Subduction and loss of continental crust during the Mesoproterozoic Sveconorwegian Orogeny

The late Mesoproterozoic Sveconorwegian Orogeny in SW Fennoscandia is characterized by tectonically bound units that record different metamorphic, magmatic, and deformation histories, interpreted to indicate separation by some unknown distance prior to orogeny. New zircon U–Pb and Lu–Hf isotope data from a 1200 km-long NE–SW transect including Archean to 1450 Ma rocks constrain the likely age and isotopic architecture of western Fennoscandia prior to the late Mesoproterozoic Sveconorwegian Orogeny. Zircon age and Hf-isotope patterns indicate that the units comprising the Sveconorwegian Province are both younger and isotopically more juvenile than the surrounding autochthonous Fennoscandian crust, and thus most likely derived from west of the present-day Norwegian coastline. The Mylonite Zone defines a major tectonic structure separating allochthonous Sveconorwegian units in its hanging wall from autochthonous Fennoscandian crust in its footwall. New and compiled metamorphic age data demonstrate that the Mylonite Zone can be traced westward through the Western Gneiss Region, aligning with Nordfjord in western Norway, where it was reused during Caledonian deformation. The proposed westward continuation of the Mylonite Zone accommodated several hundred kilometers of sinistral strike-slip movement. Eastward translation of crust probably took place sometime between 1020 and 990 Ma, coinciding with a magmatic lull, followed by a shift to more evolved isotopic compositions in the hanging wall (Telemark) and high-pressure eclogite-facies metamorphism in the footwall (Eastern Segment) to the Mylonite Zone. Following this relatively short period of compression, the entire orogen and its foreland underwent extension lasting until at least 930 Ma. The nature and fate of the ca. 500 km of crust originally separating the autochthonous and allochthonous units remain elusive. There is no evidence of arc magmatism related to Benioff-style subduction of oceanic crust, and thus we propose an amagmatic Ampferer-style subduction comprising spontaneous subduction of thinned continental crust, as proposed for the Western Alps. Subduction of continental crust and associated radioactive heat-producing elements could also account for the anomalously high temperatures in the lithospheric mantle under the Sveconorwegian Province, which cannot easily be accounted for by other mechanisms. The Sveconorwegian Province may be an anomalous feature in an otherwise larger-scale orogen, the nature of which remains obscure.