Academic dissertation for the Degree of Doctor of Philosophy in Geology at Stockholm University to be publicly defended on December 3, 2025 in William-Olsson lecture hall, Geoscience building, Stockholm University.

Supervisor

• Victoria Pease, Professor, Department of Geological Sciences, Stockholm university.

Opponent

• Stephen Box, Doctor, United States Geological Survey (USGS), United States.

Abstract

The Yukon-Koyukuk Basin (YKB) is a large wedge-shaped depression south of the Brooks Range orogen in Alaska. It consists of two sub-basins, the northern Kobuk-Koyukuk Basin and the southern Lower-Yukon Basin, separated by remnants of the Koyukuk Arc Terrane. In the Late Jurassic, this arc began colliding with the Arctic Alaska continental margin, resulting in Brookian orogeny. Hypotheses for YKB formation vary from a forearc-backarc setting to formation during post-collisional lithospheric extension of the Brookian hinterland. Although this point remains debated, part of YKB development is clearly related to Brookian orogenesis. While the beginning of the collision is well constrained by synorogenic deposits on the northern side of the Brooks Range, the evolution of the YKB remains unclear. To test which model best describes basin formation and to better assess basin evolution, this thesis investigates the sedimentary units deposited within it. Sediments are effective in recording orogenic evolution, documenting progressive mountain belt denudation, linking erosional detritus to distinctive source areas, constraining depositional ages through U-Pb mineral dating, and more.

This PhD thesis characterizes YKB sedimentary units using point counting of framework grains (quartz, feldspars, lithics) in thin section to determine composition, degree of transport, and maturity, and, together with new paleocurrent data, defines sediment dispersal patterns. Heavy mineral analysis further refines provenance and associated source regions. U-Pb analyses of volcanic and detrital zircons provide absolute age constraints and define shifts in sediment supply from early volcanic- to later metamorphic-dominated input. Zircon trace element chemistry provides insight into magmatic sources and allows evaluation of tectonic environments in which magmas formed.

Integrated data suggest active sedimentation of deposits with distinct volcanic input characterized by strong continental contribution at ~138 Ma (Valanginian), consistent with the forearc-backarc model rather than the extensional scenario. This was followed in the late Early Cretaceous (~112 Ma, Albian) by deep-marine turbiditic deposits with strong metamorphic input from the surrounding highlands, later overlain by more proximal conglomeratic deposits at ~100 Ma. During the Late Cretaceous, inferred east-west convergence between the North American and Eurasian plates possibly caused additional uplift of the Ruby Terrane. Its enhanced erosion marked a major provenance shift, with sediment supply moving from the Brooks Range in the northern Kobuk-Koyukuk Basin to the Ruby Terrane in the southern Kobuk-Koyukuk Basin and eastern Lower-Yukon Basin. Increased Ruby Terrane uplift further confined the southern Kobuk-Koyukuk Basin, resulting in rapid infill and deposition of fluvio-deltaic deposits. In addition, a structurally controlled valley cutting across the Koyukuk Arc Terrane funnelled sediment to the backarc basin, leading to the deposition of slurry beds and directly connecting the forearc and backarc domains.

Comparisons with modern systems such as the Banda Arc-Timor and Luzon Arc-Taiwan highlight strong similarities in tectonic setting, metamorphic grade, volcanic geochemistry, and basin stratigraphy. These parallels improve understanding of sedimentary basins formed in arc-continent collisional settings, providing fundamental insights for refining models of their formation and evolution.

Link to DiVA