Avalonia: a long-lived terrane in the Lower Palaeozoic?

Avalonia was undoubtedly an independent terrane throughout the Ordovician, merging with Baltica at about Ordovician–Silurian boundary times (443 Ma). We consider it to have been internally unified throughout the Lower Palaeozoic, and not the two independent ‘East’ and ‘West’ Avalonian terranes of some authors. In the early Ordovician its faunas were certainly Gondwanan. However, its earlier history is controversial: we discuss whether Avalonia split off from Gondwana in the early Ordovician, or whether it left Gondwana in the late Neoproterozoic. We conclude that the varied basement terranes underlying Avalonia were aggregated to the margin of Gondwana before 650 Ma. Some substantial transform movements occurred along the Gondwanan margin between 610 and 530 Ma, but the Avalonian area remained part of core Gondwana until about the end of the Cambrian (490 Ma), when the rift–drift initiation of the opening Rheic Ocean between Gondwana and Avalonia began.