UC Davis

For much of its length, the Denali fault juxtaposes the Wrangellia Composite terraneagainst the ancestral North American margin. However, the location of the main geologic boundary between the Wrangellia Composite terrane and the North American margin is obscured in the Alaska Range Suture Zone. In the Eastern Alaska Range, where persistent and rapid exhumation has been occurring since ~23 Ma, the origin and protolith of the rocks north of the Denali fault is unknown. We present new U-Pb detrital zircon geochronology from seven samples collected in the eastern Alaska Range located within this region of focused exhumation. The prevalence of suspected metamorphic grains, grains affected by lead loss, and newly crystallized grains due to regional plutonism provided challenges when filtering our data. Grains from each data set with U/Th ratios > 10, Uranium content > 5,000 ppm, or U-Pb ages > 3% error were removed. Additionally, neo-crystallized grains were identified and removed based on the known distribution of U-Pb zircon ages of plutons in the immediate field area. The maximum depositional age of our samples ranges from Late Devonian to Middle Triassic. These findings contradict previous mapping and suggest that many assemblages in the eastern Alaska Range will be mapped differently with future detailed mapping. It is possible rocks to the north of the Denali fault in the eastern Alaska Range have ties with the ancestral North American margin, which would be expected based on geographic location and would imply these rocks have remained in place. Alternatively, it is also possible that one of our samples has sediment sourced from the southern portion of the Alexander terrane, and two of our samples have sediment sourced from the Endicott Arm of the southeastern Yukon Tanana terrane. If these correlations are correct, this requires >1000 kilometers of transport, which requires slip on a separate structure from the Denali fault.