Johanna Salminen

Return to Rodinia? Moderate to high palaeolatitude of the São Francisco/Congo craton at 920 Ma

Abstract Moderate to high palaeolatitudes recorded in mafic dykes, exposed along the coast of Bahia, Brazil, are partly responsible for some interpretations that the São Francisco/Congo craton was separate from the low-latitude Rodinia supercontinent at about 1050 Ma. We report new palaeomagnetic data that replicate the previous results. However, we obtain substantially younger U–Pb baddeleyite ages from five dykes previously thought to be 1.02–1.01 Ga according to the 40Ar/39Ar method. Specifically, the so-called ‘A-normal’ remanence direction from Salvador is dated at 924.2±3.8 Ma, within error of the age for the ‘C’ remanence direction at 921.5±4.3 Ma. An ‘A-normal’ dyke at Ilhéus is dated at 926.1±4.6 Ma, and two ‘A-normal’ dykes at Olivença have indistinguishable ages with best estimate of emplacement at 918.2±6.7 Ma. We attribute the palaeomagnetic variance of the ‘A-normal’ and ‘C’ directions to lack of averaging of geomagnetic palaeosecular variation in some regions. Our results render previous 40Ar/39Ar ages from the dykes suspect, leaving late Mesoproterozoic palaeolatitudes of the São Francisco/Congo craton unconstrained. The combined ‘A-normal’ palaeomagnetic pole from coastal Bahia places the São Francisco/Congo craton in moderate to high palaeolatitudes at c. 920 Ma, allowing various possible positions of that block within Rodinia.

Palaeomagnetism of the Salla Diabase Dyke, northeastern Finland, and its implication for the Baltica-Laurentia entity during the Mesoproterozoic

Abstract 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°, I=73.9°, k=75.7°, α95=4.8°) of 13 sites along the large single dyke provides a virtual geomagnetic pole (VGP) position of Plat=71°N, Plon=113°E (A95=8.1°). 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.

Palaeomagnetism and U–Pb geochronology of c. 1570 Ma intrusives from Åland archipelago, SW Finland – implications for Nuna

Abstract 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.

New Evidence for Impact from the Suvasvesi South Structure, Central East Finland

The circular Suvasvesi South structure (diameter about 3.8 km) is located in Central East Finland (62°35.8′N, 28°13′E) and correlates with the Haapaselka open lake area, the southern of the two Suvasvesi lakes. Suvasvesi S was first noticed in satellite images and might form a crater doublet with the proven Suvasvesi N impact structure. We have previously presented evidence, such as presence of fractured target rocks and shatter-cone boulders on the eastern shore of Haapaselka, which suggest that the Suvasvesi South is also an impact structure. During the summer 2002 we carried out a field survey in the area of the Suvasvesi lakes, which led to additional discoveries of shatter cones in boulders. We also discovered impact melt boulders in gravel pits along the roadsides, about 5 km southeast of the structure. Microscopic studies of the thin sections of impact melt and of the shatter cone boulders reveal the presence of well developed and decorated PDFs in quartz grains, maskelynite, fluidal textures between impact melt mineral clasts and kink bands in micas. Consequently, the melt rock is considered to be of impact origin. It is unlikely that the boulders with shatter cones and impact melt blocks were derived from the northern structure, because material transported from it by ice drift would not have passed this area. The outcrops on the islets of the Suvasvesi South area are heavily fractured with subvertical NNW-SSE and ENE-WSW trending; however the fracturing may be related to the Svecofennian tectonic deformations occurring in this area. Also, the outcrops show shatter cone features with a maximum of 50 cm in size. We interpret the shatter cone features to be of impact origin because of their shape and because the orientation of their apices differ from the other deformation systems. However, thin section analysis from outcrop specimens has not shown impact evidence so far. The new findings suggest the presence of an eroded impact melt layer in the southern structure. Bathymetric and airborne magnetic data point to a distinct structure of smaller dimension than the northern one. Preliminary paleomagnetic measurements on the granitic host rocks of Suvasvesi South reveal two components, of which one (steep downwards) is probably either a hard viscous remanence of present age or a Svecofennian age feature. The other one is poorly defined, but has a southwest direction similar to that isolated for the northern structure and could be related to impact.

Preliminary Magnetostratigraphy for the Jurassic–Cretaceous Transition in Porto da Calada, Portugal

We present a stratigraphic log supporting a preliminary magnetostra- tigraphy of a Tithonian-Berriasian section in Porto da Calada (Portugal). Based on biostratigraphy and reversed and normal magnetostratigraphy, the location of the Tithonian-Berriasian boundary is tentatively located at ca. 52 m, not in dis- agreement with former proposals. Due to the occurrence of later remagnetization (diagenesis), the magnetostratigraphic definition of the Tithonian-Berriasian sec- tion at the Cabo Espichel (Portugal) location was not able to be established.

Mesoproterozoic Nuna Supercontinent and the Geomagnetic Field in Light of Recent Paleomagnetic Data from Diabase Dykes of Finland

Mesoproterozoic supercontinent Nuna (e.g. Columbia, Hudsonland) has increased in recent years enabling more reliable global continental reconstructions (e.g Hoffman 1997; Evans and Mitchell 2011; Zhang et al. 2012; Pisarevsky et al. 2014). Supercontinent Nuna included Baltica, Laurentia, Siberia, proto-Australia and Antarctica, Amazonia and West Africa, Congo-São Francisco, North China, Kalahari and India cratons. Baltica and Laurentia are thought to represent two of the most important building blocks of this supercontinent in a single geologically valid NENA (North EuropeNorth America) juxtaposition between ca. 1.75-1.27 Ga forming the core of Nuna with Siberia (e.g. Gower et al. 1990; Evans and Mitchell 2011). Recent high quality, precisely dated Mesoproterozoic paleomagnetic poles of Baltica support the NENA connection. These include the pole from Åland (1575.9 ± 3.0 Ma; U-Pb) diabase dykes (Salminen et al. 2015) and coeval pole from Satakunta diabase dykes (Salminen et al. 2014) in Finland; a pole for the Mesoproterozoic Satakunta sandstones in Finland (Klein et al. 2014); and poles for Lake Ladoga basalts and intrusives (1459 ± 3, 1457 ± 2 Ma; U-Pb) in Russia (Salminen and Pesonen 2007; Lubnina et al. 2010). One striking feature of the 1.576 Ga high quality paleomagnetic data for Åland and Satakunta is the asymmetry of polarity, i.e. the mean directions of normal (N) and reversed (R) polarities are not antiparallel at 95% confidence level and do not pass the reversal test (McFadden and McElhinny 1990). One possible reason for such an asymmetry could be an unusual behavior of the geomagnetic field at the Mesoproterozoic, which would hamper the paleomagnetic reconstructions. Antipodality of N and R directions is expected in the case where the geomagnetic field is represented by the geocentric axial dipole (GAD), whereas steepening or shallowing of inclinations can result from the contamination of GAD by zonal multipolar fields. We used 26 global dual-polarity paleomagnetic results from PALEOMAGIA database (Veikkolainen et al. 2014a) to detect possible deviations from the GAD hypothesis (Hospers 1954) applying the quantity called inclination asymmetry (Veikkolainen et al. 2014b). The asymmetry tests indicate that GAD is a relatively good fit at the Mesoproterozoic (1.7-1.4 Ga) and therefore zonal multipolar fields do not explain the observed asymmetry. One other possible reason for asymmetry is an unremoved secondary component, which could explain the asymmetry for Åland and Satakunta data. Additional support for component mixing comes from the secondary component distribution, which is streaked in part toward the N-polarity direction. A third reason can be a small but significant age difference between N and R magnetized dykes which could explain the asymmetry. However, the actual age span for the Mesoproterozoic dykes for Baltica awaits further precise age dating. In addition to results from Åland, Satakunta and Lake Ladoga we present here new high quality Mesoproterozoic paleomagnetic and geochronological results from the Häme dykes (1642 ± 2 Ma, 1647 ± 14 Ma; U-Pb) in Finland that do not show asymmetry. These results also support the NENA connection placing Baltica on equatorial latitudes at 1.64 Ga.