Cenozoic evolution of the Yakutat-North American collision zone, southeast Alaska

A better understanding of orogenic syntaxes is necessary in order to improve our knowledge of continental deformation, which impacts human life immensely. Orogenic syntaxes are kinematic transition zones and potentially concentrate large amounts of stress and strain, which can lead to frequent and high-magnitude earthquakes and mass wasting processes. The St. Elias syntaxis in the St. Elias Mountains of southeast Alaska and adjacent western Canada recently gained attention for its high exhumation and erosion rates, and by comparison to the “tectonic aneurysm” model that was developed for the eastern and western Himalayan syntaxes. The St. Elias syntaxis is the area where transform motion along the Fairweather Fault segment of the Yakutat-North American plate boundary transitions into flat-slab subduction of the thick, oceanic Yakutat crust. Detailed studies of exhumation processes at the St. Elias syntaxis are hampered by its extensive glaciation. The approach in this study therefore comprises the use of detrital material from large and small glacio-fluvial catchments and the application of multiple thermo- and geochronometric dating techniques in order to reveal the long-term exhumation history of the syntaxial region and its relation to other parts of the orogen. Cooling age populations were extracted from sand-sized samples and complete cooling histories (500–65 °C) and provenance information (U-Pb dates, lithology) were ob-tained from glacially transported cobbles and interpreted in combination with previ-ously published and new bedrock thermo- and geochronologic ages. A total of 4905 new single-grain zircon fission-track (ZFT) ages (~430–0.2 Ma) of modern, sand-sized detrital samples from 47 different catchments covering an area of almost 45,000 km2, 1350 new single-grain apatite fission-track (AFT) ages (~433–1 Ma) of modern, sand-sized detrital samples from 15 of the 47 catchments, five ZFT bedrock ages (~154–9.4 Ma), three bedrock biotite 40Ar/39Ar ages (~42–5 Ma), as well as data of 27 cobble-sized detrital samples with 21 zircon U-Pb ages (~277–31 Ma), eight amphibole 40Ar/39Ar ages (~276–16 Ma), seven biotite 40Ar/39Ar ages (~50–42 Ma), four zircon (U-Th)/He ages (~35–4.8 Ma), four AFT ages (~17–1.6 Ma), and six apatite (U-Th)/He ages (~4.2–0.6 Ma) are presented. Two large-scale terrane subduction and accretion phases influenced the upper crustal cooling of the study area; the Jurassic–Cretaceous accretion of the Wrangellia Composite Terrane to the former North American margin and the ongoing flat-slab subduction and collision of the Yakutat microplate. The Fairweather plate boundary segment has been transpressional in nature since at least 30 Ma and collision of the Yakutat microplate with the North American Plate began ~15–12 Ma. Rapid exhumation in the St. Elias syntaxis area began ~10 Ma and was confined by an unmapped, ice covered, discrete structure northeast of the northern Fairweather Fault and possibly the Fairweather Fault itself, most likely forming a one-sided, positive flower structure. The locus of rapid exhumation shifted southwest into the central syntaxis area at ~5 Ma and exhumation rate and depth were increased, causing ~10 km of exhumation ~5–2 Ma. This occurred probably due to a combination of i) an increase in the compressional component of Yakutat-North American convergence, ii) the subduction of increasingly thicker oceanic crust of the wedge-shaped Yakutat microplate, and iii) a change in erosional patterns and rates due to 6–5 Ma onset of glaciation. Pliocene exhumation might have been accommodated by a two-sided, positive flower structure centered at the northern Fairweather Fault, but with deep exhumation focused on the North American Plate. After ~2 Ma, the focus of most rapid exhumation migrated farther south to the lower plate of the syntaxial region (Yakutat microplate). Overall, the results indicate that syntaxial regions should be treated as 4D-problems with spatio-temporally heterogeneously distributed deformation and exhumation. If process rates are high, as in the case of the St. Elias and Himalayan syntaxes, then, the dynamics of these regions are likely to respond very quickly (0.5–1.0 Myr) to changes in tectonic, rheologic, and climatic settings.

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