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Thrust sheets of the Uppermost Allochthon in the Caledonides of Scandinavia are distinguished by lithological assemblages and magmatic units that are, in many ways, quite different from those in subjacent nappe complexes. Supracrustal successions are derived mainly from platformal, shelf-edge and basinal-slope environments and are characterized in particular by extensive developments of carbonate rock units that range in age from Late Riphean to Early Silurian. Metasedimentary iron ore formations are also present. Another prominent feature is the Ordovician, arc-type, granitoid plutons and batholiths that dominate the geology in certain parts of the allochthon. In addition to these lithological elements, the Uppermost Allochthon carries an Ordovician tectonothermal record and early Caledonian, NW-vergent thrust polarity that is unique in Norway. Taken together, these features are indicative of a history of development and crustal growth along the eastern margin of Laurentia, involving an outboard magmatic arc, or arcs, and Taconian accretionary orogenesis. This was followed by recycling and deposition in Late Ordovician to Early Silurian successor basins prior to Laurentia-Baltica collision and the onset of the Scandian orogeny. The Taconian thrust sheets were then detached from their Laurentian roots and incorporated into the Siluro-Devonian, Scandian orogenic wedge on the Baltoscandian margin of Baltica. Taking into account the widely reported sinistral megashear arising from the Scandian, oblique collision and plate rotation, the rock units that constitute the Uppermost Allochthon are likely to have originally been located closer to the northern Appalachian segment of the margin of Laurentia, in view of the strikingly similar lithostratigraphic, magmatic, and tectonothermal histories of these two, now widely separated terranes.

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