The Ordovician Grampian Orogeny, Western Ireland: Obduction Versus Bulldozing During Arc-Continent Collision: 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 similar to 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.Plain Language Summary Island arcs occur when one dense oceanic plate slides, or subducts, beneath another. If the subducting plate is attached to a continent, an arc-continent collision will occur, which can produce high mountains at the continental margin. This is happening in Taiwan today. This process, which has taken place over perhaps half of geological time, produces a great variety of mountain belts. The island arc and the oceanic floor in front of it may override the continent or may bulldoze into it. The reasons for this variability are not clear. We examine ancient arc-continent collisions, concentrating on the Grampian event of western Ireland, which holds critical clues. We argue that it involved collision between a hot buoyant arc cut by an oceanic ridge, which was able to ride over an old, low-lying, and sediment-covered margin. Comparison between ancient and modern examples improves understanding of a process responsible for considerable geo-hazards in Taiwan.