Ocean Magnetic Anomalies and Their Relations to Continents1
Published:January 01, 1974
Linear magnetic anomalies paralleling and symmetrical with the axes of the midocean ridges are acclaimed widely as "characteristics" of midocean ridges. Yet linear magnetic anomalies are known from only 70 percent of the seismically active midocean ridge system; they are absent along 30 percent of the ridges. Within that part of the ridge system which has linear magnetic anomalies, less than 50 percent exhibit anomalies that are symmetrical with respect to the ridge axis; possibly 80 percent show no symmetry beyond the so-called “anomaly 5”. In about 21 percent of the ridge system having linear anomalies, those anomalies are oblique to the trend of the ridge.
At least 19 areas exist where the linear anomalies either end abruptly at continental margins or can be traced into the continents. The plate-tectonics hypothesis does not explain the latter phenomenon. However, where anomalies appear to terminate against continental crust, the plate-tectonics hypothesis requires that either (1) the continent is overriding the oceanic crust or (2) a so-called transform fault is present along the continent-ocean boundary where the anomalies terminate. In most localities, evidence for either phenomenon is lacking.
Close examination of the known linear magnetic anomalies shows that they are approximately concentric around continental nuclei of Archean age—an observation which suggests strongly that the linear anomalies are partly of Proterozoic age. Some support for this suggestion is now available from radiometric dates of rocks within the ocean basins—dates which range from 785 to 1,690 m.y. Although few rocks known to be the sources of the magnetic anomalies have been dated, it is probable that some of the rocks dated as Proterozoic are anomaly sources. In fact, the ages predicted by the so-called magnetic stratigraphy are found in so few places that its usefulness in predicting the age of any part of an ocean basin appears to be essentially nil—a statement borne out by data presented in this paper. We suggest that the linear magnetic stripes of the ocean basins are not what they are claimed to be. We suggest, instead, that the linear magnetic bands are very ancient features of the earth's crust, formed during the layering of the earth early in its cooling history. We suggest further that the Vine-Matthews hypothesis be laid aside until such time that sufficient geologic facts are at hand to support the hypothesis or to provide an alternate one.
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Plate Tectonics—Assessments and Reassessments
The general theme of this publication is the assessment and reassessment of various data, observations, and ideas about the earth as they relate to the concept that has come to be known as plate tectonics. Much widely scattered material was brought together for this publication, and its 24 papers contain an abundance of worldwide references that are important in studying plate tectonics.