Abstract

Compilation of western Alaska geologic and geophysical data indicates that several rock assemblages, including Paleozoic to Precambrian metamorphic rocks, lower Paleozoic sedimentary rocks, and a Jurassic to Paleozoic sequence of mafic volcanic and marine sedimentary rocks associated with mafic and ultramafic intrusive rocks, can be traced from the north end of the Kokrines-Hodzana highlands at the headwaters of the Koyukuk River, across the strike-slip systems of central Alaska, to Goodnews Bay in the south. The Paleozoic to Precambrian metamorphic rocks consist dominantly of metasedimentary and subordinate meta-igneous sequences which form a southwest-trending belt from the headwaters of the South Fork of the Koyukuk River to Goodnews Bay that is referred to as the Ruby geanticline. These metamorphic rocks are on lapped from the east by a lower Paleozoic platform sequence which appears to shale out toward the southeast.

Belts of Jurassic to Paleozoic marine sedimentary and mafic volcanic rocks associated with mafic and ultramafic rocks mark the northwest and southeast sides of the Ruby geanticline and the southern margin of the Brooks Range. The pre-strike-slip faulting configuration of the Jurassic to Precambrian rocks suggests they formed by intermittent rifting of the lower Paleozoic to Precambrian crust of western Alaska along at least two arms (the present southern margin of the Brooks Range and the northwest flank of the Ruby geanticline). A possible third rift basin may lie on the southeast flank of the Ruby geanticline. These marginal, rifted basins were subsequently collapsed during a Jurassic-to-Early Cretaceous orogeny. The rifted margins of the Brooks Range province and the Ruby geanticline became the suture/root zones from which the contents of the rift basins were thrust to the north and southeast respectively.

The compressional orogeny of the Jurasic and Early Cretaceous was overlapped in the Early and Late Cretaceous by an 80-to-110-million-year-old calc-alkalic and alkalic intrusive event and the development of extensive, continually subsiding troughs, in which great thicknesses of volcanic rocks, graywacke, mudstone, and conglomerate accumulated. South of the Kaltag fault, intrusive and volcanic activity from 58 to 79 million years ago followed infilling of the sedimentary troughs. Initiation of the major strike-slip faulting of western Alaska probably began at, or after, the very end of the Cretaceous.