Neoproterozoic palaeogeography in the North Atlantic Region: Inferences from the Akkajaure and Seve Nappes of the Scandinavian Caledonides

Abstract

The Scandinavian Caledonides exposes increasingly far travelled nappes upwards (westwards). The Lower- to Middle Allochthon are widely regarded as indigenous to the pre-Caledonian margin of Baltica (Fennoscandia), while higher thrust sheets (e.g. lower Upper Allochthon) have more controversial ancestries. Recent studies have suggested that thick early-Neoproterozoic (Tonian-Cryogenian) metasedimentary sequences hosted in thrust sheets of the Scandinavian, Greenland, British and Svalbard Caledonides represent two cycles of sedimentation along the peri-Rodinia (Laurentia-Baltica) margin of the supercontinent; alimented by erosion of the Grenvillian and Sveconorwegian orogenic belts. To test and expand this model, we report zircon U-Pb and whole rock Sm-Nd data from orthogneisses and siliciclastic metasediments in a transect from the (para)autochthonous Dividal Group, through the crystalline Middle Allochthon Akkajaure Nappes and into the overlying Seve Nappes (lower Upper Allochthon).The Akkajaure Nappe Complex (Middle Allochthon) is dominated by Precambrian crystalline rocks, of granitic to dioritic composition, that yield ages of 1788 ± 6, 1806 ± 15, and 1876 ± 10 Ma. These rocks are thrust over an imbricated basement (Lower Allochthon), which contains rocks of similar age, including a rhyolite dated at 1790 ± 6 Ma. The ages and Nd isotope chemistry of these rocks are identical to those from other Fennoscandian basement units and imply derivation from west of the Lofoten islands. A sample of sediment, originally attributed to the lowest unit of the overlying Seve Nappe Complex, has a detrital population dissimilar to all other Seve-Kalak Nappe samples and has a significant 1800. Ma volcanic component.This sediment was intruded by granite at 1797 ± 4. Ma. Its provenance and rapid depositional timing indicates it is better considered as part of the tectonically captured Fennoscandian basement (Middle or Lower Allochthon).Metasediments from higher Seve nappes, in Jämtland, were deposited after 730. Ma and contain a detrital zircon population indistinguishable from many sediments within the wider North Atlantic Region (NAR). Based on the similarity in detrital zircon age populations, depositional timings, and the apparent lack of material from the local Fennoscandian crystalline basement, these Jämtland Seve sediments are interpreted as peri-Rodinian deposits, which were carried along the passive margin of Baltica during the opening of Iapetus (after 610. Ma). The strong similarity of detritus across NAR sedimentary units and the Rodinian paleogeography this implies, indicates that the exotic versus endemic derivation of nappes is more relevant in terms of which side of Iapetus these rocks were sitting after rifting; Baltica side (endemic) versus Laurentia side or microplate (exotic).The detrital zircon population from the early Cambrian part of the (para)autochthonous Dividal Group is identical to that from the Jämtland Seve and modern river sands flowing off the nappes. This indicates that the Seve Nappes were in sufficient proximity to Baltica by the Cambrian to shed detritus into the Dividal Group. This supports the notion of Ordovician arrival of peri-Rodinian units onto Baltica.