Inversion of magnetovariation event to causative current. III. Test of channelled current model, SABC conductor inverted as chennelled currents and structure evaluation
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Cited by 5 publications
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This review presents results from magnetovariation fields recorded by two-dimensional arrays of magnetometers. The emphasis is on the conductive structures mapped and studied and their tectonic implications. Eleven arrays were operated in North America between 1967 and 1985. A region of highly conductive uppermost mantle and/or lower crust, extending at least from 33øN to 54øN, has been shown to extend beneath much of the continent west of the Rocky Mountains. Two recent investigations are discussed in more detail: those of conductive structures under the Canadian Rockies and EMSLAB (Electromagnetic Sounding of the Lithosphere and Beyond) array results for Washington and Oregon, including a conductive strip beneath the Cascades volcanoes. Correlations with high heat flow and seismic parameters make it reasonable to attribute these regional conductors to partial melting and/or hot saline water. Within the craton the North American Central Plains conductor is discussed; this narrow, crustal feature is associated with a major fracture zone and age boundary, which may be a plate boundary of Proterozoic time. An array beneath the auroral electrojet, for study of the external currents, is noted. Results from four array studies in southern Africa are considered. Two of these relate to the Southern Cape Conductive Belt, which correlates not with high heat than the value for seawater (0.25 ohm m) to 106 ohm m or more for very dry crystalline rocks. This range of seven orders of magnitude may be compared with the single order of magnitude that covers seismic wave velocities in the Earth. These two disciplines differ in another way. Seismic disturbances propagate according to wave equations, whereas the low-frequency electromagnetic fields used in magnetovariation studies propagate in conducting media, such as even the most resistive rocks, according to diffusion equations which are a special case pages 141 -15 7 Paper number 89RG00095 142 ø REVIEWS OF GEOPHYSICS / 27,1 of Maxwell's equations. For the magnetic field B the diffusion equation is V 2 B = i tOOl. tB (1) where to is frequency and • is permeability. For such diffusing fields the skin depth, at which B falls to 1/e of the surface values in a uniform half-space, is useful. In units useful in the Earth this skin depth is 30.2 x/-•/c• km, with the period T in hours and conductivity c• in siemens per meter (1 S = 1 ohm'•).Conductive structure in the Earth can be investigated by means of an array of magnetometers on the surface recording three components of natural time-varying magnetic fields, in the period range from 1 min to 1 day. The natural magnetovariation (MV) fields are produced in part by primary currents of various three-dimensional geometries in the ionosphere and magnetosphere [Rostoker, 1972;Kisabeth and Rostoker, 1977] and in part by secondary, induced currents in the solid Earth. The latter, internal currents flow preferentially in the more conductive rocks, which can be located by mapping the MV fields.A magnetometer array may be one dimensional (1D), ...
This review presents results from magnetovariation fields recorded by two-dimensional arrays of magnetometers. The emphasis is on the conductive structures mapped and studied and their tectonic implications. Eleven arrays were operated in North America between 1967 and 1985. A region of highly conductive uppermost mantle and/or lower crust, extending at least from 33øN to 54øN, has been shown to extend beneath much of the continent west of the Rocky Mountains. Two recent investigations are discussed in more detail: those of conductive structures under the Canadian Rockies and EMSLAB (Electromagnetic Sounding of the Lithosphere and Beyond) array results for Washington and Oregon, including a conductive strip beneath the Cascades volcanoes. Correlations with high heat flow and seismic parameters make it reasonable to attribute these regional conductors to partial melting and/or hot saline water. Within the craton the North American Central Plains conductor is discussed; this narrow, crustal feature is associated with a major fracture zone and age boundary, which may be a plate boundary of Proterozoic time. An array beneath the auroral electrojet, for study of the external currents, is noted. Results from four array studies in southern Africa are considered. Two of these relate to the Southern Cape Conductive Belt, which correlates not with high heat than the value for seawater (0.25 ohm m) to 106 ohm m or more for very dry crystalline rocks. This range of seven orders of magnitude may be compared with the single order of magnitude that covers seismic wave velocities in the Earth. These two disciplines differ in another way. Seismic disturbances propagate according to wave equations, whereas the low-frequency electromagnetic fields used in magnetovariation studies propagate in conducting media, such as even the most resistive rocks, according to diffusion equations which are a special case pages 141 -15 7 Paper number 89RG00095 142 ø REVIEWS OF GEOPHYSICS / 27,1 of Maxwell's equations. For the magnetic field B the diffusion equation is V 2 B = i tOOl. tB (1) where to is frequency and • is permeability. For such diffusing fields the skin depth, at which B falls to 1/e of the surface values in a uniform half-space, is useful. In units useful in the Earth this skin depth is 30.2 x/-•/c• km, with the period T in hours and conductivity c• in siemens per meter (1 S = 1 ohm'•).Conductive structure in the Earth can be investigated by means of an array of magnetometers on the surface recording three components of natural time-varying magnetic fields, in the period range from 1 min to 1 day. The natural magnetovariation (MV) fields are produced in part by primary currents of various three-dimensional geometries in the ionosphere and magnetosphere [Rostoker, 1972;Kisabeth and Rostoker, 1977] and in part by secondary, induced currents in the solid Earth. The latter, internal currents flow preferentially in the more conductive rocks, which can be located by mapping the MV fields.A magnetometer array may be one dimensional (1D), ...
Summary This paper describes the first detailed 3-D study of the Alberta basement using the magnetotelluric (MT) method. Long-period MT data were used to generate a 3-D electrical resistivity model of the crust and upper mantle beneath Alberta. The Western Canada Sedimentary Basin was imaged as a low resistivity layer. A number of crustal and upper mantle conductors were imaged and include: (1) the Red Deer conductor, (2) the Kiskatinaw conductor, (3) a conductor coincident with the Kimiwan oxygen isotope anomaly, (4) the southern Alberta—British Columbia conductor, (5) the Loverna block conductor, and (6) a conductor beneath the Birch Mountain kimberlite field. Conductors (3) and (6) are newly identified in this study. All of these conductors can be related to past tectonic events associate with the assembly of Laurentia and subsequent kimberlite magmatism. The depth of the lithosphere-asthenosphere boundary across Alberta varies from 150 km to 300 km as defined from the 100 Ωm contour. Furthermore, the Snowbird tectonic zone in Alberta is characterized by a thick, resistive lithosphere. Similarly, the diamondiferous Buffalo Head Hills kimberlite field is imaged with a thick, resistive lithosphere, both may indicate depletion. In contrast, the Birch Mountain kimberlite field is underlain by a pronounced conductor that extends through the lithosphere and may be the result of metasomatism.
A synthesis of electromagnetic studies in the Lithoprobe Alberta Basement Transect: constraints on Paleoproterozoic indentation tectonics
Electromagnetic surveys of the crust and upper mantle underlying the Western Canada Sedimentary Basin have revealed two conductivity anomalies that are arguably related to Paleoproterozoic tectonic processes, recording a signature diagnostic of euxinic (foreland?) basin succession and perhaps defining a deep-marine transtensional basin. As well, a regionally pervasive electrical anisotropy throughout the crust crosses Proterozoic tectonic boundaries and is aligned in many areas with the regional tectonic fabric (as expressed by aeromagnetic data) created during the final stages of Proterozoic tectonic assembly of western Laurentia. There is also an order of magnitude increase in the upper mantle conductivity underlying Churchill Province Archean crust relative to that underlying the neighbouring Proterozoic crust. The interpretations of the electromagnetic observations are analogous with the Tertiary to modern-day tectonic development of southeast Asia. The foreland basin succession would reflect the closing of the South China Sea and the deep-marine basin may be analogous to the Andaman Sea and North Sumatra basin. Electrical anisotropy may be indicative of the successive lateral accretion of oceanic crust beneath the continental shelf followed by shortening. The enhanced mantle conductivity mechanism is unknown, although it may be associated with metasomatic modification of the subcontinental lithosphere subsequent to collisional thickening and followed by delamination or convective removal of some lithosphere. While the Indo-Eurasian-Laurentia tectonic analogy is incomplete, it is sufficiently compelling in explaining the electromagnetic and other data that it is a useful basis for understanding the Proterozoic assembly of Laurentia.Résumé : Des relevés électromagnétiques de la croûte et du manteau supérieur sous le bassin sédimentaire de l'Ouest canadien ont révélé deux anomalies conductrices qui peuvent être reliées aux processus tectoniques en cours au Paléo-protérozoïque, fournissant ainsi une signature diagnostique d'une succession de bassins euxiniques (d'avant-pays) et peut-être définissant un bassin marin profond de tension transversale. Aussi, une anisotropie électrique régionale péné-trant à travers toute la croûte recoupe les limites tectoniques du Protérozoïque et, dans plusieurs régions, est alignée avec la texture tectonique régionale (telle qu'exprimée par les données aéromagnétiques) créée durant les phases finales de l'assemblage tectonique de la Laurentie occidentale au Protérozoïque. Il y a aussi une augmentation d'un ordre de magnitude dans la conductivité du manteau supérieur sous-jacent à la croûte archéenne de la province de Churchill par rapport à la conductivité du manteau supérieur sous-jacent à la croûte protérozoïque avoisinante. Les interprétations des observations électromagnétiques sont analogues au développement du sud-est de l'Asie, du Tertiaire au temps présent. La succession des bassins d'avant-pays correspondrait à la fermeture de la mer de Chine méridionale et le bassin mar...
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