The geophysical significance of geomagnetic variation anomalies |
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| Authors: | DI Gough |
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| Institution: | Institute of Earth and Planetary Physics, University of Alberta, Edmonton, Alta., Canada |
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| Abstract: | Analytic procedures in classical geomagnetic deep sounding and in two-dimensional magnetometer array studies are outlined. Three types of magnetic variation anomaly are considered, and anomalies of one geographical area. Continental-edge anomalies remain somewhat ambiguous as to the contribution of structure in the upper mantle; the geophysical significance in geothermal terms is understood, if the effect exists. Subduction-zone anomalies in the Peruvian Andes and in central Japan are considered in relation to the ascent of an andesitic melt fraction from the lithosphere slab, a process which accounts also for uplift and support of the mountains. In western North America anomalies are closely related to heat flow and indicate complex tectonic activity with considerable fine structure in general agreement with seismological parameters. The Basin and Range Province has a highly conductive upper mantle and still higher conductivities are found under the Wasatch fault belt and under the southern Rockies. Under the northern Rockies the evidence is for only a thin conductive layer in the upper mantle and in general for much lower heating than in mid-latitudes of the United States. Crustal anomalies are discussed in relation to the current concentration effect. It is suggested that some of them may mark metamorphic belts in crystalline basement rocks. This association has been demonstrated for the North American Central Plains anomaly. |
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