GEOPHYSICAL EVIDENCE THAT OCEANIC ROCKS UNDERLIE THE YUKON FLATS BASIN, ALASKA

Compilation of new and existing aeromagnetic and gravity data from the 46,000 km2 Yukon Flats basin, northeastern Alaska, reveals that much of the basin is underlain by Devonian to Jurassic oceanic rocks related to the Tozitna and Angayucham terranes in Alaska and the Slide Mountain terrane in Canada. This compilation and interpretation was done in support of a new evaluation of the basin’s petroleum potential. We produced a red-green-blue composite color map consisting of: (1) reduced-to-the-pole magnetics, (2) magnetic potential and (3) “basin-stripped” gravity. Domains on the composite map share common geophysical characteristics and can be related spatially to bedrock units exposed on the basin borders.

Examination of the geophysical domains indicates that dense and locally magnetic rocks underlie most of Yukon Flats basin. The same geophysical signature is mapped over bedrock exposures of the Devonian to Jurassic Angayucham-Tozitna terrane (ATt), a lithologically diverse assemblage of brittley deformed, generally low-grade metamorphic rocks of oceanic affinity exposed over broad areas on the borders of the Yukon Flats basin. Metagabbro, metabasalt, chert, and argillite are common lithologies in the ATt. In the subsurface, the ATt geophysical signature extends from the south, along the Tintina fault system, northward to the Brooks Range. Along the eastern edge of the Yukon Flats basin, the ATt appears to overlie moderately dense and non-magnetic Proterozoic(?) and Paleozoic continental margin rocks exposed in the adjacent Porcupine Plateau. The western edge of the ATt in subsurface is difficult to distinguish due to the presence of magnetic granites similar to those exposed in the Ruby geanticline. However, surface geologic mapping in this area indicates that the ATt rests structurally above, and presumably has been thrust over, rocks of the Ruby geanticline. We can’t show with our data whether continental rocks of the Ruby geanticline and Porcupine plateau meet under the ATt in the Yukon Flats basin; there may be sections of the basin, like the Yukon-Koyukuk basin, where rocks of oceanic affinity extend to the base of the crust.