Orphan terranes and tectonic names

The period from the Early Ordovician to the Late Silurian involved the assembly of much of the continental lithosphere that now surrounds the North Atlantic.  British geologists refer to this as the Caledonian orogeny, a term coined long before the events that welded the bulk of the British Isles were even dreamt of, let alone understood.  They are now in the embarrassing position (although most show few signs of grave discomfiture) of using the same term for at least two completely unrelated tectonic events.  Clinging to the old name, they now refer to mountain-building events around 470 Ma, during which accretion of an arc terrane to Laurentia resulted in the famous “fountain of nappes” of the Dalradian and Moinian Supergroups, as the “Grampian phase of the Caledonian orogeny”.  Now, I am all in favour of retaining a sense of history in nomenclature, but the fact is that northern Scotland is now known to have been part of Laurentia for a good billion years before this event.  Moreover, the offending island arc was first recognised on the eastern seaboard of North America, where it was dubbed the Taconic Arc; hence the Taconic orogeny there.  About 60 to 70 Ma later, the Avalonia terrane (also named first by North American geologists from a peninsula in Newfoundland) collided with this earlier orogenic belt in Laurentia.  North American geologists, for reasons of their own, refer to the deformation and metamorphism that ensued as the Acadian orogeny.  The British Isles experienced exactly the same event, yet it is referred to as the “Acadian phase of the Caledonian orogeny” – not the Cumbrian, as one might expect from the parochial considerations that prefer “Grampian” to Taconic, for the Iapetus suture that divides terranes north and south in Britain probably lies beneath northern Cumbria.  How confusing this is, and how unnecessary!

 The plot thickens in Scandinavia, long renowned for the pandemonium of orogenies dating from Palaeoproterozoic times.  There, tectonic events around 470 Ma are the “Finnmarkian phase of the Caledonian orogeny”, and those which closed the Lower Palaeozoic are the “Scandian phase”.  Norse, Swedish and Finnish geologists can be excused for sticking with their palaeotoponymy, because Scandinavian lithosphere was a separate entity from Laurentia during these times – Baltica.  The comforting isolation of Baltica had been thought to have ended with its accretion to Laurentia when the “Old Red” continent (Laurussia) formed.  Not entirely so.  Norway is now the proud custodian of a bit of the Taconian orogen (Yoshinobu, A.S. et al. 2002.  Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny. Geology, v. 30, p.883-886).  However, that does not make a unification of Baltica’s tectonic nomenclature with Laurentia sensible, because the sliver seems to have travelled a vast distance from its parent.  Hence “orphan”, because it was emplaced as one of the many nappes of western Scandinavia.  British geologists should take no comfort from this, and it is about time that they accepted a common tectonic history for the whole of Laurentia, otherwise their parochially-named orogenies might justifiably be called “bastards”!


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