A PRELIMINARY AEROMAGNETIC VIEW OF THE PORCUPINE RIVER REGION, NORTHEASTERN ALASKA

The Porcupine Plateau of northeastern Alaska likely holds important clues about the tectonic evolution of the North American Arctic region. The region contains bedrock of both North American and Arctic affinities together with fundamental geologic structures such as the Kaltag-Porcupine fault system that sutured disparate lithotectonic assemblages together and accommodated large-scale horizontal transport of crustal fragments. The region also contains the Old Crow batholith, a Paleozoic granitoid body with associated occurrences of tin, tungsten, uranium, and rare earth element mineralization. The geology of this remote part of interior Alaska is not well understood because of a lack of modern geologic mapping and geophysical data. A new aeromagnetic survey (300 m line spacing, 230 m average terrain clearance) was flown along the Alaska–Yukon border in 2017 to give context to anticipated geologic framework studies in the region. Shallow bedrock throughout the survey area is dominated by sedimentary strata that have little to no aeromagnetic expression. Instead, aeromagnetic anomalies are primarily sourced by deeply buried rocks and isolated near surface igneous rocks. Hypothetical crustal-scale structures such as the Kaltag-Porcupine fault system, which is thought to project into Alaska from the Yukon along the Porcupine River corridor, are not clearly expressed aeromagnetically. However, basement northwest of the Porcupine River is more strongly magnetized than that to the south and numerous anomalies have a northeast alignment. Long-wavelength, northeast-trending aeromagnetic highs (sourced by deeply buried rocks) are present on both sides of the Porcupine River, although their frequency is greater in rocks associated with the Arctic Alaska terrane to the north. The anomaly sources are unknown, but an isolated exposure of a strongly magnetized mafic Proterozoic intrusion provides one possible explanation. Volcanic rocks south of the river produce an isolated, northeast-trending set of anomalies. Basalts of the Miocene Porcupine volcanic field, considered to post-date significant deformation events, produce numerous short-wavelength anomalies. Various margins of the irregularly-shaped volcanic field have northeast-striking trends, suggesting structural control on emplacement.