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Absolute timing of Caledonian orogenic wedge assembly, Central Sweden, constrained by Rb-Sr multi-mineral isochron data
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0001-9630-2633
Stockholm University, Faculty of Science, Department of Geological Sciences.
Number of Authors: 32019 (English)In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 344, p. 339-359Article in journal (Refereed) Published
Abstract [en]

Multi-mineral Rb-Sr isochron ages for mylonites of the Caledonian nappe pile (Jamtland, Sweden) constrain the orogenic evolution of the central Scandinavian Caledonides. An age of 437.8 +/- 3.9 Ma from a metamorphic leucosome in the central nappe stack dates crystallization of migmatized gneiss. Thirteen isotopic ages derived from mylonites across all structural levels are between 434.6 +/- 6.9 Ma and 426.3 +/- 3.1 Ma. These results, complemented by a dense network of kinematic field data, are interpreted to reflect the timing of protracted top-to-the-ESE general shear under decreasing amphibolite- to upper-greenschist-facies conditions across all nappes. Three samples, derived from the top, middle and bottom of the nappe pile, yield additional deformation ages between 416.0 +/- 3.6 Ma and 410.1 +/- 3.0 Ma. These ages are interpreted to represent post-assembly imbrication of the nappe stack under decreasing metamorphic conditions. The new tectonochronologic data are consistent with a tectonic model that explains nappe stacking in Jamtland by three stages of ESE-directed in- and out-of-sequence thrusting. In the first stage, postdating earlier subduction-exhumation of the high-grade metamorphic Seve Nappe Complex, the latter was emplaced in-sequence in the lower/mid-crust above nappes in its footwall (foreland) at c. 430 Ma. This event is related to the collision of Baltica with an island arc. Concurrently at c. 430 Ma, determined by paleogeography and structurally unrelated to the first stage, second-stage out-of-sequence thrusting emplaced the Kali Nappe Complex atop the developing nappe stack during final Baltica-Laurentia collision. Eventually, imbrication and exhumation of the composite Jamtland nappe stack in the uppermost middle crust during the third stage at c. 414 Ma was caused by continuous underthrusting of Baltica beneath Laurentia. This three-stage thrust model reflects the complex interactions of Baltica-arc-Laurentia collisions that were accommodated by two merging subduction zones.

Place, publisher, year, edition, pages
2019. Vol. 344, p. 339-359
Keywords [en]
Collisional orogeny, Nappe assembly, Out-of-sequence thrusting, In-sequence thrusting, Geochronology, Scandinavian Caledonides, Seve Nappe Complex, Jamtland
National Category
Earth and Related Environmental Sciences
Identifiers
URN: urn:nbn:se:su:diva-172944DOI: 10.1016/j.lithos.2019.06.033ISI: 000483419900022OAI: oai:DiVA.org:su-172944DiVA, id: diva2:1352317
Available from: 2019-09-18 Created: 2019-09-18 Last updated: 2019-12-04Bibliographically approved

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