Forearc basins in southern Alaska were changed by the subduction of a spreading ridge and thick oceanic crust. This altered the usual tectonic setup and created unique sedimentary and volcanic records. The spreading ridge caused uplift and deposition of different types of sediments in the basin. A slab window beneath the basin led to local high points and volcanic centers. These findings show how flat-slab subduction can significantly impact the structure and content of forearc basins.