Electrical Anisotropy Below Slow- and Fast-Moving Plates: Paleoflow in the Upper Mantle?

Bahr, Karsten; Simpson, Fiona

doi:10.1126/science.1066161

Cited by 83 publications

(49 citation statements)

References 18 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Decomposed impedance strikes with a NE-SW trend to periods of $10,000 s lead Bahr and Simpson (2002) to suggest that the olivine in the upper mantle below southern Fennoscandia was strongly aligned despite the slow movement of the plate, thus inferring convective processes or fossil anisotropy were also contributing. However, forward modeling of the regional conductance map shows that crustal structure at least in the centralsouthern portion of the project area was sufficient to explain the great majority of the strikes and apparent anisotropy (Korja et al, 2002b;Lahti et al, 2005) (Figure 14).…”

Section: Fossil Transpressional Regimesmentioning

confidence: 96%

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Wannamaker

2005

Surv Geophys

134

View full text Add to dashboard Cite

Electrical anisotropy, the effect of current density in a medium being a function of the orientation of the electric field, is being recognized increasingly as an important effect in explaining Earth electromagnetic observations. A consideration of anisotropy, however, in most cases is an admission of spatial aliasing in earth structure, wherein the averaging volume of diffusive EM fields may be greater than the characteristic dimensions of a family of oriented structures, thus leading to a response which is equivalent to a bulk anisotropic medium. Even for two-dimensional geometries, there can be strong non-parallelism of principal axes of vertical magnetic field relative to the impedance over broad areas, as well as impedance phase variations which leave normal quadrants, if there are multiple directions of anisotropy or anisotropy strike distinct from bulk geometric (2D) strike. This paper concentrates on experience with regional field studies in continental settings where bulk anisotropy is apparent. Upper crustal anisotropy may result from preferred orientations of fracture porosity, or lithologic layering, or oriented heterogeneity. Lower crustal anisotropy may result from preferred orientations of fluidized/melt-bearing or graphitized shear zones, but does not necessarily reflect current state of stress per se. In the upper mantle, the prior causes all may act in pertinent domains, but added to these is the possibility of strong electrical anisotropy due to hydrous defects within shear-aligned olivine crystals (solid-state conduction). Several field examples from continental MT investigations will be discussed, which roughly fall into active transpressional, active transtensional, and fossil transpressional regimes. A general challenge in interpreting data with apparent anisotropic effects is to establish the tradeoff between heterogeneity and anisotropy in the inversion of EM responses.

show abstract

Section: Fossil Transpressional Regimesmentioning

confidence: 96%

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Wannamaker

2005

Surv Geophys

134

View full text Add to dashboard Cite

show abstract

“…Others have now made similar interpretations elsewhere in Canada and worldwide (Boerner et al 1999;Simpson 2001;Bahr and Simpson 2002;Hamilton et al 2006;Padilha et al 2006), although some of these interpretations are coming under closer scrutiny (Lahti et al 2005;Nieuwenhuis et al 2013). Also pioneered in Canada, and fully consistent with the multidisciplinary ethos of Lithoprobe, are qualitative comparisons between electrical and seismic anisotropy, with the first being the key publication by Ji et al (1996) in which the obliquity between the seismic and electrical anisotropy directions was interpreted as an indicator of mantle ductile shearing.…”

Section: Results With Global Consequencesmentioning

confidence: 81%

“…This is shown in Hamilton et al (2006) for South Africa, Miensopust et al (2011) for Botswana, and Adetunji et al (2014) for Ontario. An example of the dangers in using period as a proxy for depth is the interpreted imaging of the asthenospheric flow direction beneath Scandinavia from MT phases at a period of 2049 s by Bahr and Simpson (2002), whereas these long-period MT phase anisotropy effects were shown to be due to crustal artefacts by Lahti et al (2005). In addition, if there is significant difference in the resistivity in orthogonal lateral directions, due to either extrinsic (structure) or intrinsic (grain) anisotropy, the two different modes of MT will sample different depths at the same period (Jones 2006).…”

Section: Depth Of Penetrationmentioning

confidence: 99%

The electrical resistivity of Canada’s lithosphere and correlation with other parameters: contributions from Lithoprobe and other programmes

et al. 2014

View full text Add to dashboard Cite

Over the last 30 years, through Lithoprobe and other programmes, modern, high-quality magnetotelluric (MT) measurements probing deep into the lithosphere and underlying asthenosphere have been made at over 6000 sites across Canada in all provinces and territories, except Nova Scotia. Some regions are well covered, particularly Alberta, southern British Columbia, and western Ontario, whereas others remain poorly covered, such as Quebec and large swaths of Nunavut. Prior publications from individual studies have added significantly to the wealth of Canada's geoscience knowledge, and have demonstrated that MT can contribute significantly to understanding of the tectonic processes that have shaped Canada. However, to date no continent-scale maps of lithospheric electrical parameters have been constructed from the extensive MT database. Herein we review the contributions made by the MT components of Lithoprobe, and present new continental-scale maps of various electrical parameters at crustal and upper mantle depths for the whole of Canada. From those maps, combined with regional estimates of temperature, we develop derivative information on petrological-geophysical properties, including predictions of temperature and water content. We find that at 100 km depth the Canadian Shield is cold and dry, and the Cordillera is warmer but mostly dry, i.e., little water is present in the peridotite. Exceptions are beneath the Prairies, the Wopmay Orogen, and northeast Nunavut where there does appear to be water in the nominally anhydrous minerals. Also, southwest British Columbia appears colder than the rest of the Cordillera due to the subducting Juan de Fuca plate. In contrast, at 200 km depth almost all of Canada is dry.Résumé : Au cours des trente dernières années, grâce à Lithoprobe et à d'autres programmes, des mesures magnétotelluriques (MT), modernes et de grande qualité, ont été prises à plus de 6000 sites à travers le Canada, dans toutes les provinces et territoires sauf en Nouvelle-Écosse; ces mesures sondaient profondément dans la lithosphère et l'asthénosphère sous-jacente. Certaines régions sont bien couvertes, surtout l'Alberta, le sud de la Colombie-Britannique et l'ouest de l'Ontario, alors que d'autres sont mal couvertes, par exemple le Québec et de grandes parties du Nunavut. Des publications antérieures, basées sur des études individuelles, ont grandement contribué à enrichir les connaissances géoscientifiques canadiennes et elles ont démontré que les mesures MT peuvent grandement contribuer à la compréhension des processus tectoniques qui ont formé le Canada. Toutefois, à ce jour, aucune carte des paramètres électriques lithosphériques, à l'échelle du continent, n'a été construite à partir de la vaste base de données MT. Dans le présent article, nous revoyons les contributions des composantes MT de Lithoprobe et nous présentons de nouvelles cartes à l'échelle du continent de divers paramètres électriques aux profondeurs de la croûte et du manteau supérieur pour l'ensemble du Canada. À partir de ces cartes, ...

show abstract

“…In a recent paper, Simpson (2013) showed that electrical anisotropy generated by hydrogen diffusivity in aligned olivine at mantle depth has an anisotropy factor of \4.5. Therefore, in other cases where much stronger electrical anisotropy in the mantle has been inferred from magnetotelluric models (e.g., Bahr and Simpson 2002;Gatzemeier and Moorkamp 2005), the anisotropy needs to be explained by other mechanisms, or isotropic heterogeneities have to be considered to explain the data (e.g., Korja et al 2002). Caricchi et al (2011) showed that anisotropy of electrical conductivity can be generated by interconnected melt channels in the spreading direction and low melt connectivity in the ridge parallel direction.…”

Section: Mantle Flow and Anisotropymentioning

confidence: 97%

“…Electrical anisotropy at lower crustal and upper mantle depths has been inferred from MT studies and been related to mantle flow (Simpson 2001;Bahr and Simpson 2002;Gatzemeier and Moorkamp 2005). One explanation for the anisotropy is the alignment of olivine crystals with respect to plate movement (lattice-preferred alignment).…”

Section: Mantle Flow and Anisotropymentioning

confidence: 99%

On the Coupling of Geodynamic and Resistivity Models: A Progress Report and the Way Forward

Heise

Ellis

2015

Surv Geophys

View full text Add to dashboard Cite

Magnetotelluric (MT) studies represent the structure of crust and mantle in terms of conductivity anomalies, while geodynamic modelling predicts the deformation and evolution of crust and mantle subject to plate tectonic processes. Here, we review the first attempts to link MT models with geodynamic models. An integration of MT with geodynamic modelling requires the use of relationships between conductivity and rheological parameters such as viscosity and melt fraction, which are provided by laboratory measurements of rock properties. Owing to present limitations in our understanding of these relationships, and in interpreting the trade-off between scale and magnitude of conductivity anomalies from MT inversions, most studies linking MT and geodynamic models are qualitative rather than providing hard constraints. Some recent examples attempt a more quantitative comparison, such as a study from the Himalayan continental collision zone, where rheological parameters have been calculated from a resistivity model and compared to predictions from geodynamic modelling. We conclude by demonstrating the potential in combining MT results and geodynamic modelling with examples that directly use MT results as constraints within geodynamic models of ore bodies and studies of an active volcano-tectonic rift.

show abstract

Electrical Anisotropy Below Slow- and Fast-Moving Plates: Paleoflow in the Upper Mantle?

Cited by 83 publications

References 18 publications

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

Anisotropy Versus Heterogeneity in Continental Solid Earth Electromagnetic Studies: Fundamental Response Characteristics and Implications for Physicochemical State

The electrical resistivity of Canada’s lithosphere and correlation with other parameters: contributions from Lithoprobe and other programmes

On the Coupling of Geodynamic and Resistivity Models: A Progress Report and the Way Forward

Contact Info

Product

Resources

About