Influence of regional shear zones on the lithological pattern in central Sweden

The geochemical and isotopic characteristics of metamorphosed Svecofennian mafic dykes from the Dannemora area in the NE part of the Bergslagen region in central Sweden were investigated and compared to mafic intrusive rocks in their vicinity. The dykes, with an inferred age of c. 1860–1870 Ma, are calc-alkaline, sub-alkaline and basaltic in composition and have a mixed subduction and within-plate geochemical affinity. They are the result of mixing of at least three mantle source components with similar basaltic major element composition, but different concentrations of incompatible trace elements. Magma M1 is strongly enriched both in Rare Earth Elements (REE) and High-Field-Strength Elements (HFSE); magma M2 is highly enriched in Large-Ion Lithophile Elements (LILE, except Sr) with only moderate enrichment in HFSE and REE (particularly low in Heavy Rare Earth Elements); and magma M3 is enriched in Sr and has a flat REE profile. Magma M3 also has a somewhat more positive (depleted) initial εNd value of +1.8, compared to +0.4 to +0.5 for magmas M1 and M2. The magma evolution was controlled by a mixture of fractionation (mainly affecting the compatible elements) and mixing, best seen in the incompatible element concentrations and the Nd isotope data. The basaltic overall composition indicates little or no wholesale contamination by upper continental crust, but the dykes have undergone later metasomatic changes mainly affecting the alkali elements.

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“…2009) and Singö shear zone (SSZ in Fig. 1; Tirén & Beckholmen, 1990) to the north and by the TIB (Högdahl et al . 2004) to the south.…”

Section: Geological Settingmentioning

confidence: 99%

Source character, mixing, fractionation and alkali metasomatism in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden

2013

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“…One of these synforms, the Hamrånge synform (Figure 2), located in the eastern part of the domain, consists of well‐preserved amphibolite facies supracrustal rocks including a 1.88 Ga [ Bergman et al , 2008] metavolcanic unit [ Lundegårdh , 1956; Sukotjo , 1995]. To the west and south supracrustal rocks of more uncertain affinity are abundant in a wedge‐shaped area corresponding to the transitional area referred to as the Ockelbo domain by Tirén and Beckholmen [1990] (Figure 2). The southern boundary of this wedge roughly coincides with an area interpreted as a separate tectonic domain [ Hermansson et al , 2007] or a kinematically unspecified deformation zone [ Tirén and Beckholmen , 1990; Stephens et al , 1994].…”

Section: Geological Settingmentioning

confidence: 99%

“…To the west and south supracrustal rocks of more uncertain affinity are abundant in a wedge‐shaped area corresponding to the transitional area referred to as the Ockelbo domain by Tirén and Beckholmen [1990] (Figure 2). The southern boundary of this wedge roughly coincides with an area interpreted as a separate tectonic domain [ Hermansson et al , 2007] or a kinematically unspecified deformation zone [ Tirén and Beckholmen , 1990; Stephens et al , 1994].…”

Section: Geological Settingmentioning

confidence: 99%

“…[18] On the basis mainly of magnetic and gravity signatures (Figures 4a and 4b), a deformation zone with unknown kinematics has previously been inferred between Sandviken and Rättvik [Tirén and Beckholmen, 1990] or Gävle and Rättvik [Stephens et al, 1994] (Figures 1 and 2). The spatial discrepancy is related to the ENE -WSW Gävle graben, filled with 1.5-1.3 Ga Jotnian sandstones and 1.25 Ga diabase dykes and sills [Söderlund et al, 2006], which obscure both the surface and geophysical expressions of this zone.…”

Section: The Gävle-rättvik Zone (Grz)mentioning

confidence: 99%

“…The boundary between the Ljusdal domain, roughly equivalent to the Bothnian unit [ Lahtinen et al , 2005] and the Bergslagen province is, however, poorly defined. This transitional area including mixed lithologies is bounded by ductile shear zones and has previously been suggested to be a separate geologic domain [ Tirén and Beckholmen , 1990]. The transitional nature is enigmatic since the Ljusdal domain and the Bergslagen province show significant differences in metamorphic grade, structural style, and magmatic age of comparable subduction‐related granitoids.…”

Section: Introductionmentioning

confidence: 99%

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Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield

2009

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The Paleoproterozoic part of the Fennoscandian Shield is composed of crustal components formed in different tectonic settings and generally separated by well‐defined shear zone systems. An anomalous transitional boundary has been investigated by integrating structural analysis and geochronology with published geophysical data. The nature of this boundary is interpreted to be a consequence of an apparent stacking in the lower and middle crust initiating 1.87–1.86 Ga dextral shear along the Gävle‐Rättvik Zone (GRZ) and adjacent shear zones, resulting in an arcuate northern boundary of the Bergslagen province. This boundary coincides with geophysical anomalies and temporal and metamorphic breaks. Owing to continuous convergence the pure‐shear overprint component increased on the GRZ and caused a shift of dextral shear to the Hagsta Gneiss Zone with recorded shear at 1809 ± 2 Ma. Most likely, both these structures are related to coeval shear zones farther to the east as a part of an ∼1500 km long crustal, or possibly terrane, boundary.

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Proterozoic extensional tectonics of the central Fennoscandian Shield: Results from the Baltic and Bothnian Echoes from the Lithosphere experiment

Korja¹,

Heikkinen²

1995

Tectonics

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A three-dimensional crustal structure of the Bothnian Sea is deduced from the near-vertical reflection data of the crosscutting lines 1, 6, 7, and C of the BABEL (BAltic and Bothnian Echoes from the Lithosphere) experiment. For this study the common depth point stacked sections have been migrated with a velocity 6.5 krn/s and displayed as stacked envelope sections to show their enhanced large-scale crustal structures. The seismic sections show listric shear zones which dip toward SE and flatten at major detachment zones 35-40 km and 48 km in depth. The detachment zones are associated with changes in P wave velocity from 6.8 krn/s to 7.0 krn/s and from 7.4 krn/s to 8.2 krn/s and coincide with the middle and lower crustal boundary and the Moho boundary, respectively. The geometry and crosscutting relationship of the detachment zone are well developed on the E-W directed line 7, from where the Moho detachment zone can be interpreted as to be younger. Different shear zone systems operated in the parallel N-S directed lines 1 and 6, and therefore it is not easy to correlate crustal structures between lines 1 and 6. In the thinned southern part of study area, the lower crest has a highly reflective lamellar structure and a strongly reflective subhorizontal Moho. Increased reflectivity of the lower crest is attributed to stretching and mafic underplating of the crest associated with the Subjotnian diabase dykes and rapakivi magmatism. In the upper crest the rapakivi granite and diabase association is characterized by a rather transparent area within which strong reflections mimic graben and horst structures. The distribution of the bimodal association seems to be controlled by the low-angle listric shear zones. The spatial distribution of outcropping lithologies combined with deeply penetrating shear zones suggests a simple shear extensional deformation. The first stage of deformation, which concentrated in the upper and middle crust, records the collapse of the overthickened Svecofennian crust. The second stage involving the entire crest, may record either the continuation of the postcollisional collapse or a separate anorogenic extensional period. The seismic structures predict major post-Svecofennian deformation/faulting and tilting in the central Fennoscandian Shield. The extensional tectonics operating in the Fennoscandian Shield in the Proterozoic is comparable to modem extensional tectonics.

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Influence of regional shear zones on the lithological pattern in central Sweden

Cited by 8 publications

References 2 publications

Source character, mixing, fractionation and alkali metasomatism in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden

Source character, mixing, fractionation and alkali metasomatism in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden

Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield

Proterozoic extensional tectonics of the central Fennoscandian Shield: Results from the Baltic and Bothnian Echoes from the Lithosphere experiment

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