1,721,041 research outputs found
Intensity and direction of lattice-preferred orientation of olivine: are electrical and seismic anisotropies of the Australian mantle reconcilable?
No full textIt has been hypothesised that seismic and electrical anisotropy at the base of the lithosphere are caused by strain-induced lattice-preferred orientation (LPO) of olivine [100] axes parallel to present-day plate motion. This would imply that seismic and electrical anisotropy observations can provide geodynamicists with fundamental information for characterising mantle flow. The qualitative agreement between the fast direction of SV-waves and direction of maximum electrical conductance modelled deeper than 150 km below the North Central craton of Australia appear to support a common alignment mechanism, and the observed, anisotropic electrical conductances can be generated by hydrogen diffusivity in a water-poor (< 1000 ppm H/Si) olivine mantle. A quantitative test is proposed for the hypothesis that electrical anisotropy is generated by anisotropic hydrogen diffusion rates (D) in olivine. Electrical anisotropy factors are computed using random resistor network models assuming that D[100] approximate to 20 x D[010] approximate to 40 x D[001]. Electrical and seismic anisotropies calculated from olivine LPO angular distribution functions modelled for a range of shear strains under a simple shear deformation demonstrate that the intensity of olivine [100] alignments (and associated shear strains) that would be required to explain the electrical anisotropy in the mantle below central Australia are significantly greater than predicted by Rayleigh wave anisotropies. The poor agreement between the observed electrical anisotropies and the electrical anisotropies that would be predicted from the Rayleigh wave anisotropies indicates that either (i) electrical anisotropy in the upper mantle below central Australia is not generated by hydrogen diffusivity alone or (ii) the seismic anisotropy is underestimated. The orientation of the olivine [100] axes maxima is inferred to be similar to 30degrees rotated relative to the direction of present-day absolute plate motion (APM) that is determined relative to the hotspot reference frame (HS2-NUVEL1). Both the APM direction that is determined relative to a reference frame defined by requiring no-net rotation of the lithosphere (NNR-NUVEL1) and GPS-derived plate motion vectors fit the geophysical observations of upper mantle anisotropy better. This may support the contention that hotspots are not stationary relative to the deep mantle. (C) 2002 Elsevier Science B.V. All rights reserved
A three-dimensional electromagnetic model of the southern Kenya Rift: Departure from two dimensionality as a possible consequence of a rotating stress field
No full textA generic, three-dimensional (3-D) model has been developed which explains the three-dimensionality exhibited by magnetovariational (MV) and magnetotelluric (MT) data from southern Kenya, In this model, observed variations in electromagnetic strike with period and location, impedance phase splitting, and peaks in tipper magnitude are all understood in terms of two regionally two-dimensional (2-D) structures striking NW-SE and N-S, respectively. The observed period and location dependence of the electromagnetic strike may arise as an indirect consequence of a rotating stress field, with regional-scale structures formed at different stages of the stress-strain history of Kenya being preserved as conductive lineaments. These conductive structures are not all confined to the upper crust. Thus, whereas stress data provide constraints on rifting at the upper crustal scale, the MT impedance tensor data provide constraints at lithospheric scales. The constraints and resolution provided by the MV and MT data have been rigorously investigated using 3-D forward modeling. Decoupling of the period and site dependence of electromagnetic strike aids resolution and constraint of conductors, rendering attempts to fix an average strike in space and frequency inexpedient. An anomalous apparent "strike" at the center of the Rift Valley reflects neither the N-S strike of the rift, nor the NW-SE striking shear fabric, but is shown to be a virtual strike arising as a result of coupling between the respective strike directions. The NW-SE trending conductivity anomaly straddles the rift and both its flanks, extends to at least middle to lower crustal depths, and appears substantially more electrically anomalous than the rift itself. A hypothesis that melt exists in the mantle directly below the rift at latitude 1.8 degrees S is not supported by the MT data
A comparison of electromagnetic distortion and resolution of upper mantle conductivities beneath continental Europe and the Mediterranean using islands as windows
No full textTo investigate the potential of utilising islands as windows through which oceanic mantle conductivities might be imaged, simultaneous magnetotelluric (MT) measurements were carried out continuously over a 3-month duration at two continental stations-one in the Swabian Alps, southern Germany, the other in the Bourgogne, central France-and on the Mediterranean islands of Montecristo (MCHR)and Mallorca (MALL). The MT transfer functions to 1.5 cycles per day (cpd) and single-site Z/H transfer functions for the first four harmonics of the solar quiet (Sq) daily field variations are presented. The transfer functions derived at the island sites are corrected for the distortive and inductive effects arising from the Mediterranean bathymetry relative to the transfer functions calculated at the continental stations using three-dimensional thin sheet modelling. The corrections are formulated in terms of the magnetic perturbation tenser, W. Static shifts in the MT transfer functions are corrected using the Z/H transfer functions, and by comparison with hypothesised resistivity jumps at the transition zones. Models are derived jointly from the residual MT and Z/H transfer functions. Constraints on the resistivities of the upper mantle are investigated. Despite the widely different tectonic settings, no significant differences in the conductivities of the upper mantle below 100 kin are detected. The disadvantages of utilising widely spaced islands as windows to the mantle are highlighted by departures from one-dimensionality in the complex tectonic setting of the Mediterranean region. (C) 2002 Elsevier Science B.V. All rights reserved
Fluid trapping at the brittle–ductile transition re‐examined
No full textThe brittle-ductile transition has been suggested to provide a mechanical trap to deep crustal fluids. The mechanism was advanced as a way of reconciling the geophysical case for a wet lower crust, founded on the revelation of deep crustal electrical conductors and seismic reflectors, with the problem of maintaining interconnected, low-density fluids in stable crust for geologically significant timescales. Although some deep crustal conductors are now attributed to graphite, the hypothesis of fluid trapping at the brittle-ductile transition has been widely adopted in electromagnetic literature, with no regard to tectonic regime, and in association with standardized temperatures of 300-450 degrees C. Meanwhile, petrologists continue to argue that the lower crust is dry. This paper re-examines the arguments on which the hypothesis of fluid trapping at the brittle-ductile transition has been founded, and concludes that there is a geophysical case for a dry lower crust based on electromagnetic studies. The magnetotelluric (MT) technique yields electrical conductances (conductivity-thickness products) that are direction dependent (or anisotropic). The necessity of considering direction-dependent conductances, rather than a bulk conductance, is demonstrated using data from Saxothuringia, Germany. A quantitative model is developed to facilitate joint interpretation of the maximum conductance and the anisotropy of conductance (ratio of maximum to minimum conductance). The model yields quantitative arguments against fluids being the principal cause of deep crustal electrical conductivity, because unreasonably thick layers and unreasonably high porosities are required
Geomagnetic evidence for a continuously connected plume conduit extending to at least the 660-km discontinuity below Hawaii
No full textHydrogen diffusivity and electrical anisotropy of a peridotite mantle
No full textLong-period magnetotelluric (MT) data have indicated that electrical conductivity in the upper mantle is highly anisotropic. Rates and anisotropies for self-diffusion of hydrogen in single crystals of mantle minerals are related to electrical resistivity by the Nernst-Einstein relationship. Assuming that the dominant mechanism for electrical conduction in the mantle is hydrogen diffusion, the electrical anisotropy of a peridotite should be controlled by its mineral composition and by the lattice-preferred orientation (LPO) of its constitutive minerals. Macroscopic electrical anisotropies arising from diffusion of hydrogen in upper mantle rocks displaying strain-induced LPO of olivine, enstatite and diopside are calculated using resistor networks in which each resistor has a statistical probability of representing a mineral grain with a particular misorientation relative to the olivine [100] maximum density direction. The orientations of the grains are defined by angular distribution functions describing LPO (1) generated by viscoplastic self-consistent modelling at a range of shear strains and (2) measured in a naturally deformed peridotite. The naturally deformed peridotite displays a strong LPO, but the predicted mean electrical anisotropy factor is less than 3. Geophysical data indicate higher electrical anisotropies for the mantle. This suggests that grain boundary processes that are controlled by shape-preferred orientation of crystals and/or macroscopic heterogeneities further enhance the electrical anisotropy of the mantle. Ambiguities in the conduction mechanism highlight the need for direct laboratory measurements of ionic conductivities in mantle assemblages that can be compared with those calculated from the Nernst-Einstein equation
Magnetotelluric data from before, during and after the September 2017 magnetic storm at 7 sites in Scotland
No full textMagnetotelluric (MT) time series including the September 2017 magnetic storm at 7 sites in the Scottish Highlands collected by Fiona Simpson (University of Southampton) and Karsten Bahr (University of Göttingen) using Göttingen RAP dataloggers, Magson fluxgate magnetometers and Filloux-type electrodes.
Data acquisition methodology is described in F. Simpson and K. Bahr, 2005. Practical Magnetotellurics, Cambridge University Press, London pp. 254, 2005, ISBN: 9781108462556, DOI: 10.1017/CBO9780511614095
This dataset is described in:
F. Simpson and K. Bahr, 2020a. Nowcasting and validating Earth’s electric field response to extreme space weather events using magnetotelluric data: application to the September 2017 geomagnetic storm and comparison to observed and modelled fields in Scotland, Space Weather, https://doi.org/10.1051/swsc/2020049
F. Simpson and K. Bahr, 2020b. Estimating the electric field response to the Halloween 2003 and September 2017 magnetic storms across Scotland using observed geomagnetic fields, magnetotelluric impedances and perturbation tensors, Journal of Space Weather and Space Climate, https://doi.org/10.1029/2019SW002432</span
Magnetotelluric data for the Halloween 2003 magnetic storm in the vicinity of Uppsala and Eskdalemuir geomagnetic observatories (synthesized using geomagnetic observatory data from INTERMAGNET)
No full textMagnetotelluric data for the Halloween 2003 magnetic storm in the vicinity of Uppsala (UPS), Sweden and Eskdalemuir (ESK), Scotland geomagnetic observatories synthesized from geomagnetic observatory data from INTERMAGNET. The data were generated to facilitate comparison of the ground effects of the Halloween 2003 magnetic storm in Sweden and Scotland. The data demonstrate the greater risk of hazardous storm-time electric fields being generated in southern Sweden compared to central Scotland and are further described in the gold open access paper:
F. Simpson and K. Bahr, 2020a. The role of tectonic-plate thickness and mantle conductance in determining regional vulnerability to extreme space weather events: possible enhancement of magnetic source fields by secondary induction in the asthenosphere. Space Weather, 18(12), [e2020SW002587]. https://doi.org/10.1029/2020SW002587
The synthesis technique that enables electric fields to be estimated from geomagnetic observatory data is described and validated in the following gold open access papers:
F. Simpson and K. Bahr, 2020b. Nowcasting and validating Earth's electric field response to extreme space weather events using magnetotelluric data: application to the September 2017 geomagnetic storm and comparison to observed and modelled fields in Scotland, Space Weather, 18, e2019SW002432, https://doi.org/10.1029/2019SW002432.
F. Simpson and K. Bahr, 2020c. Estimating the electric field response to the Halloween 2003 and September 2017 magnetic storms across Scotland using observed geomagnetic fields, magnetotelluric impedances and perturbation tensors, JSWSC, swsc200019, 10, (48), https://doi.org/10.1051/swsc/2020049.</span
Nowcasting and validating Earth's electric‐field response to extreme space‐weather events using magnetotelluric data: application to the September 2017 geomagnetic storm and comparison to observed and modelled fields in Scotland
No full textIn the UK, geomagnetically induced currents (GICs) are calculated from thin‐sheet electrical conductivity models. In the absence of conductivity models, time derivatives of magnetic fields are sometimes used as proxies for GIC‐related electric fields. An alternative approach, favored in the US, is to calculate storm‐time electric fields from time‐independent impedance tensors computed from an array of magnetotelluric (MT) sites and storm‐time magnetic fields recorded at geomagnetic observatories or assumed from line‐current models. A paucity of direct measurements of storm‐time electric fields has restricted validation of these different techniques for nowcasting electric fields and GICs. Here, we present unique storm‐time electric‐field data from 7 MT sites in Scotland that recorded before, during and after the September 2017 magnetic storm. By Fourier transforming electric‐field spectra computed using different techniques back to the time domain, we are able to make direct comparisons with these measured storm‐time electric‐field time series. This enables us to test the validity of different approaches to nowcasting electric fields. Our preferred technique involves frequency‐domain multiplication of magnetic‐field spectra from a regional site with a local impedance tensor that has been corrected for horizontal magnetic‐field gradients present between the local site and the regional site using perturbation tensors derived from geomagnetic depth sounding (GDS). Scatter plots of scaling factors between measured and nowcasted electric fields demonstrate the importance of coupling between the polarization of the storm‐time magnetic source field and Earth's direction‐dependent deep electrical conductivity structure
Resistance to mantle flow inferred from the electromagnetic strike of the Australian upper mantle
No full textSeismic anisotropy is thought to result from the strain-induced lattice-preferred orientation of mantle minerals, especially olivine, owing to shear waves propagating faster along the a-axis of olivine crystals than along the other axes. This anisotropy results in birefringence, or 'shear-wave splitting', which has been investigated in numerous studies. Although olivine is also anisotropic with respect to electrical conductivity (with the a-axis being most conductive), few studies of the electrical anisotropy of the upper mantle have been undertaken, and these have been limited to relatively shallow depths in the lithospheric upper mantle. Theoretical models of mantle flow have been used to infer that, for progressive simple shear imparted by the motion of an overriding tectonic plate, the a-axes of olivine crystals should align themselves parallel to the direction of plate motion. Here, however, we show that a significant discrepancy exists between the electromagnetic strike of the mantle below Australia and the direction of present-day absolute plate motion. We infer from this discrepancy that the a-axes of olivine crystals are not aligned with the direction of the present-day plate motion of Australia, indicating resistance to deformation of the mantle by plate motion
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