To display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust, a new colored isostatic residual gravity map of the conterminous United States has been prepared using the gravity data set compiled for the Gravity Anomaly Map of the United States (Society of Exploration Geophysicists, 1982). The new isostatic residual gravity map is based on an Airy-Heiskanen model of local compensation, in which the surface load requiring compensation is defined by 5-min topographic and bathymetric data sets. A colored first-vertical-derivative map of isostatic residual gravity further enhances the short-wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field. For the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance, the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort. Most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features. Anomalies on isostatic residual gravity maps should not be casually interpreted in terms of “undercompensation” or “overcompensation” because large-amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium. Many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies. We present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment. In general, highs correlate with intruded or accreted mafic material or with upthrusted crustal sections, whereas lows occur over low-density sedimentary or volcanic sections, felsic intrusive bodies, or down-warped crustal sections. Although some longer-wavelength anomalies, such as the broad gravity high centered over Montana, could be manifestations of density contrasts deep in the mantle, many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers, so that their source bodies need not be deep. The fact that certain of these broader anomalies have well-defined boundaries which correlate with near-surface geologic features increases the likelihood that their sources lie entirely within the lithosphere. If so, then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions. We have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns. Contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the United States. These Fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories.

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A new isostatic residual gravity map of the conterminous United States with a discussion on the significance of isostatic residual anomalies

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

SUMMARY The INDEPTH project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the Tibetan Plateau. In the Lhasa Block, seismic reflection surveys in 1994 detected a number of bright-spots at 15‐20 km depths that indicate zones of crustal fluids (aqueous fluids or partial melt). Coincident magnetotelluric (MT) data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots. Using constrained inversion, the MT data require a minimum crustal conductance of 6000 S. This abnormally high electrical conductance can be best explained by a layered model with fluids: partial melt, aqueous fluids or a combination of partial melt and aqueous fluids. The non-uniqueness of the MT method means that a wide range of melt fraction‐thickness combinations for the above models could all explain the 6000 S conductance. To distinguish between these three models, other geophysical and geological data are required. Reflection seismic data suggest that a high fluid content (>15 per cent) is present at the top of the layer. The amplitude-versus-offset data suggest that the top of this layer may be aqueous fluids rather than partial melt. Passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt. Petrological studies suggest that concentrations of aqueous fluids above 0.1 per cent at mid-crustal depth cannot be sustained. Taken together, these data show that the high conductivity in Southern Tibet is most probably the result of a relatively thin layer of aqueous fluids (100‐200 m) overlying a thicker zone of partial melt (>10 km).

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Partial melt or aqueous fluid in the mid-crust of Southern Tibet? Constraints from INDEPTH magnetotelluric data

Deep electrical resistivity structure of the northwestern U.S. derived from 3-D inversion of USArray magnetotelluric data

Summary The application of d.c. resistivity, magnetotelluric—both natural and artificial source—and geomagnetic variation techniques has provided lithospheric electrical resistivity models which generally display a relatively low resistivity region at lower crustal/upper mantle depths. In this paper, model parameters are summarized for the lower crust in tectonically inactive regions and discussed in more detail with respect to specific studies undertaken in Europe, N. America and Africa. Special attention is drawn to the resolution of the model parameters and to the fact that the depth to the low resistivity layers and the ratio of their thickness to their resistivity can normally be well determined. The petrological interpretation is non-unique. Although electrical resistivity is strongly temperature dependent, the temperature within the lower crust in inactive regions is too low to give an in situ decrease in resistivity of the major rock phases. A more acceptable interpretation involves compositional variations—both lateral and vertical—within the crust associated with an increased amount of free fluids (e.g. water) or of free carbon (e.g. graphite). The correlation of the low resistivity structures in the lower crust with sedimentary layers involved in large-scale overthrusting or related tectonic processes is an attractive idea while the generation of low resistivity shear zones during metamorphism could also be an effective explanation of the observations. Some of the elongated anomalies of low resistivity which are discussed may correspond to Proterozoic plate boundaries and subduction zones.

Electrical resistivity in continental lower crust

A long, narrow belt of rocks of very high electrical conductivity has been discovered and mapped by means of large arrays of recording magnetometers, over a distance of 1400 km from southeastern Wyoming to the edge of the Canadian Shield in Saskatchewan. Evidence that the conductive belt might be associated with conductive minerals in metamorphosed and fractured rocks in the basement has been discussed in earlier papers. Recent results on the Precambrian geology at both ends of the conductor, in the Churchill Province of the Shield and in southeastern Wyoming, support the hypothesis that the anomaly in electrical conductivity traces a major fracture zone in the lithosphere of Precambrian North America. This paper presents and relates various lines of evidence which together tend to substantiate such a fracture zone or mobile belt. From the age and composition of rocks near the southern end of the structure, Hills and others suggest that a Proterozoic subduction zone is located there. It is possible that t...

P. A. Camfield

Papers

A possible Proterozoic plate boundary in North America