Geologic Significance of Doonerak Structural High, Central Brooks Range, Alaska

Abstract

Rocks in the Doonerak anticline resemble correlative sequences in the northeastern Brooks Range more than they resemble the directly adjacent sequence in the north-central Brooks Range. Mafic dikes in the anticline have potassium-argon ages of about 380 m.y. (Devonian) and 470 m.y. (Ordovician). They intrude low-grade metamorphic rocks and associated volcanic rocks that are therefore probably of Cambrian or Ordovician age. These early Paleozoic units are overlain unconformably by Carboniferous to Triassic strata. The Doonerak anticline is bounded on both the north and south by faults that can be interpreted in at least two ways. A structural-window hypothesis postulates that the two faults are actually parts of the same fault and reflect long-range thrusting from south to north over the Doonerak high. This fault would constitute a sole thrust under the north-central Brooks Range, and all Upper Devonian through Triassic rocks there would be allochthonous. Consequently, both late Paleozoic and Mesozoic strata might be preserved at depth beneath the stacked thrust sheets of that area. An alternative hypothesis favored by the writers calls for two separate faults, the southern one thrusting Middle and lower Upper Devonian rocks northward onto the phyllitic rocks on the south flank of the anticline. A distinct northern fault is interpreted as a south-directed thrust that moved Upper Devonian and younger rocks southward over the Carboniferous rocks on the north flank of the anticline. This interpretation implies that the rocks of the north-central Brooks Range are mainly autochthonous and that the rocks directly beneath them would be like Middle Devonian and older strata now present in the core of the Brooks Range, south of the Doonerak anticline.