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Metamorphic Zonation by Out-of-Sequence Thrusting at Back-Stepping Subduction Zones: Sequential Accretion of the Caledonian Internides, Central Sweden
Stockholm University, Faculty of Science, Department of Geological Sciences.
Stockholm University, Faculty of Science, Department of Geological Sciences.ORCID iD: 0000-0003-3347-9284
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Number of Authors: 52018 (English)In: Tectonics, ISSN 0278-7407, E-ISSN 1944-9194, Vol. 37, no 10, p. 3545-3576Article in journal (Refereed) Published
Abstract [en]

Exhumation of the high-grade metamorphic Seve Nappe Complex and its emplacement between lower-grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jamtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top-to-the-ESE, foreland-directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation-parallel boudinage, and abundant top-to-the-ESE and rare, scattered top-to-the-WNW shear-sense indicators imply foreland-directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in- and out-of-sequence thrusts that generated the metamorphic zonation. Our model envisions in-sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out-of-sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high-grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.

Place, publisher, year, edition, pages
2018. Vol. 37, no 10, p. 3545-3576
National Category
Geology
Research subject
Geology
Identifiers
URN: urn:nbn:se:su:diva-162921DOI: 10.1029/2018TC005088ISI: 000450992300009OAI: oai:DiVA.org:su-162921DiVA, id: diva2:1272275
Funder
The Geological Survey of Sweden (SGU), 1760Available from: 2018-12-18 Created: 2018-12-18 Last updated: 2019-04-01Bibliographically approved
In thesis
1. Assembly of the Caledonian Orogenic Wedge, Jämtland, Sweden
Open this publication in new window or tab >>Assembly of the Caledonian Orogenic Wedge, Jämtland, Sweden
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Collisional orogeny creates the largest mountain belts on Earth. The Caledonides of Scandinavia are a deeply eroded, ancient mountain belt, which today exposes a deep section through the former orogenic interior. The orogenic internides hold important geological information necessary to understand the geodynamic processes shaping collisional plate boundaries. This thesis explores the kinematics and timing of orogenic wedge formation in Jämtland, central Sweden. An integrated approach of structural field mapping, microstructural analysis, Rb–Sr radiogenic dating and rock magnetism yielded new and comprehensive tectonochronologic data. A regionally extensive network of kinematic field data demonstrated pervasive ductile top-to-the-ESE shearing across the entire tectonostratigraphy. Rb–Sr multi-mineral isochron ages constrained the absolute timing of ductile deformation to c. 430 Ma and c. 415 Ma. Local structural and magnetic data showed that final nappe emplacement and exhumation had occurred before extensional deformation initiated. The new data presented in this thesis contradicted a tectonic model previously proposed for Caledonian nappe stacking. These findings were used to develop an alternative tectonic model consistent with both the new and other available structural, petrological and chronological data. The new model for orogenic wedge assembly comprises three stages of foreland-directed, top-to-the-ESE thrusting. It reflects the complex interactions caused by the merging of two subduction zones accommodating Baltica–arc–Laurentia collisions during Ordovician to Devonian time.

Place, publisher, year, edition, pages
Stockholm: Department of Geological Sciences, Stockholm University, 2019. p. 27
Series
Meddelanden från Stockholms universitets institution för geologiska vetenskaper ; 376
Keywords
collisional orogeny, Scandinavian Caledonides, Seve Nappe Complex, Köli Nappe Complex, in-sequence thrusting, out-of-sequence thrusting, imbrication, nappe stacking, orogenic wedge assembly, Rb–Sr geochronology, rock magnetism, anisotropy of magnetic susceptibility
National Category
Geology
Research subject
Geology
Identifiers
urn:nbn:se:su:diva-167123 (URN)978-91-7797-668-4 (ISBN)978-91-7797-669-1 (ISBN)
Public defence
2019-05-23, De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
The Geological Survey of Sweden (SGU), 1760
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Manuscript.

Available from: 2019-04-26 Created: 2019-04-01 Last updated: 2019-04-29Bibliographically approved

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