The Palaeozoic geography of Laurentia and western Laurussia: A stable craton with mobile margins

The large Palaeozoic continent of Laurentia was largely in North America, but included parts of modern Europe. It was independent from late Neoproterozoic times at about 570 Ma until it merged with Avalonia–Baltica in the 430–420 Ma Silurian Caledonide Orogeny, after which it formed the major western sector of the combined Laurussia Supercontinent. Laurussia in turn became part of the even larger Pangea Supercontinent in the Late Carboniferous, as documented by the oblique Laurussia–Gondwana collision seen in the Laurentian sector in the Ouachita Orogeny. Laurentia's margins and the many peri-Laurentian terranes are reviewed. Those parts of northeast Siberia which today form parts of the North America Plate, but were not part of Laurentia or Laurussia in the Palaeozoic, are also reviewed. A revised Apparent Polar Wander Path (APW) for the Laurentian Craton is presented for all of the Palaeozoic. Laurentia was at equatorial palaeolatitudes throughout and rotated little, apart from shortly after its collision with Avalonia–Baltica in the Silurian Caledonide Orogeny; however, in contrast, its position and orientation were much less affected in the Ouachita Orogeny at the time of Pangean assembly. The Laurentian Craton was variably flooded at many times with epeiric seas, which formed optimal numbers of ecological niches which in turn encouraged animal speciation and evolution. A summary is presented of the Palaeozoic geological history of Laurentia and its surrounding areas, and the Laurentian sector of Laurussia during and after its integration within Pangea, together with new palaeogeographical maps from the Cambrian to the end of the Permian. On those maps there are plotted areas of land, shallow shelf, deeper shelf and oceans derived from much pre-existing data, as well as reefs, volcanic and plutonic rocks and some selected faunas and floras. The substantial number of terranes at the margins of the continent through time are briefly reviewed, notably those in Mexico, the Appalachians, and northwestern parts of Europe which were once parts of Laurentia. The many terranes containing Palaeozoic rocks in the northwestern part of the North American Plate, and forming much of the Cordillera in northwest Canada and Alaska as well as northeast Siberia, are itemised: some were periLaurentian, some peri-Siberian, and others originally oceanic in the Palaeozoic. The concept of an Arctida Microcontinent is discussed. That microcontinent had originally been postulated as existing from the Neoproterozoic to the Devonian, and to have consisted of the composite Arctic Alaska and the Seward, York and Farewell terranes in Alaska, and the Pearya Terrane of Ellesmere Island, as well as the Chukotka Peninsula, New Siberian Islands, Severnaya Zemlya, northern Taimyr and adjacent areas now in the northeast of modern Siberia. Many parts of that area contain faunas of both Siberian and Laurentian aspect, which are reviewed and analysed. It is concluded that there was a smaller independent continent in the Lower Palaeozoic, which was originally somewhere between Siberia and Laurentia in the Cambrian, but which did not include the New Siberian Islands, Kolyma and Omolon (which were parts of Siberia), Severnaya Zemlya and northern Taimyr (the independent Kara Microcontinent), or the Farewell Terrane (independent until the Mesozoic). The eastern end of that Arctic Alaska–Chukotka Microcontinent docked with northwestern Laurussia (Ellesmerian margin) in the Devonian, but it did not reach its present position within North America until after rotation in the Cretaceous. The Cordilleran terranes of Wrangellia, Alexander and some smaller units are confirmed as having existed as another microcontinent independent from North America until the Mesozoic. However, there appear to be no terranes now in western North America which originated from Baltica. The Pearya Terrane, now forming northern Ellesmere Island, was probably involved in the most northerly sector of the Silurian Caledonide Orogeny.