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  • 1
    Electronic Resource
    Electronic Resource
    The impact cratering record of Fennoscandia (1996)
    Springer
    Earth, moon and planets 72 (1996), S. 377-393 
    Details
    ISSN: 1573-0794
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The current database of craterform structures in Fennoscandia contains 22 structures of impact origin and about fifty other structures which lack sufficient evidence for impact. The discovery rate of new structures has been one or two per year during the past ten years. The proven impact structures are located in southern Fennoscandia and the majority have been found in Proterozoic target rocks. The age of the structures varies from prehistoric to ≤ 1000 Ma and their diameters (D) from 0.04 km to 55 km. Nine of the structures contain impact melt. A characteristic feature of the Fennoscandian impact record is a relatively large number of small (≤ 5 km) but old (〉 200 Ma) structures: this is a result of success of geophysical methods to discover small but old impact structures in an eroded shield covered with relatively thin overburden. Some of the large circular structures in satellite images and/or in geophysical maps may represent deeply eroded scars of very old impacts, but due to the lack of shock metamorphic features, impact-generated rocks or identified ejecta layers, they cannot yet be classified as impact sites. Two huge structures are proposed here as possible impact sites on the basis of circular satellite images and distinct geophysical anomalies: the Lycksele structure in northern Sweden (D ~ 120 km, see also Witschard, 1984) and the Valga structure in Latvia/Estonia (D ~ 180 km). However, endogeneous explanations, like buried granites, basement domings, or fault-bounded blocks are also possible for these structures. Hints, such as distal ejecta layers or impact produced breccia dykes, of an Archaean or Early Proterozoic impact structure have not been found in Fennoscandia so far. New ways of searching for these structures are proposed with particular emphasis on high-resolution integrated geophysical methods. The impact cratering rate in Fennoscandia is ~ 2.0 · 10−14 km−2 a−1 (for craters with D 〉 3 km) corresponding to about two events per every 100 Ma for the last 700 Ma. Due to erosion, this is a minimal estimate but is higher than the global rate probably due to strong research activity for finding impact structures in Fennoscandia.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00117542
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Late Precambrian Keweenawan asymmetric reversals (1981)
    [s.l.] : Nature Publishing Group
    Nature 294 (1981), S. 436-439 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] First we assume that the geomagnetic field during Keweenawan time consisted of an axial geocentric dipole (main dipole, MN, R) and an axial offset dipole (m) (see also refs 12-14). The geocentric dipole has two polarity states, normal and reversed (Fig. 2). The magnetic dipole moments^during these ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/294436a0
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  • 3
    Electronic Resource
    Electronic Resource
    The petrophysical classification of meteorites (1993)
    Springer
    Studia geophysica et geodaetica 37 (1993), S. 65-82 
    Details
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Summary Petrophysical properties (susceptibility, intensity of the Natural Remanent Magnetisation (NRM) and bulk density) of 489 meteorite samples from 368 meteorites are discussed. The samples, obtained from Finnish meteorite collections, represent all chemical-petrological meteorite classes and their groups. This meteorite petrophysical database has many potential applications in the geophysical studies of extraterrestrial bodies (planets and their moons, asteroids, meteorite parent bodies, etc.). Here we use the database to classify meteorites rapidly and non-destructively by applying the petrophysical classification scheme developed by Kukkonen and Pesonen [10]. For example, the main classes and many groups form distinct clusters in petrophysical relation diagrams such as susceptibility vs. density or NRM vs. susceptibility. The petrophysical classification method was tested on 24 meteorites from Czechoslovak, 3 from Swedish and one from Australian collections. The chemical-mineralogical classifications of these meteorites were previously known. The subjective classification method was also compared with a mathematical cluster analysis. The subjective classification technique was successful in 64% to 93% of the cases whereas the mathematical analysis was successful in 57% to 82% of the cases. The failures can be attributed to (i) non-uniqueness problems (cluster plots overlap) and (ii) effects of porosity, self-demagnetisation, electrical conductivity and frequency on measured values, or to biasing caused by small sample size.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01613921
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  • 4
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    Palaeomagnetism of the Salla Diabase Dyke, northeastern Finland, and its implication for the Baltica-Laurentia entity during the Mesoproterozoic (2009)
    In:  Geological Society Special Publication 323: 199-217.
    Details
    Publication Date: 2009-09-08
    Description: New palaeomagnetic and rock magnetic results are presented for the 1122{+/-}7 Ma Salla Diabase Dyke in NE Finland. A positive baked-contact test proves that the dyke has a primary natural remanent magnetization carried by magnetite. The characteristic remanent magnetization direction (D=42.2{degrees}, I=73.9{degrees}, k=75.7{degrees}, {alpha}95=4.8{degrees}) of 13 sites along the large single dyke provides a virtual geomagnetic pole (VGP) position of Plat=71{degrees}N, Plon=113{degrees}E (A95=8.1{degrees}). Although secular variation may not have been fully averaged out, the new VGP provides an important result to define the late Mesoproterozoic position of Baltica. The VGP is not close to any known Proterozoic palaeopoles of Baltica, and therefore the pre-Sveconorwegian apparent polar wander path (APWP) of Baltica must be modified. The pre-Sveconorwegian (c. 1.3-1.0 Ga) APW swathes of Baltica, Laurentia (including the Logan Loop) and Kalahari cratons show similar shape, but new well-dated palaeomagnetic poles for c. 1.25-1.12 Ga interval from these continents are required to test the similarity. The Salla dyke VGP provides hints that the Mesoproterozoic Baltica-Laurentia connection in the Hudsonland supercontinent assembly lasted until 1.12 Ga.
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  • 5
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    Palaeomagnetism and U-Pb geochronology of c. 1570 Ma intrusives from Aland archipelago, SW Finland - implications for Nuna (2015)
    Geological Society (of London)
    Details
    Publication Date: 2015-05-30
    Description: We report new palaeomagnetic and isotope age data of Early Mesoproterozoic (i.e. Subjotnian) intrusions from the Åland archipelago, SW Finland. The palaeomagnetic results reveal dual-polarity magnetizations with a pronounced reversal asymmetry occurring in dykes. We explain the asymmetry by an unremoved secondary component, which is affecting more N-polarity dykes. Other explanations, such as the age difference of magnetization between normal and reversed polarity dykes, are discussed. The primary nature of magnetization in dykes for both normal (N) and reversed (R) groups is verified by positive baked contact tests. A dyke showing reversed polarity from Korsö is dated 1575.9±3.0 Ma (U–Pb) in this study. This and previous U–Pb data tighten the magmatic activity in Åland to 1580–1570 Ma. We combined new palaeomagnetic data with those from earlier studies to provide a new key-palaeomagnetic pole for Baltica. Our data positions Baltica on equatorial latitudes, supporting the NENA (North Europe–North America) connection between Baltica and Laurentia at 1.59–1.58 Ga. Palaeomagnetic data support that NENA was valid at 1.75, 1.58, 1.46, and 1.26 Ga, forming the core of Mesoproterozoic Nuna (a.k.a. Columbia) supercontinent.
    Print ISSN: 0305-8719
    Electronic ISSN: 2041-4927
    Topics: Geosciences
    Published by Geological Society (of London)
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  • 6
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    Distribution of magnetic particulates in a roadside snowpack based on magnetic, microstructural and mineralogical analyses (2013)
    Oxford University Press
    Details
    Publication Date: 2013-09-10
    Description: Vehicle traffic is at present one of the major sources of environmental pollution in urban areas. Magnetic parameters are successfully applied in environmental studies to obtain detailed information about concentrations and quality of iron-bearing minerals. A general aim of this research was to investigate the magnetic, microstructural and mineralogical properties of dust extracted from the roadside snowpack accumulated on the side of an urban highway, northern Helsinki. Vertical snow profiles were taken at different distances (5, 10 and 15 m) from the road edge, during winter season 2010–2011. The temporal distribution of mass magnetic susceptibility () of the road dust shows that the concentration of magnetic particles increases in the snowpack during winter. Roadside snowpack preserves a large fraction of the magnetic particulate until the late stages of melting and this could be considered as one of the main factors responsible for the resuspension phenomenon observed in Nordic countries. The vertical distribution of and SIRM (saturation isothermal remanent magnetization)/ ratio may indicate the migration of magnetic particles down in the snowpack during melting conditions. Ultrafine to coarse-grained (superparamagnetic to multidomain) magnetite was identified as the primary magnetic mineral in all the studied road dust samples. The examined road dust contains significant amount of dia/paramagnetic minerals (e.g. quartz, albite, biotite) and the content of magnetite is relatively low (below 1 weight percent, wt%). The roadside snowpack is enriched in anthropogenic particles such as angular and spherical iron-oxides, tungsten-rich particles and sodium chloride. This study demonstrates the suitability of snow as an efficient collecting medium of magnetic particulates generated by anthropogenic activities.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 7
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    Lightning-induced remanent magnetization--the Vredefort impact structure, South Africa (2013)
    Oxford University Press
    Details
    Publication Date: 2013-09-10
    Description: Earlier studies at the large Vredefort impact structure since 1960 have shown that values of natural remanent magnetizations (NRMs) and, hence, Koenigsberger's Q values (ratio of remanent over induced magnetization), for different rock lithologies are elevated compared to the values for similar rock types around the world. Three origins for the high Q values have been suggested, namely shock by meteorite impact, enhanced plasma field and lightning strikes. We have studied whether laboratory lightning experiments can produce enhanced NRMs in the Vredefort target rocks. For comparison, we also included rocks from the Johannesburg dome, which is not a meteorite impact site. The results revealed increased NRMs, susceptibility and Q values of the rocks from both Vredefort and Johannesburg domes. Rock magnetic measurements and scanning electron microscope analyses of lightning pulsed and unpulsed samples showed that the lightning included changes in magnetic properties of the rocks. We suggest that in some samples lightning have changed magnetic mineralogy by oxidizing magnetite to maghemite. Indication of this oxidation came from the low-temperature variation of the remanent magnetization where we observed several hallmarks of maghemitization in samples treated by lightning strikes. Further indications of mineralogical changes include increased Curie points above the magnetite's Curie point (580 °C) and appearance of pronounced lower temperature (200–400 °C) phases in susceptibility versus temperature curves. These changes are interpreted to indicate partially oxidized magnetite (maghemitization) coupled with grain fragmentations and by this way grain size reduction. High-temperature hysteresis and REM (= NRM/saturation isothermal remanent magnetization) studies support these conclusions. Our results were analogous with the ones for lodestones and protolodestones where partially oxidized magnetite is thought to make magnetization more intense.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 8
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    Palaeosecular variation, field reversals and the stability of the geodynamo in the Precambrian (2014)
    Oxford University Press
    Details
    Publication Date: 2014-10-12
    Description: Palaeosecular variation (PSV), as estimated from the scatter of remanent magnetization directions or poles, can be used to shed light to processes in the geodynamo, and potentially, to model the growth of the solid inner core. To understand the temporal aspects of the geomagnetic field behavior in the far past, we have calculated the scatter of palaeomagnetic poles (S) from a set of 55 high-quality observations derived from a new Precambrian paleomagnetic data compilation. Our analysis supports former Phanerozoic and Precambrian analyses of PSV, which favour a lower reversal rate, a higher stability of the geodynamo and a substantially smaller size of the inner core prior to 1.5 billion years ago.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
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    The impact cratering record of Fennoscandia (1996)
    Springer
    Details
    Publication Date: 1996-02-01
    Print ISSN: 0167-9295
    Electronic ISSN: 1573-0794
    Topics: Geosciences , Physics
    Published by Springer
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  • 10
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    Probing impacts in Fennoscandia (1991)
    American Geophysical Union
    Details
    Publication Date: 1991-01-01
    Print ISSN: 0096-3941
    Electronic ISSN: 2324-9250
    Topics: Geosciences
    Published by American Geophysical Union
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