Paleomagnetism of Late Archean metavolcanics and metasediments, Abitibi orogen, Canada: tholeiites of the Kinojevis Group

1983 ◽  
Vol 20 (3) ◽  
pp. 436-461 ◽  
Author(s):  
John Wm. Geissman ◽  
David W. Strangway ◽  
Ann M. Tasillo-Hirt ◽  
Larry S. Jensen
Keyword(s):  
Geomagnetic Field ◽  
Regional Metamorphism ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
High Coercivity ◽  
Polar Wander ◽  
Magnetic Viscosity ◽  
Secular Variations ◽  
Temperature Time

Iron-rich and magnesium-rich basaltic tholeiites of the latest Archean Kinojevis Group, central Abitibi Belt, northeastern Ontario, contain a natural remanent magnetization that is generally multivectorial. The components in all units reside in essentially pure magnetite, not the original titanomagnetites (where x for Fe3−xTixO4 probably was 0.60–0.65). Any original TRM was lost by chemical reconstitution of the magnetic oxides in response to primary deuteric conditions, long-term burial, regional metamorphism to prehnite–pumpellyite facies, and possibly intrusion by Matachewan dikes. Data from contact tests with Matachewan dikes indicate that the units are indeed capable of retaining a very latest Archean – earliest Proterozoic field (e.g., D = 194.9°, I = −14.3°, k = 8.1, α95 = 7.9°; n = 45 vectors, 38 samples). High-coercivity, high-blocking-temperature directions from samples from other flows, corrected for nearly penecontemporaneous downwarping, are in only crude agreement with those of Matachewan dikes, possibly suggesting that these components reflect a general Late Archean – Early Proterozoic field for the Superior Province. The paleomagnetic data from Kinojevis tholeiites indicate the emplacement, burial, and tight downwarping of the 10 km or so of Kinojevis stratigraphy were nearly synchronous with Matachewan intrusion. The Kinojevis data by themselves cannot be taken as statistically reliable indicators of the pre- (or immediately post-) Matachewan geomagnetic field nor can temperature–time relations for magnetic viscosity be used to predict the preservation of a statistically reliable TRM in any of these units. Individual magnetization components were blocked over geologically short periods of time, whereas the ensemble of data from discrete flows must record secular variations, field excursions, and possibly long-term polar wander.

Paleomagnetism of the Fond du Lac Formation and the Eileen and Middle River sections with implications for Keweenawan tectonics and the Grenville problem

1981 ◽  
Vol 18 (5) ◽  
pp. 829-841 ◽  
Author(s):  
Doyle R. Watts
Keyword(s):  
Remanent Magnetization ◽  
Lake Superior ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
Tectonic Deformation ◽  
River Section ◽  
Orthogonal Projections ◽  
Chemical Leaching ◽  
Polar Wander ◽  
The Times

Overlying the Keweenawan lavas of northern Wisconsin and Michigan is a thick sequence of terrestrial sandstone, shale, and siltstone that has undergone some tectonic deformation associated with movement along thrust faults and the development of the Lake Superior syncline. Thermal and alternating field demagnetization, chemical leaching, and multivector analysis using orthogonal projections reveal a trivector structure of the natural remanent magnetization (NRM) of the Fond du Lac Formation and Middle River section (Amnicon and Orienta Formations), and a bivector structure of the NRM of the Eileen section (Eileen Formation). The components may be classified by their physical properties as revealed by demagnetization. A population of high blocking temperature components, K1, is found in all three sections and gives poles as follows: Fond du Lac, 16°N, 160°E; Middle River, 25°N, 148°E; Eileen, 20°N, 156°E after structural correction is applied. A population of intermediate blocking temperature components, K2, is post-tectonic and found only in the Fond du Lac Formation and Middle River section. Poles calculated from K2 fall among the Grenville type poles (Fond du Lac, 9°S, 145°E; Middle River, 24°S, 162°E). A third population of components, K3, has low blocking temperature and coercivity and is isolated only by chemical leaching. K3 has steep positive inclination, northern declination, and is post-tectonic. It is interpreted as a recent magnetization.Any interpretation of the path of apparent polar wander for North America must accommodate the sequence of magnetization K1 to K2. The timing of tectonism in the Keweenawan basin is bracketed by the times of acquisition of K1 and K2. These results reconfirm some recent interpretations that include Grenville poles on the polar wander track of interior Laurentia.

Paleomagnetism of Archean rocks from northwestern Ontario: Wabigoon gabbro, Wabigoon Subprovince

1983 ◽  
Vol 20 (12) ◽  
pp. 1805-1817 ◽  
Author(s):  
David J. Dunlop
Keyword(s):  
Remanent Magnetization ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
Virtual Geomagnetic Pole ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Northwestern Ontario ◽  
Paleomagnetic Pole ◽  
Geomagnetic Pole ◽  
Chemical Remanent Magnetization

The Wabigoon gabbro of the Archean Wabigoon greenstone belt in northwestern Ontario preserves a univectorial natural remanent magnetization (NRM) with D = 246°, I = 12° (k = 19.5, α95 = 10.5°, N = 11 sites). The precision is reduced if sample means are averaged, however (k = 9.3, α95 = 9.2°, N = 29 samples). The paleomagnetic pole falls either at 160°W, 11°S (δp = 5.3°, δm = 10.6°), corresponding to an age of ~1300 Ma on the Laurentian apparent polar wander path, or the reverse of this, 20°E, 11° N, corresponding to a late Archean age (~2800 Ma). No ~1300 Ma igneous or metamorphic event is known in the area; a major west-northwest-trending dike about 9 km south of the gabbro yields a virtual geomagnetic pole at 122°W, 45°N and seems to be of Abitibi age (~2150 Ma) rather than Mackenzie age (~1250 Ma). A few gabbro samples and some greenstones from the intrusive baked zone have hybrid remanences in which a higher blocking temperature Kenoran-age (~2600 Ma) NRM is superimposed on the gabbro characteristic NRM. However, the Kenoran component may be a younger chemical remanent magnetization (CRM) residing in hematite. The hypothesis that the gabbro characteristic remanence is itself a hybrid of Kenoran and Keweenawan (~1100 Ma) NRM's, which would explain both the high between-sample scatter and the lack of a ~1300 Ma remagnetizing event, is considered but rejected because fewer than 10% of the gabbro samples exhibit multivectorial swings during alternating field or thermal cleaning. Two geomagnetic field reversals are recorded at interior sites, but only one or none is recorded near the margin of the intrusion. The different cooling histories of margin and interior, as well as the bulk of the other evidence, favour magnetization during initial cooling in late Archean time.

Paleomagnetism and structural history of the Ghost Range intrusive complex, central Abitibi Belt, Ontario: further evidence for the Late Archean geomagnetic field of North America

1982 ◽  
Vol 19 (11) ◽  
pp. 2085-2099 ◽  
Author(s):  
John Wm. Geissman ◽  
David W. Strangway ◽  
Ann M. Tasillo-Hirt ◽  
Larry S. Jensen
Keyword(s):  
North America ◽  
Blocking Temperature ◽  
High Coercivity ◽  
Thermal Methods ◽  
Virtual Geomagnetic Pole ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Group 3 ◽  
Group 2 ◽  
Group 1

On the basis of their remanence properties, the lithologies of the Late Archean (2710–2703 Ma) Ghost Range Complex, an east–west-trending layered mafic–ultramafic extrusive sequence in the central Abitibi Greenstone Belt, can be divided into three groups. Group 1 units contain a high-coercivity, high-blocking temperature (greater than 520 °C) magnetite-dominated remanence characteristic of the complex (D = 280°, I = 2°, k = 5.5, α95 = 11.8°, virtual geomagnetic pole = 13°E, 7°S; isolated by both AF and thermal methods), in good agreement with the few previous results bearing on the Late Archean apparent polar wander path for North America. Group 2 units contain a low-coercivity, low-blocking-temperature (less than 310°C) scattered remanence residing in pyrrhotite. Often, both remanences coexist in a single lithology at a given site. Group 3 units contain distributed coercivity and blocking-temperature remanences, again residing in magnetite, that are more scattered but statistically identical to the mean group 1 direction. The order of magnetic blocking appears to have been group 1 before group 3 before group 2. The geologic setting of the Ghost Range suggests that it has remained essentially stable since emplacement and therefore the group 1 direction appears to reliably represent a Late Archean paleomagnetic pole.

Viscosité magnétique potentielle, aimantation détritique et viscosité réelle d'un sédiment lacustre quaternaire

1987 ◽  
Vol 24 (9) ◽  
pp. 1903-1912 ◽  
Author(s):  
Daniel Biquand ◽  
François Sémah
Keyword(s):  
Remanent Magnetization ◽  
Rock Magnetism ◽  
Viscosity Index ◽  
Natural Remanent Magnetization ◽  
Blocking Temperature ◽  
Magnetic Viscosity ◽  
Paleomagnetic Study ◽  
Temperature Spectra

The magnetic viscosity of sediments, as indicated by thermal demagnetization of natural remanent magnetization (NRM), depends on two main parameters: (i) the specific magnetic viscosity of the material and (ii) the efficiency of the primary magnetization process. In an attempt to determine the relative importance of these two variables, we studied a Lower Pleistocene lacustrine sequence bearing a primary reversed detrital remanent magnetization (DRM).Using natural samples and small cores made of crushed sediment, our study is based on the thermodynamic theory of rock magnetism developed by L. Néel, who established an equivalence between time and temperature, that is, between viscous remanent (VRM) and thermoremanent (TRM) magnetization processes. The determination of the blocking temperature spectra from 20 to 152 °C allows us to calculate the maximum theoretical VRM acquired in situ at each horizon, while the detailed thermal study of the NRM permits an appraisal of the DRM quality. This leads us to define a geological viscosity index, which accounts, in a rather convincing manner, for the behaviour of the samples observed during the classical paleomagnetic study. For the section studied, it appears that the variations of this index are closely correlated with the efficiency of the DRM acquisition process.

scholarly journals A Feasibility Study of Microbialites as Paleomagnetic Recorders

2021 ◽  
Vol 9 ◽  
Author(s):  
Ji Jung ◽  
Julie A. Bowles
Keyword(s):  
Geomagnetic Field ◽  
Remanent Magnetization ◽  
Inner Core ◽  
Salt Lake ◽  
Natural Remanent Magnetization ◽  
High Coercivity ◽  
Magnetic Carrier ◽  
Deep Time ◽  
Geologic Record ◽  
Magnetic Field Variations

Microbialites–layered, organosedimentary deposits–exist in the geologic record and extend back in deep time, including all estimated times of inner core nucleation. Microbialites may preserve magnetic field variations at high-resolution based on their estimated growth rates. Previous studies have shown that microbialites can have a stable magnetization. However, the timing and origin of microbialite magnetization were not well determined, and no study has attempted to evaluate whether actively growing microbialites record the geomagnetic field. Here, we present centimeter-scale magnetization and magnetic property variations within the structure of modern microbialites from Great Salt Lake (GSL), United States, and Laguna Bacalar, Mexico, Pleistocene microbialites from GSL, and a Cambrian microbialite from Mongolia. All samples record field directions close to the expected value. The dominant magnetic carrier has a coercivity of 35–50 mT and unblocking temperatures are consistent with magnetite. A small proportion of additional high coercivity minerals such as hematite are also present, but do not appear to appreciably contribute to the natural remanent magnetization (NRM). Magnetization is broadly consistent along microbialite layers, and directional variations correlate with the internal slope of the layers. These observations suggest that the documented NRM may be primarily detrital in origin and that the timing of magnetization acquisition can be close to that of sediment deposition.

LONG-TERM TRENDS IN COSMIC RAYS AND GEOMAGNETIC FIELD SECULAR VARIATIONS

2021 ◽  
Vol 44 ◽  
pp. 79-80
Author(s):  
A.G. Elias ◽  
◽  
B.S. Zossi ◽  
A.R. Gutierrez Falcon ◽  
E.S. Comedi ◽  
...  
Keyword(s):  
Cosmic Rays ◽  
Data Center ◽  
Geomagnetic Field ◽  
The Earth ◽  
Geomagnetic Cutoff Rigidity ◽  
Secular Variations ◽  
Geomagnetic Cutoff ◽  
Long Term Trends ◽  
Long Term Trend

Cosmic rays are modulated by solar and geomagnetic activity. In addition, the flux that arrives to the Earth is sensitive to the inner geomagnetic field through its effect on the geomagnetic cutoff rigidity, Rc. This field has been decaying globally at a rate of ~5% per century from at least 1840. However, due to its configuration and non-uniform trend around the globe, its secular variation during the last decades has induced negative and positive Rc trends depending on location. In the present work, the database from the World Data Center for Cosmic Rays (WDCCR) is used to analyze long-term trend variations linked to geomagnetic secular variations. This database includes more than 100 stations covering, some of them, almost seven decades since the 1950’s. Those stations spanning more than 20 years of data are selected for the present study in order to adequately filter solar activity effects.

scholarly journals Review of Long-Term Trends in the Equatorial Ionosphere Due the Geomagnetic Field Secular Variations and Its Relevance to Space Weather

Atmosphere ◽  
2021 ◽  
Vol 13 (1) ◽  
pp. 40
Author(s):  
Ana G. Elias ◽  
Blas F. de Haro Barbas ◽  
Bruno S. Zossi ◽  
Franco D. Medina ◽  
Mariano Fagre ◽  
...  
Keyword(s):  
Space Weather ◽  
Secular Variation ◽  
Geomagnetic Field ◽  
Upper Atmosphere ◽  
Anthropogenic Activity ◽  
Weather Forecasts ◽  
Term Change ◽  
Secular Variations ◽  
Long Term Trends

The Earth’s ionosphere presents long-term trends that have been of interest since a pioneering study in 1989 suggesting that greenhouse gases increasing due to anthropogenic activity will produce not only a troposphere global warming, but a cooling in the upper atmosphere as well. Since then, long-term changes in the upper atmosphere, and particularly in the ionosphere, have become a significant topic in global change studies with many results already published. There are also other ionospheric long-term change forcings of natural origin, such as the Earth’s magnetic field secular variation with very special characteristics at equatorial and low latitudes. The ionosphere, as a part of the space weather environment, plays a crucial role to the point that it could certainly be said that space weather cannot be understood without reference to it. In this work, theoretical and experimental results on equatorial and low-latitude ionospheric trends linked to the geomagnetic field secular variation are reviewed and analyzed. Controversies and gaps in existing knowledge are identified together with important areas for future study. These trends, although weak when compared to other ionospheric variations, are steady and may become significant in the future and important even now for long-term space weather forecasts.

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