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Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion
Department of Earth Sciences, Uppsala University, Villavägen 16, Uppsala, Sweden.
Department of Earth Sciences, Uppsala University, Villavägen 16, Uppsala, Sweden.
Department of Geology, Lund University, Sölvegatan 12, Lund, Sweden.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
2020 (English)In: Sweden: Lithotectonic Framework, Tectonic Evolution and Mineral Resources / [ed] M. B. Stephens and J. Bergman Weihed, Geological Society of London, 2020, p. 517-548Chapter in book (Other academic)
Abstract [en]

Nappes of continental outer and outermost margin affinities (Middle Allochthon) were transported from locations west of the present Norwegian coast and thrust eastwards onto the Baltoscandian foreland basin and platform. They are of higher metamorphic grade than underlying thrust sheets and most are more penetratively deformed. These allochthons are treated here in three groups. The lower thrust sheets comprise Paleoproterozoic crystalline basement (e.g. Tännäs Augen Gneiss Nappe) and greenschist facies, Neoproterozoic, siliciclastic metasedimentary rocks (e.g. Offerdal Nappe). These are overthrust by a Cryogenian−Ediacaran succession intruded by c. 600 Ma dolerites (Baltoscandian Dyke Swarm) with an affinity to mid-ocean ridge basalt containing normal to enriched incompatible element contents (Särv Nappes). The upper sheets are dominated by higher-grade allochthons (Seve Nappe Complex) with similar, mainly siliciclastic sedimentary protoliths, more mafic magmatism and some solitary ultramafic bodies. Within this early Ediacaran continent−ocean transition zone (COT) assemblage, generally metamorphosed in amphibolite facies, some nappes experienced migmatization, and eclogites are present. Evidence of ultrahigh-pressure metamorphism has been obtained from garnet peridotites and eclogites; recently, microdiamonds have been discovered in paragneisses. Subduction of the COT started by the late Cambrian and accretion continued through the Ordovician, prior to the Baltica–Laurentia collision. Thrusting of all these Middle allochthons onto the foreland basin exceeds a distance of 400 km.

Place, publisher, year, edition, pages
Geological Society of London, 2020. p. 517-548
Series
Memoirs of the Geological Society of London, ISSN 0435-4052, E-ISSN 2041-4722 ; 50
National Category
Geology
Research subject
Ore Geology
Identifiers
URN: urn:nbn:se:ltu:diva-78289DOI: 10.1144/M50-2018-73Scopus ID: 2-s2.0-85087027640ISBN: 978-1-78620-460-8 (print)OAI: oai:DiVA.org:ltu-78289DiVA, id: diva2:1420991
Available from: 2020-04-01 Created: 2020-04-01 Last updated: 2020-08-26Bibliographically approved

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