Provenance analyses: a tool to constrain tectonic changes. Parallelism between the ancient Yukon-Koyukuk Basin (Alaska) and modern Southeast Asia.

We present heavy minerals, detrital zircon data and U-Pb tuff ages from the sedimentary units of the Yukon-Koyukuk Basin (YKB) in Alaska, to investigate and understand its evolution within a tectonic framework. Two basins in Southeast Asia, with similar tectonic settings, serve as modern analogues for comparison. The YKB started to form in middle to late Jurassic after the collision between an intraoceanic volcanic arc and the Arctic Alaska margin, which eventually led to the thrusting of the oceanic basement over the passive continental margin. The basin is flanked by the metamorphic rocks of the Seward Peninsula to the west, the Brooks Range to the north and the Ruby Terrane to the east.

Our results are from the whole stratigraphic column of the basin, from the lowermost Kv unit to the uppermost Ks unit:

• Kv unit: basaltic and andesitic lava flows interbedded with volcanogenic sediments. K-Ar ages vary from 134 Ma and 118 Ma. A single U-Pb zircon age from a tuff is about 138 Ma.
• Kvg unit: volcaniclastic greywacke with Albian molluscs are interbedded with tuffaceous layers. Two of these are dated at 112 Ma and 110 Ma (Albian).
• Kmc unit: conglomerate and greywacke with a strong mafic and calcareous imprint. Cretaceous molluscs are widespread.
• Kqc unit: quartz rich deposits with plant fossils of Cretaceous age.
• Ks+Kms units: late Cretaceous fluvial to shallow marine and deep marine sandstone and shale layers.

We use QEMSCAN® (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) for heavy mineral (HM) analysis, which, along with point counting, petrographic description, paleocurrent data and sedimentary features analysis, helps to constrain sediment provenance. Kv is the only unit reflecting a volcanic source, while the others display the progressive unroofing of the deeper and higher-grade metamorphic rocks surrounding the basin. U-Pb SIMS dating of tuffs and detrital zircons provide, for the first time, an absolute chronology for basin evolution.

The combined data indicate the YKB formed in a forearc/backarc setting prior to 138 Ma (deposition of the volcanic-rich Kv unit). With the progressive denudation of the Brooks Range, sedimentation shifted towards greater metamorphic input, leading to the deposition of the Kvg unit at 110 Ma. Ultimately, after the Late Cretaceous uplift of the Ruby Terrane, the main sediment source changed again, and the stratigraphically higher units were deposited from the Cenomanian to at least the Maastrichtian. The Savu Sea and the Taiwan margin in Southeast Asia show similarities in structure, metamorphic grade, sediment composition and tectonic setting, suggesting these are effective modern analogues that can enhance our understanding of both ancient and modern arc-continent collisional systems.