PRESENT STATUS OF NEOPROTEROZOIC-ORDOVICIAN PLATE FORENSICS IN THE NORWEGIAN CALEDONIDES

The Scandinavian Caledonides comprise far-travelled allochthons thrust on Baltica during Siluro-Devonian (Scandian) collision with Laurentia. The Lower and Middle Allochthons contain platformal, continental-rise and ocean-margin rocks believed indigenous to Baltica. By contrast, the Upper and Uppermost Allochthons, dealt with here, contain rocks derived from the Laurentian margin or diverse parts of Iapetus.

The Laurentian margin in North America is characterized by a Cambrian - Mid Ordovician carbonate bank, overridden by Taconian thrust sheets involving siliciclastic strata and obducted ophiolites. Fossiliferous Laurentian carbonates are unknown in Norway. However, recent chemostratigraphic analysis of marbles in Nordland has identified the same carbonate bank, imbricated during NW-thrusting and unconformably overlain by Late Ordovician and Silurian strata prior to Scandian orogeny. In SW Nordland, sheets of high-grade rocks metamorphosed at ~470 Ma were thrust NW over low-grade units including ophiolites and siliciclastic strata, and this 'Taconian' boundary was then stitched by plutons at ~445 Ma.

The region of west Central Norway comprises 497-481 Ma ophiolite fragments overlain by volcanic-sedimentary strata extending at least to latest Ordovician. At Hølonda, ophiolite obduction was succeeded, first by barren sediments, then by those with a Late-Arenig (~469 Ma) to Llanvirn fauna of clear Laurentian affinity. Parts of this warm-water fauna also occur in volcanic-sedimentary strata on the island of Smøla, truncated by Late Ordovician (~446 Ma) plutons. The fauna implies that mature arc magmatism and, in some opinions, ophiolite obduction, occurred at or near the Laurentian margin at equatorial latitudes.

The above relationships are in sharp contrast with two localities farther south and east, where successions may relate to land areas at high latitude. At Otta, unconformably on an obducted ophiolite, an Early Llanvirn fauna of cool-water Celtic stock is identical to that of the Gander terrane in the northern Appalachians. At Nordaunevoll, Tremadocian U-rich graptolitic shale shares chemical characteristics with black shales in Baltica, Ganderia and Avalonia. Thus, the "Red Indian Line" marking the Taconian boundary between Laurentia and Ganderia may exist in Norway.