Abstract
The St. Elias Orogen is the result of ~10 Myr of oblique collision and flat-slab subduction in the Gulf of Alaska between North America (NA) and the Yakutat microplate (YAK). Extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and orogenic evolution in the vicinity of flat-slab subduction.

Joint tomographic inversion of coincident reflection and refraction profiles constrains YAK crustal velocity and thickness. The offshore YAK crust ranges in thickness from ~15 to ~30km, considerably thicker than normal oceanic crust. The crustal thickness and velocity structure support an oceanic plateau origin for the YAK microplate. Crustal velocity and structure are continuous across the YAK shelf except for a regional dip of the top of YAK crust of ~3 degrees to the west. Moho arrivals across the profile do not mimic the dipping trajectory of the basement, indicating that the YAK slab is wedge-shaped, thinning in the direction of subduction. This geometry leads to the following implications for the YAK-NA collision: first, uplift and deformation have intensified through time as successively thicker, more buoyant YAK crust attempts to subduct; second, current topography, exhumation and deformation patterns are partially controlled by underlying slab structure at depth.