Crustal structure of the Yakutat terrane and the evolution of subduction and collision in southern Alaska

Abstract

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.