A new model is proposed for the problematic preservation of an Ordovician forearc basin, which records a complete sedimentary record of arc-continent collision during the Grampian (Taconic) orogeny in the west of Ireland. The South Mayo Trough represents an arc and forearc complex developed above a subduction zone in which the slab dipped away from the Laurentian passive margin. The collision of this arc with Laurentia caused the Middle Ordovician Grampian orogeny. However, the South Mayo Trough, in the hanging wall of this collision zone, remained a site of marine sedimentation during the entire process. Early sediments show derivation from...