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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 an island-arc complex, an ophiolitic backstop, and polymetamorphic trench sediments. These are conformably overlain by marine deposits derived from a more evolved arc complex and an emerging juvenile orogen. This transition is dated as being coeval with the Grampian metamorphism of the Laurentian footwall. The problem remains as to why subsidence continued in a basin on the hanging wall. It is proposed that the suppression of the expected topography is due to the nature of the Laurentian continental margin. Geophysical and geological evidence suggests that this was a volcanic margin during Neoproterozoic rifting. It is argued that the subduction of this margin caused the formation of eclogites, which reduced its buoyancy. Simple numerical models are presented which show that this is a viable mechanism for the suppression of topography during early stages of arc-continent collision and hence for the preservation of forearcs.

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