| 1. |
- Ogenhall, Erik, 1967-, et al.
(författare)
-
Deformation history of the Palaeoproterozoic Svecofennian supracrustal rocks at Hamrånge, south-central Sweden
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The west-central part of the Palaeoproterzoic Fennoscandian Shield consists of different lithological units: the marine Bothnian basin, granitoids formed in an active continental margin (the Ljusdal Domain, LjD), and a continental back-arc magmatic region (the Bergslagen province). Geochemical signatures indicate that the supracrustal Hamrånge Group (HG) represents an additional unit - a fragment of an oceanic island arc that has been accreted to the continental margin granitoids. Structural data show that the 1.9 Ga supracrustal rocks in the lower part of the stratigraphy of the HG have been affected by three episodes of deformation (D1-D3). On the other hand, the ≤1.86 Ga quartzite in the uppermost part probably has been affected only by the latter two deformational phases. The nature of D1 is poorly understood, but may represent the early accretion of the island arc to the continental margin. A NW-SE compressional phase during D2 resulted in F2-folds and thrusts in the supracrustal rocks, and thrusts are also observed in the footwall granitoids. Microstructures demonstrate that metamorphism outlasted and possibly peaked after D2, and structural data imply a period of flattening subsequent to the D2-thrusting, but prior to D3. During D3 the area was affected by a regional N-S compression causing upright E-plunging F3-folding. This F3-folding matured to steep ductile shear zones, which belong to a system of 1.82-1.80 Ga NW-SE shear zones transecting the central part of the Fennoscandian Shield. The structural evolution recorded in the Hamrånge area, suggesting D2-thrusting towards NW and subsequent N-S (D3) shortening, is in accordance with the accepted tectonic models of the Fennoscandian Shield. The structural configuration of the area is partly comparable, at a smaller scale, to a major tectonic unit in the 0.5 Ga Scandinavian Caledonides, the Lower Seve Nappe (LSN), suggesting similarities in structural evolution of orogenies through time. An important difference is the relative timing of deformation and metamorphism. However, significant differences in tectonostratigraphy suggest that the HG is not far-transported and that the HG and tectonically underlying granitoids of the LjD originally had a close spatial relationship.
|
|
| 2. |
- Ogenhall, Erik, 1967-
(författare)
-
Geological Evolution of the Supracrustal Palaeoproterozoic Hamrånge Group: A Svecofennian Case Study
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis utilizes several geological methods to investigate the origin and evolution of the supracrustal rocks in the Palaeoproterozoic Hamrånge Group (HG) in the south-central Swedish Svecofennian. The first paper is based on whole-rock geochemistry to show the plate tectonic setting of volcanic rocks within the HG. This indicates that the environment was probably an oceanic volcanic arc. Geochronology, used in paper two, shows that the volcanism was active at 1888±6 Ma and that the sediments forming the stratigraphically overlying quartzite were deposited after 1855±10 Ma, with provenance ages overlapping both the volcanic rocks and the 1.86-1.84 Ga continental margin Ljusdal granitoids. In the third paper, thermobarometry was applied to samples from the HG, the migmatitic Ockelbo sub-domain to the south, and the 1.81 Ga Hagsta Gneiss Zone (HGZ) that separate these two units. The results show distinct differences in the metamorphic conditions that have affected the HG and the Ockelbo sub-domain, supporting previous interpretations that the HGZ is an important crustal structure, possibly a terrane or domain boundary. Paper four deals with the structural geology of the Hamrånge area. The study shows that the volcanic rocks and the underlying mica schist have been subjected to three deformation episodes (D1-D3), while the uppermost quartzite was most likely only affected by D2 and D3. While structures related to D1 are rarely seen, D2 resulted in a penetrative foliation, strong lineations and NW-vergent folding and thrusting. D3 is a result of a N-S compression that formed regional E-W folds and steep, ca. NW-SE shear zones, e.g. the HGZ. The results presented in this thesis, integrated with previously published data, outline a model for the geological evolution of the Hamrånge area: At 1.89 Ga a volcanic arc formed that subsequently collided with a continental margin resulting in the first deformation episode, D1, and probably a metamorphic event. This was possibly followed by an extensional period, after 1855±10 Ma, forming a basin that accumulated sediments later to form the quartzite stratigraphically on top of the volcanic rocks. The second deformation episode, D2, formed a fold-thrust belt when the supracrustal HG was thrusted to the NW, on top of the 1.86-1-84 Ga Ljusdal Domain. Flattening and a second metamorphic period followed this thickening of the crust. The last ductile deformation, D3, caused by regional tectonic forces, resulted in F3-folds that matured into ca. 1.8 Ga large-scale, steep shear zones transecting the Fennoscandian Shield.
|
|
| 3. |
|
|
| 4. |
- Bergman, Stefan, et al.
(författare)
-
The Storsjön-Edsbyn Deformation Zone, central Sweden : Research report of a project entitled: "The tectonometamorphic history of a major shear zone in central Sweden - integrated geological-geophysical study"
- 1994
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A major, polyphase NNW-SSE-striking deformation zone in central Sweden, 15-30 km wide and >200 km long, apparently separates two major Proterozoic provinces. Four main phases of deformation have been recognized along the deformation zone; associated mylonites are characterized by their mineralogy, microstructures, kinematic patterns, magnetic signatures and relative ages.Locally, metamorphic patterns developed during regional Svecofennian deformation and early shearing were significantly disturbed by later movements on mostly steep, greenschist facies deformation zones. As a result, previously unknown intermediate-high-pressure rocks were juxtaposed against low-pressure rocks more typical for the region. Also the geometry of the major, early Svecofennian Ljusdal Batholith, appears to be controlled by the shear zone and a previously unknown splay.Rotation of planar structures indicates >10-20 km dextral displacement along plastic mylonites. Stress- and strain analysis of mesoscopic shear zone populations of different generations suggests that most displacements are compatible with N-S to NE-SW-oriented bulk regional shortening.Most deformation occurred before intrusion of c. 1.2 Ga mafic dykes and sills. The existence of this deformation zone, however, significantly affected the depositional, metamorphic and structural evolution of the Caledonian orogen in this region.
|
|
| 5. |
- Sjöström, Håkan, 1945-, et al.
(författare)
-
Deformation Zones in Eastern Bergslagen (Uppland-Sörmland)
- 2001
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- 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.
|
|
| 6. |
- Sjöström, Håkan, et al.
(författare)
-
Svecofennian Metamorphic and Tectonic Evolution of East Central Sweden : Research report of a project entitled: Svekofennisk metamorf och tektonisk utveckling i östra mellansverige
- 1998
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In this report we present results from a metamorphic and structural investigation of east central Sweden, and integrate the results with age data into a model of the tectonometamorphic evolution of the region.Metamorphic grades range from medium grade in well-preserved supracrustal areas, like Hamrånge and Los, to granulite facies in intensely migmatized gneisses at Hornslandet, where pyroxene granulites and charnockites have been recorded. Thermobarometry results confirm earlier interpretations of regional low pressure-high temperature (LPM) character of the metamorphism. Results indicating medium or high pressures appear to be related to inappropriate whole-rock chemistry and in particular the CaO content in the samples. Garnets very rarely show growth zoning patterns due to the high temperature during metamorphism but imposed retrograde zoning along garnet margins is common, reflecting cooling of the crust. A PT-path obtained from a growth zoned garnet at Arlanda indicates isobaric temperature increase. Preliminary dating of monazites in pyroxene granulites yield cooling ages of 1846 and 1842 Ma respectively, which may reflect the intrusion of the Ljusdal granitoid rather than regional LPM. In the Los-Edsbyn area an age of 1803±1 Ma has been obtained on metamorphic zircon.The structural evolution comprises at least two major phases of folding and shearing, referred to two deformation episodes, D 1 and D2. Shear zones were formed during both D1 and D2. The former are folded by regional F2 folds and are characterised by complete annealing during the LPM. A preliminary age determination indicates a minimum age of 1826 Ma for D1 shear zones. The kinematics of these zones are unknown. D2 shear zones often truncate limbs of regional F2 folds and there appears to be a close relationship between folding and shearing. The kinematics of the D2 shear zones and their probable relation to F2 folds suggests that previous interpretations for the structural evolution should be revised, both considering principal stress orientation during D2 and the timing of D1 and D2.There is no systematic variation in metamorphic grade related to the existence of steep, regional shear zones. Metamorphic variations appear to reflect thermal variations in the crust possibly related to mafic underplating. Comparison of present crustal thickness and the distribution of granulites in the area and southern Finland indicate postmetamorphic modification of the crust.
|
|
