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Thrust-emplaced tectonic units dominate the structure of the Scandinavian Caledonides. The lower units made up the Baltoscandian margin of the early Paleozoic continent Baltica. The thrust sheets that probably formed the outer part of this margin are treated here as suspect terranes. Three such terranes (1 through 3) are distinguished. The higher tectonic units, occurring as far-transported (>400 km) thrust sheets in the Upper and Uppermost Allochthons, are composed of exotic terranes derived from outboard of Baltica. Terranes containing, at least in part, oceanic sequences (4 through 9) have been distinguished from those comprising metamorphic complexes of less-certain affinity (10 through 12); a terrane (13) composed predominantly of exotic continental lithosphere forms the highest unit in the tectonostratigraphy.

A complex polyphase accretionary history is apparent for the outboard terranes. Late Cambrian to Early Ordovician collision of the Baltoscandian margin with an outboard arc complex resulted in the Finnmarkian orogenic activity. Erosion and, thereby, loss of outboard units emplaced by early Caledonian thrusting on the outer margin of Baltica is thought to have played a major role during later uplift. Middle Ordovician clastic sequences in one of the outboard terranes (4) and in a foreland basin are related to the uplift and erosion associated with the Finnmarkian event.

Several of the outboard terranes (5, 6, and 11) are inferred to have accreted to Laurentia (terrane 13?) during Early Ordovician continent-arc collision. This Laurentian accretionary episode resulted in the early Caledonian orogenic activity in these terranes and preceded rifting, opening of one or more marginal basins, and closure of Iapetus; proximity to Baltica was achieved only during Late Ordovician time. The early Caledonian tectonothermal activity recorded in several of the outboard terranes was, therefore, unrelated in space and possibly also in time to the Finnmarkian episode along the Baltoscandian margin. Subsequent continent-continent collision during the Silurian and Devonian resulted in the Scandian orogenic activity, both along the Baltoscandian margin and in the outboard terranes, and birth of the megacontinent Laurasia.

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