The Contribution of the Finnmarkian Orogeny to the Framework of the Scandinavian Caledonides

Researches in the last decade have demonstrated that the Scandinavian Caledonides are a complex montage built-up during three successive and distinctive orogenic cycles. The earliest of these — the Finnmarkian Orogeny had a unique significance in the evolutionary pattern. In Late Precambrian times the Laurento-Baltic super-craton was rifted and the opening and spreading of the Iapetus Ocean initiated. Clearly marked continental miogeoclines were developed on either sides of the ocean during the Cambrian, and major subduction was initiated, seaboard of Scandinavia, in late Cambrian times. Ensimatic island arc complexes were developed at this stage and the subduction polarity was arguably westward. This major stage of oceanic contraction resulted in the destruction of vast quantities of ocean floor by subduction and the obduction of major ophiolites onto the deforming rise prism. This latter was subjected to polyphasal deformation and metamorphism culminating in continentwards thrusting over the western part of the Baltic Shield. The type development for the Finnmarkian Orogeny is in the northern part of Norway, though evidence can be found throughout the belt, where either Finnmarkian metamorphics or ophiolites are truncated by a major first- order unconformity capped by Ordovician sediments of varied age. The morphogenic stage of the Finnmarkian Orogeny was essentially a feature of the Lower and Middle Ordovician.