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/sup 0/C, substantially higher than solidus temperatures and outside the range of validity of heat flow models. The resulting plots of log conductivity sigma 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, tectonically 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/sup 0/C, partial melting seems likely onlymore » in regions of highest heat flow where the conductive temperature profiles are inappropriate. Another result is that at a given temperature, crustal high-conductivity layes (HCL) are more conductive by another order of magnitude and show more scatter than do LCL's.« less

Electrical conductivity, temperatures, and fluids in the lower crust

The continental middle to lower crust (CLC) remains one of the most enigmatic parts of the Earth about which comparatively little is known. Remote-sensing geophysical and geochemical data illustrate our gaps in complete knowledge of the state and composition of the CLC. Generally, it is thought that the CLC, when compared to the upper crust, is somewhat more uniform in its properties. However, studies undertaken in the last decade have revealed that the CLC can be as heterogeneous as the upper crust, but that some regions of surprising homogeneity exist. In this chapter, I will describe attempts to image one particular physical parameter of the CLC, namely its electrical conductivity a. Electromagnetic (EM) sounding methods, as with most geophysical methods, are sensitive to the structure that exists today, and as such are dependent on the current state and composition of the CLC, rather than its state at formation or as a consequence of tectonic or metamorphic activity as are petrological studies on exhumed samples from the CLC. All EM methods give responses that are volumetric averages of the Earth's conductivities sensed by the diffusive EM fields, and as such are in the same class of geophysical techniques as seismic surface wave studies, potential field methods, and geothermal investigations. These are distinct from seismic reflection and refraction methods which deal with non-diffusive waves. However, with EM methods the governing field equations (§2) and wide range of the physical parameter being sensed (electrical conductivity, §1.1) ensure a far greater resolving power to anomalies than with the other diffusive geophysical techniques. EM and seismic methods are the only geophysical techniques for which probing of the deep crust is assured; with all others there are inherent screening effects and depth ambiguities. Imaging the electrical conductivity structure beneath the surface has a number of uses, classified mainly by the depth of investigation: (1) Identifying zones of mineralization, which are of economic importance. (2) Detecting fluids in the deep crust, such as those above downgoing slabs, e.g., the Juan de Fuca plate (Kurtz et al., 1986a, 1990; EMSLAB, 1989). (3) Resolving one physical property of crustal zones. As expressed by Dohr et al. (1989): " ... magnetotellurics works like a broad paint brush, colouring the layers bounded by the seismic reflections" .

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Electrical conductivity of the continental lower crust

The most revealing description of electromagnetic (EM) distortions due to near-surface inhomogeneities and topography is in terms of galvanic and inductive effects. In either case, the distorted electric and magnetic fields can be best visualized as a vectorial sum of primary and secondary fields. Secondary electric fields due to electric charge build-up in the galvanic case persist to the longest periods. In contrast, the secondary electric and magnetic fields due to inductive, vortex currents disappear at long periods. The static shift of magnetotelluric (MT) apparent resistivity sounding curves is a classic example of the galvanic effect. Methods to correct for unwanted distortions such as the static shift can be classified into six categories: use of invariant response parameters, curve shifting, statistical averaging, spatial filtering, use of distortion tensors, and computer modeling. Although invariant impedance calculations are simple to make, they cannot, in general, recover the undistorted impedance. Short period curve shifting is best done with auxiliary soundings such as time domain EM; however, this requires multiple surveys. The shifting of long period MT sounding branches is useful if a standard curve is known and can be matched. Statistical averaging of neighboring MT soundings that are conformal but static shifted has proven very effective at removing random distortions if adaquate data are available. The new EMAP (Electromagnetic Array Profiling) method combats the inherent spatial high pass characteristics of EM distortions by low pass operations in data collection and processing. EMAP proposes the continuous, in-field measurement of electric field dipoles to avoid spatial aliasing. Distortion tensor stripping of topographic distortions is possible since terrain is deterministic but stripping the effects of uncertain subsurface inhomogeneities may be misleading. A new decomposition of the MT impedance tensor under the assumption of surficial three-dimensional (3-D) galvanic effects imposed on a one- or two-dimensional (1-D and 2-D) regional setting promises a way to recover the regional structure. There is a continual need for 3-D computer modeling to test new methods and to calculate topographic and regional effects. Computer modeling has established the value of 2-D modeling of the data identified as transverse magnetic (TM) in some 3-D environments. Ideally, EM distortion correction requires continuous, or at least many, data and the application of more than one correction-modeling scheme.

Near-surface and topographic distortions in electromagnetic induction

Several regional seismic refraction and magnetotelluric (MT) profiles have been completed across the Cascade Range and surrounding geologic provinces in California, Oregon, and Washington. Joint geologic interpretation of the two data sets provides constraints on the nature of the crust not available from either data set alone. Analysis of three MT and two seismic refraction profiles in Oregon and a coincident MT and refraction profile in northern California show a high degree of correlation between resistivity and velocity models. The main feature that is evident in both data sets is a highly conductive (2–20 ohm m) zone that occurs at depths of 6–20 km and largely within a midcrustal velocity layer of 6.4–6.6 km/s, overlying a lower crust with velocities of 7.0–7.4 km/s. Although this conductor and the midcrustal zone of 6.4–6.6 km/s velocities are generally rather horizontal, important structures do occur. For instance, near the boundary of Western Cascades and High Cascades the MT midcrustal conductor rises to within 6 km of the surface. In addition, on the coincident MT-refraction profile in northern California a significant westward downdip occurs on both the MT deep conductor and the 6.4-km/s velocity layer, with both occurring at very similar depths. However, in the Columbia Plateau of Washington, no deep crustal conductors occur shallower than 25 km; also, the velocity structure is quite different, with a 6.8-km/s midcrust and a 7.5-km/s lower crust. Complex accretionary structures occur on MT models for the southern Washington Cascades, quite unlike the more horizontal structures of the Oregon and California Cascades. The accretionary structures in the southern Washington Cascades have been shown to be related to stress release in the area of Mount St. Helens. In order to explain the similar structures in the MT and refraction models for Oregon and California, we propose a model involving the effects of metamorphic zonation to produce the velocity structure, combined with metamorphically produced fluids and partial melt to produce the deep conductor. The higher midcrustal velocities and larger depth to deep MT conductors under the Columbia Plateau are explained by a more mafic crustal composition and lower heat flow.

Peter S. Mitchell

Papers

Magnetotelluric results opposing magma origin of crustal conductors in the rio grande rift