Electrical Conductivity Models of the Crust

We have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles. Regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models; we avoided those data where purely conductive heat flow suggested temperatures more than about 1000°C, substantially higher than solidus temperatures and outside the range of validity of heat flow models. The resulting plots of log conductivity σ versus 1/T demonstrate that even low‐conductivity layers (LCL) have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits. In addition, technically active regions show systematically higher conductivities than do shield areas. Because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions, these materials probably account for the relatively higher conductivities of rocks in situ in the crust; free water in amounts of 0.01–0.1% in fracture porosity could explain crustal conductivities. Other possibilities are graphite, hydrated minerals in rare instances, and sulfur in combination with other volatiles. As most of the temperatures are less than 700°C, partial melting seems likely only in regions of highest heat flow where the conductive temperature profiles are inappropriate. Another result is that at a given temperature, crustal high‐conductivity layers (HCL) are more conductive by another order of magnitude and show more scatter than do LCL's. Because the differences between HCL's and LCL's are independent of temperature, we must invoke more than temperature increases as a cause for large conductivity increases; increased fluid concentration in situ seems a probable cause for enhanced conductivities in HCL's. From the point of view of these observations, it does not matter whether the fluids are in communication with the surface or trapped at lithostatic pressures.

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The interpretation and tectonic implications of the geoelectric structure of Southern Scotland

Ingham

Hutton

1982

Geophys J Int

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Summary Recent magnetotelluric observations in a traverse across Southern Scotland from the Grampian Highlands across the Midland Valley to the Southern Uplands have led to a two‐dimensional model in which layers of anomalously high conductivity exist at varying depths along the traverse. From available heat flow measurements and thermal conductivity estimates of probable crustal and upper mantle materials, temperature—depth curves have been derived for the three regions and these in turn have been used to interpret the electrical conductivity in terms of the possible causes of high conductivity. In the Highlands and Midland Valley the conducting zones have been identified with ionic or electronic conduction in hydrated rocks and the process of dehydration at the boundary between amphibolitic and granulitic facies. The increase in depth of the conducting zone towards the north lends support to the hypothesis that the Highland Boundary Fault is the location of an ancient subduction zone. The thick conductor under the Southern Uplands is also interpreted as arising from both electronic conduction in hydrated rocks and dehydration at the amphibolitic–granulitic transition but with the additional possibility of a partial melt zone at depths of about 70 km.

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Electrical Conductivity Models of the Crust

Cited by 7 publications

References 8 publications

On mapping multiple electrical discontinuities associated with deep seated anisotropic oblique zones using magnetotelluric frequency sounding

On mapping multiple electrical discontinuities associated with deep seated anisotropic oblique zones using magnetotelluric frequency sounding

Electrical conductivity, temperatures, and fluids in the lower crust

The interpretation and tectonic implications of the geoelectric structure of Southern Scotland

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