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Deformation Zones in Eastern Bergslagen (Uppland-Sörmland)
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
2001 (English)Report (Other academic)
Abstract [en]

In this report we present results from a structural investigation of deformation zones in Eastern Bergslagen. Key areas for investigation have been the NNE-SSW Österbybruk/Skyttorp Zone (ÖSZ), the N-S Gimo Zone (GZ) and the Banded Series in the southeastern Stockholm archipelago.

Both the ÖSZ and the GZ is here interpreted to be splays from the Singo Shear Zone (SSZ) system where they originally enveloped tectonic lenses oriented approximately parallel to the SSZ. Solidification of a c. 1.8 Ga granite within the SSZ possibly caused a strain distribution southwards from the SSZ leading to the large scale clock-wise rotation and development of Everging footwall folds to the SSZ. The E-verging fold have steep axes and rotate both the regional foliation as well as the stretching lineation.

The ÖSZ and the GZ show E-up kinematics (with a sinistral horizontal component) related to the E-vergent folding. An earlier(?) W-up pattern along the western margin of ÖSZ is here interpreted to be related to a dextral/NE-down shear along the SSZ prior to the folding. The GZ located closer to the SSZ shows generally a higher metamorphic grade than the ÖSZ. Microstructures and chlorite-bearing shearbands of the ÖSZ indicate upper greenschist facies conditions during deformation, while striped gneisses and a C-S fabric with stable hornblende and biotite along both planes indicate amphibolite facies conditions along the GZ. The Banded Series (BS) is a high-T deformation zone exposed on several islands in the Stockholm archipelago (Uto-Orno-Morto-Runmaro-Skarprunmarn). The shear zone evolved from folds and has in turn been folded by regional-scale S-folds. The kinematic pattern iscomplex due to the existence of tectonic lenses, recrystallisation and annealing, and partly due to the later folding of the BS. The tectonic lenses contains hinges of rootless folds with generally NE plunging axes parallel to a very pronounced stretching lineation indicating intense stretching during shearing. Red Kf-augens common within the BS is here interpreted to be derived from dismembered early pegmatites, while white, less common augens were derived form neosomes. These conditions indicate that the BS was temporally related to c. 1816-1821 Ma pegmatites and peak metamorphism.

Additional studies of the intensely folded Värmdölandet-Ingarö-Södertörn area has revealed that an important sinistral/W-up shear occurred coeval with an eastward to southeastward hinge escape of the E-W mainland folds. A regional approximately N-S shortening has probably caused both the hinge escape as well as a conjugate, co-existing shear system with a dextral/NE-down (SSZ) and sinistral/NW-up (BS) shear system.

Place, publisher, year, edition, pages
Uppsala: Uppsala University, 2001. , p. 80
Series
Forskningsrapport, Sveriges geologiska undersökning
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-315879OAI: oai:DiVA.org:uu-315879DiVA, id: diva2:1078312
Projects
Deformationszoner i östra Bergslagen (Uppland-Sörmland): Karaktär och signifikans under svekokarelsk och efterföljande tektonometamorf utveckling
Funder
The Geological Survey of Sweden (SGU)
Available from: 2017-03-03 Created: 2017-03-03 Last updated: 2017-03-03Bibliographically approved

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