Greater Avalonia—latest Ediacaran–Ordovician “peribaltic” terrane bounded by continental margin prisms (“Ganderia,” Harlech Dome, Meguma): Review, tectonic implications, and paleogeography

Distinctive uppermost Ediacaran–Ordovician rocks (Avalonian overstep sequence) were key in definition of Avalonia, which comprises large areas of the NE Appalachians and the Caledonian and Variscan mountains (Britain and western Europe). This siliciclastic-dominated cover succession unconformably overlies a Neoproterozoic pre-Avalonian basement collage (arc, continental, oceanic fragments; Gwna-type melanges [now Rhode Island–Maritime Canada–North Wales]). Rather than an arc–transform transition, our new model proposes that the Avalonian basement was amalgamated after inferred late Ediacaran ridge-trench collision that initiated the northerly–southerly trending Avalonian transform fault (Atf, new). Avalonia is commonly regarded as originating as an arc marginal to Gondwana in the early Paleozoic and detached only in the Ordovician. We review multiple lines of evidence that show it was an insular microcontinent; this require revisions of Ediacaran–early Paleozoic paleogeography and plate tectonics models. Detrital zircon isotopic data show an Avalonia–Baltica link to the Neoproterozoic Timanian orogen, with Avalonia a peribaltic, not perigondwanan, terrane. The Avalonian basement detached from Baltica and rotated parallel to the Atf, with its origin similar to the modern Scotia Sea plate (i.e., North Scotia Ridge and transform) by accumulation of Neoproterozoic arc and continental fragments on the transform. Deposition of the cover sequence beginning ca. 552 Ma in pull-apart basins on the basement marked origin of the Avalonia ribbon-microcontinent. The cover succession, with ten unconformity-bounded depositional sequences, extends for ca. 5000 km (eastern Massachusetts–Silesia). Avalonia is bounded on its NW and SE by siliciclastic rocks (i.e., Gander and Meguma belts with an Ordovician arc in Gander) that were continental margin prisms coeval with the overstep sequence. Endemic Avalonian lower Lower Cambrian (Terraneuvian–lower Series 2, ca. 538–510 Ma) faunas, dropstones, absence of archaeocyaths, and minor shallow-water carbonates suggest an isolated location at ≥50° S. Only in the latest Early Cambrian (ca. 506 Ma) do Avalonian faunas show NW African, “Gondwanan” affinities with Avalonia-Gondwana convergence on the Atf. A longer (ca. 28 Ma, Middle Cambrian–Tremadocian) faunal similarity links temperate Avalonia and Baltica as terranes on the same plate. An insular Avalonia is consistent with a Middle Cambrian, ca. 49° S latitude (Cape Breton Island) location—its only reliable Cambrian paleomagnetic datum. Meso- and Paleoproterozoic zircons do not record nearby West Gondwana margins, but were eroded from Avalonian basement. Purported “Ganderian” plutonic zircons and whole rock signatures in SW New Brunswick indicate heterogeneity between Avalonian basement blocks, not a “Ganderia” affinity before a purported transfer of parts of Avalonia to “Ganderia.” Coeval extensional and collisional igneous rocks along the Atf is seen in regions of transform faulting and do not allow reference of Avalonian areas to “Ganderia.” Avalonia should not be shown as part of West Gondwana or separating from it in the Ordovician.