[1] We present a two-dimensional velocity model to constrain crustal thickness and composition of the Yakutat terrane in the northern Gulf of Alaska. The model was constructed using seismic reflection and refraction data along a $455 km onshore-offshore profile. Our model shows that the crystalline crust composing the Yakutat terrane is wedge-shaped, with crustal thickness increasing west to east from $15 km to $30 km. Crustal velocity and structure are continuous across the terrane, with lower crustal velocities >7 km/s, suggesting that the Yakutat terrane is an oceanic plateau across its entire offshore extent rather than a composite oceanic-continental terrane as previously proposed. The thickest Yakutat crust is entering the adjacent St. Elias orogen where elevated exhumation rates and concentrated seismicity in this vicinity are likely influenced by incipient Yakutat-North America collision. Our model includes a $8 km thick low-velocity crustal cap extending across the eastern portion of the profile where shallow basement is imaged on marine seismic reflection data. We interpret this cap as a lithified, metamorphosed remnant accretionary prism, providing evidence of a previous attempt at Yakutat subduction along its eastern margin prior to current emplacement at the southern Alaska margin.

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Mantle shear velocity (Vs) structure beneath the Transportable Array (TA) in Alaska and northwestern Canada is imaged by joint inversion of Rayleigh wave dispersion and teleseismic S wave travel times. The study connects previously unsampled parts of northern and western Alaska with portions of southern Alaska imaged with earlier seismic arrays. The new Vs tomography shows contrasting lithospheric structure in the plate interior with lower Vs shallow upper mantle indicative of thinner thermal lithosphere south of the Brooks Range and along the transform margin. Higher Vs down to~200 km beneath the Brooks Range and northern coast is consistent with the presence of a cold stable lithospheric root that may help guide intraplate deformation to the south. In the subduction-to-transform transition, a potential slab fragment is imaged beneath the Wrangell volcanic field where modern subduction has slowed due to the thick buoyant crust of the Yakutat terrane.Plain Language Summary We use a groundbreaking seismic data set from the EarthScope project to investigate the structure of the upper mantle beneath Alaska and northwestern Canada to better understand the effects of ongoing subduction and distinctive blocks within the continental lithosphere. Measurements of seismic body and surface waves are used to construct seismic images from the surface down to 800-km depth. The images reveal cold thick blocks beneath northern Alaska and the Yukon Territory adjacent to warmer thinner blocks beneath younger geologic provinces to the south, suggesting that cold strong lithosphere in the north helps guide the extent of intraplate deformation driven by the southern plate boundary. The model also identifies a potential slab fragment beneath the Wrangell volcanic field, suggesting slab contributions to volcanic activity and a growing slab tear.

[1] The St. Elias orogen is the result of ∼10 Myr of oblique convergence and flat-slab subduction in the Gulf of Alaska between North America and the Yakutat microplate. Extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge. Reflection seismic profiles across the offshore Pamplona zone fold-thrust belt, the frontal St. Elias orogenic wedge, provide constraints for quantifying Pleistocene deformation recorded in the glaciomarine Yakataga formation. The total amount of Pleistocene shortening observed varies from ∼3 to 5 mm/yr, compared to the current GPS-derived YakutatNorth America convergence rate across the St. Elias orogen of ∼45 mm/yr. Growth strata and kinematic fold analysis allow comparison of relative timing of fault activity, which reveals temporal and spatial shifting of active deformation during the glacial period: faulting localized adjacent to the coastline and at the current submarine deformation front. The abandoned, currently inactive region is colocated with the major glacial depocenter in the region, the Bering Trough. These observations imply that glacial processes such as sediment loading and focused erosion during advance-retreat cycles has a direct effect on the evolution of individual faults within the Pamplona zone and the overall deformation pattern in the offshore St. Elias margin. This information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during Pleistocene glacial-interglacial periods. Citation: Worthington, L. L., S. P. S. Gulick, and T. L. Pavlis (2010), Coupled stratigraphic and structural evolution of a glaciated orogenic wedge, offshore St. Elias orogen, Alaska, Tectonics, 29, TC6013,

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Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8-1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2-0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50-80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale. O rogenesis reflects the balance of crustal material entering a mountain belt to undergo shortening and uplift versus material leaving the orogen through exhumation, erosion, and sediment transport (1-5). Perturbations in the influx/efflux from the orogen are expected to result in predictable changes in deformation within the orogen as it attempts to reestablish equilibrium (3). The long-term sink for sediment transported out of mountain belts is often in the deep sea, particularly in large submarine fans where sediments accumulate at anomalously high rates (>10 cm/ky) compared with deep-sea pelagic sedimentation (6-8). Even higher sedimentation rates (>100 cm/ky) proximal to glacially eroded regions (9-14) imply that wet-based glaciers are extremely efficient agents of erosion. Observations and modeling have argued that erosion rates can influence tectonic processes (15)(16)(17)(18)(19), but the timescales of adjustment, and the role of landscape disequilibrium, remain unclear. For example, exceptionally high local sedimentation rates (100-1000 cm/ky) recorded on the century timescale (13) SignificanceIn coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This...

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