on the west in the central Alaska Range and on the east in Canada. In the eastern Alaska Range ultramafic rocks have not been observed south of the Denali fault but they are present locally along the fault zone and in the older terrane directly north of the fault. All rocks older than Late Cretaceous south of the Denali fault system have been cut by steep 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, with fold axes plunging at shallow angles generally northwest. The geologic data suggest that the oceanic terrane 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 been deformed many times as later oceanic plates impinged against the continental margin. The Denali fault, which is an ancient subduction zone, has been reactivated as a ridge-arc dextral 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 and trends southeasterly toward the Fairweather fault in the Gulf of Alaska, is another major right-lateral strike-slip structure that may have developed as recently as in t e middle Pleistocene. The Denali fault system appears to be presently inactive southeast of the Denali-Totschunda junction. End_of_Article - Last_Page 2502------------