Geomagnetic Deep Sounding and Upper Mantle Structure in the Western United States
TL;DR: In this paper, Fourier analysis of a polar substorm and of a storm was performed with a two-dimensional array of 42 three-component variometers between latitudes 36" and 43" N and longitudes 101" W and 116" W.
Abstract: Summary Magnetic field time variations were observed in September 1967, with a two-dimensional array of 42 three-component variometers between latitudes 36" and 43" N and longitudes 101" W and 116" W. Fourier analysis of a polar substorm and of a storm shows that the former has a smooth spectrum and the latter a complex spectrum with many maxima. Upper mantle conductivity structure can be seen qualitatively in the original variograms, but is far more sharply defined in maps of Fourier spectral component amplitudes and phases. A ridge of high conductivity runs at a depth no greater than 200 km under the Southern Rocky Mountains between the Great Plains and the Colorado Plateau, which marks a low-conductivity region within the Cordillera. A strong conductivity anomaly runs north-south along the Wasatch Front through central Utah, and indicates the presence of an upwelling of highly conductive material at depth no greater than 120 km along the edge of a step structure which brings the conductive mantle to shallower depth under the Basin and Range Province than under the Colorado Plateau. Long-period maps from the storm suggest a rise in the conductive mantle between the northsouth structures, from the Colorado Plateau southward to the Basin and Range. The daily variation shows the conductivity structures and indicates their great extent in depth. The geomagnetic deep sounding anomalies are found to be in excellent agreement with existing heat flow data, and this supports correlation of electrical conductivity with temperature. There is also good correlation with the available seismic velocity information for the upper mantle. 1. Introdaction Geophysical observations of several kinds indicate that the upper mantle of the Earth under North America is laterally inhomogeneous. Upper mantle seismic velocities of 8.0 km s-' or larger are characteristic of the eastern United States and the Great Plains Province, while velocities decrease to values of 7.9 km s-' or lower west of the Rocky Mountains (Herrin & Taggart 1962). A similar pattern is shown by travel-time anomalies of seismic waves at vertical incidence. P and S waves arrive early at stations in the eastern United States; late arrivals are predominant in the western United States (Cleary & Hales 1966; Doyle & Hales 1967; Hemn & Taggart 1968). As the differences between the P travel-time residuals and the gravity anomalies in the central and western U.S. cannot be explained by the Birch (1961) relation between velocity and density, Hales & Doyle (1967) suggested that tempera
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TL;DR: In this article, the electrical conductivity of a synthetic basalt has been determined at atmospheric pressure in air at temperatures between 700 ø and 1600øC, and it is suggested that partial fusion in the seismic low-velocity zone of the upper mantle may account for the correlation of increased electrical conductivities with development of this zone.
Abstract: The electrical conductivity of a synthetic basalt has been determined at atmospheric pressure in air at temperatures between 700 ø and 1600øC. The conductivity increases by about 2 orders of magnitude in the melting interval between 1130 ø and 1263øC. It is suggested that partial fusion in the seismic low-velocity zone of the upper mantle may account for the correlation of increased electrical conductivity with development of this zone.
131 citations
TL;DR: A detailed study of the North American Central Plains (NACP) conductive body earlier discovered striking northward from the Black Hills roughly along longitude 104" W in the United States was carried to within 90 km of the exposed Canadian Shield of north central Saskatchewan.
Abstract: Summary An array of 41 three-component magnetometers recorded geomagnetic disturbances during August and September 1972, between latitudes 42" and 54" N and longitudes 98" and 109" W. The objective was a detailed study of the North American Central Plains (NACP) conductive body earlier discovered striking northward from the Black Hills roughly along longitude 104" W in the United States. Source fields were provided by polar magnetic substorms and by other events with incident fields more uniform over the array. Northward mapping of the conductor was carried to within 90 km of the exposed Canadian Shield of north central Saskatchewan. Between latitudes 43" N and 48" N maximum depth estimates place the conductor within the lithosphere; the currents probably flow at smaller depths, i.e. within the crust. At the Shield edge the axis of the conductor is parallel to the strike of fold belts and fault zones in metamorphic rocks, including mylonites, characteristic of intense grinding and crushing. Graphitic conductors are known in many fracture zones. The conductivity anomaly links these structural elements in the Churchill Province Shield to the metamorphic belt mapped by Lidiak in the South Dakota basement, and to the Black Hills. South of the Black Hills the conductor turns southwest to the northern end of the Southern Rockies. There is evidence which suggests, but does not demonstrate, a conductive link there between the mantle conductor under the Southern Rockies and the NACP crustal conductor. It is postulated that the linear crustal structure may be a major continental fracture zone now mapped over a total length of 1800 km, of which 300 km is exposed in the Shield. Two of three earthquakes located in southern Saskatchewan have epicentres close to the axis of the conductive body.
113 citations
TL;DR: A long, narrow belt 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.
Abstract: 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...
98 citations
TL;DR: In this article, the variation field of a polar substorm was recorded by an array of 42 variometers in the western United States and was separated by surface integral methods into parts of external and internal origins at four times in the time domain and at four periods in the period domain.
Abstract: Summary
The variation field of a polar substorm was recorded by an array of 42 variometers in the western United States. This field has been separated by surface integral methods into parts of external and internal origins, at four times in the time domain and at four periods in the period domain. It is shown that the anomalies in the vertical and east-west horizontal variation fields are of internal origin and that the external fields vary smoothly over the array. The separated fields in both domains show internal currents induced in north-south striking conductive structures by the east-west horizontal field. Phase differences between the normal and anomalous fields are about 30°, and indicate large, highly-conducting structures in which self-induction controls the currents. The in-phase normalized anomalous fields at period 89 minutes have been approximated by two-dimensional models made up of upheavals of semi-circular section and a step in the surface of a perfectly-conducting half-space. A semicircular upheaval of radius 150 km from an unperturbed level at depth 360 km models the anomaly related to the Southern Rockies. A step of height 120km at the Wasatch Front, together with a semi-cylinder of radius 100 km under the Wasatch fault belt, give a good approximation to the observed anomaly at the Wasatch Front. The actual structures may be somewhat shallower and smoother. The real conductivity is estimated at 2.10−12 e.m.u., a value which would be expected at temperatures near 1500°C. Such temperatures are reasonable at the depths concerned.
85 citations
TL;DR: In this paper, a two-dimensional array of 46 variometers in the northwestern United States and southwestern Canada between latitudes 44° and 51° N and longitudes 100° and 121° W was used to describe conductive structures in the upper mantle and crust.
Abstract: Summary
Time-varying magnetic fields were recorded during the summer of 1969 with a two-dimensional array of 46 variometers in the north-western United States and south-western Canada between latitudes 44° and 51° N and longitudes 100° and 121° W. Magnetograms and maps of Fourier spectral components of three magnetic events are used to describe conductive structures in the upper mantle and crust. The most prominent of the anomalies are found in the North American Central Plains and over the Northern Rockies. The Central Plains anomaly runs from the eastern edge of the Black Hills northward along the boundary between Montana and the Dakotas to the Williston Basin. Its large magnitude, with anomalous fields larger than the normal fields, and its small half-width indicate a crustal conductor which concentrates current induced in a large region. In the western part of the array, attenuation of the vertical variation fields is attributed to a westward rise in the highly-conducting mantle. Two small anomalies in the vertical and horizontal fields, over the eastern front of the Northern Rockies and the Rocky Mountain Trench, may be associated with ridges or steps on the upper-mantle conductor or with crustal features.
83 citations
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TL;DR: The velocity of compressional waves has been determined by measurement of travel time of pulses in specimens of rock at pressures to 10 kilobars and room temperature as mentioned in this paper, mainly igneous and metamorphic rocks, furnished three specimens oriented at right angles to one another.
Abstract: The velocity of compressional waves has been determined by measurement of travel time of pulses in specimens of rock at pressures to 10 kilobars and room temperature. Most of the samples, mainly igneous and metamorphic rocks, furnished three specimens oriented at right angles to one another. The present paper gives experimental details, modal analyses, and numerical tables of velocity as function of direction of propagation, initial density, and pressure. Discussion of various aspects of the measurements is reserved for part 2.
2,185 citations
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01 Jan 1970TL;DR: A survey with temporary geomagnetic field stations (Askania Variographs) has been conducted in the southwestern United States in order to study local differences of geOMagnetic variations as discussed by the authors.
Abstract: A survey with temporary geomagnetic field stations (Askania Variographs) has been conducted in the southwestern United States in order to study local differences of geomagnetic variations. Anomalous large Z-variations, accompanied with a slight reduction of the variations in D and H, have been recorded along the California coastline for long-period (Sq) and short-period (bays, ssc's) variations. This Californian coastal anomaly has been interpreted as edge-effect of the Pacific Ocean and its effect upon the induction within the highly conductive substratum in the upper mantle, thereby allowing conclusions about the deep conductivity structure Less pronounced anomalous variations further inland in central California (Sierra anomaly) seem to be caused by conductivity differences in the continental surface layers. A third anomaly between Tuscon, Arizona, and Sweetwater, Texas (Texas anomaly), has been interpreted as a change in the deep conductivity structure which occurs along the eastern margin of the Rocky Mountains.
608 citations
TL;DR: In this paper, the authors describe how during bays and similar magnetic variations the vectors representing changes in the geomagnetic field tend to lie on or close to a plane and the orientation of this plane varies from one observatory to another.
Abstract: Summary
During bays and similar magnetic variations the vectors representing changes in the geomagnetic field tend to lie on or close to a plane. The orientation of this plane varies from one observatory to another. At coastal observatories it almost invariably tilts upward towards the nearest deep ocean.
327 citations
TL;DR: In this paper, it was shown that the vectors representing geomagnetic changes over intervals less than one hour tend to be confined to a plane at most temperate latitude stations, in many cases this plane is almost horizontal, but sometimes it is steeply inclined.
Abstract: Summary
The vectors representing geomagnetic changes over intervals less than one hour tend to be confined to a plane at most temperate latitude stations. In many cases this plane is almost horizontal, but sometimes it is steeply inclined. This appears to be caused by currents induced asymmetrically in the Earth, possibly due to the highly conducting oceans.
317 citations
TL;DR: The possibility of obtaining some knowledge of the distribution of electrical conductivity within the earth, from the observed variations of the earth's magnetic field, was first considered by Schuster (1889), in developing his theory of the daily magnetic variations as mentioned in this paper.
Abstract: The possibility of obtaining some knowledge of the distribution of electrical conductivity within the earth, from the observed variations of the earth’s magnetic field, was first considered by Schuster (1889), in developing his theory of the daily magnetic variations He separated these variations into parts of external and internal origin, and then applied the theory of electromagnetic induction in a uniform sphere, due to Lamb (1883), to show that the “internal” part could be attributed to electric currents induced in the earth by the “external” part Chapman (1919) made a more complete analysis of the diurnal variation field, and showed that it was consistent with the earth having a core of conductivity k = 36 x l 0-13 emu, surrounded by a non-conducting shell of about 250 km thickness Chapman and Whitehead (1922) found, however, that the relatively highly conducting oceans probably have an appreciable effect on the internal field, and thus introduce some uncertainty in the estimate of k
290 citations