IS BRITAIN DIVIDED BY AN ACADIAN SUTURE?

The Appalachian orogen and its transatlantic continuation in the Caledonides of Europe were developed during the progressive Paleozoic closure of the Iapetus Ocean, resulting in multiple episodes of deformation, metamorphism, and magmatism between late Cambrian and Middle Devonian. In Britain and Ireland, the Iapetus is typically interpreted as closing along the Solway–Navan line in the mid-Silurian. The later Acadian orogeny is recorded by widespread cleavage development in slate belts, beneath a regional mid-Devonian unconformity. The Acadian orogeny in Britain has been attributed to sinistral transpression associated with plate convergence farther south, in the Rheic Ocean.

In contrast, in the Appalachians, the Acadian orogeny led to regional nappe formation, Barrovian metamorphism, and granitoid intrusion in New England. Its effects are less prominent as traced northward through the Appalachians, but it is recorded by regional mid-Devonian unconformities, and by more localized high-grade metamorphism in southern New Brunswick, Cape Breton Island, and Newfoundland. In the Appalachians, the Acadian orogeny is typically attributed to the accretion at the Laurentian active margin of a Gondwana- or Baltica-derived microcontinent, West Avalonia.

Detrital zircon distributions in Paleozoic successions south of the Solway–Navan line in Britain show that Laurentia-derived zircon spread rapidly southwards across this line into the Leinster–Lakesman terrane in the Wenlock, but was interrupted between 12 and 26 Myr at a boundary between the Welsh basin and the English Lake District, here termed the Môn line. Early Devonian calc-alkaline magmatism is widespread to the northwest of this line but is absent to the southeast. We interpret the Môn line as a second suture of Devonian age. We correlate this boundary with faults that mark the southeastern limit of Acadian magmatism and metamorphism in the Appalachians: the Dover Fault in Newfoundland, the Eastern Highlands Shear Zone in Cape Breton Island (Nova Scotia), the Kennebecasis fault in New Brunswick and the Lake Char fault in southern New England. Oblique subduction at this boundary helps to explain the distribution of both deformation and magmatism during the Early Devonian in Britain and Ireland.