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The Ordovician Grampian Orogeny, Western Ireland: Obduction Versus “Bulldozing” During Arc‐Continent Collision

The Grampian Orogeny of western Ireland was formed during the mid‐Ordovician Grampian‐Taconic collision between a 5,600‐km system of oceanic arcs and the Laurentian margin. It is remarkable in that it preserves a complete sedimentary record of the arc‐continent collision process in the South Mayo Trough, which can be linked with deformation and metamorphism in the Dalradian sediments of the Laurentian margin and with the geochemical evolution of the colliding arc. The 1,650‐km‐long northern section of the Grampian‐Taconic allochthon records obduction of ophiolites, forearc, and arc during this event. We review factors that control the tectonic evolution of arc‐continent collision orogens and argue such obduction requires a ridge to intersect the arc and forearc a short period (probably ~10 my) before collision with a hyperextended, thermally mature margin. We propose that where such circumstances do not exist, the arc can either thrust over or “bulldoze” the margin. In the case of thrusting, the suture would dip moderately, and arc activity may continue during collision. A bulldozing arc produces a steep suture, and deformation is concentrated in the margin. Short‐lived Barrovian‐style metamorphism in the sediments of the margin is more likely when obduction occurs.