Abstract 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.

The ideas of sea-floor spreading and plate tectonics were just exploding upon the oceanographic community when I (T.A.) was a young graduate student at the Scripps Institution of Oceanography in the late 1960s. This was an exciting time; it seemed that the whole world lay waiting for reinterpretation. Recognizing the power of the Vine-Matthews hypothesis, Bill Menard had asked his draftswoman, Isabel Taylor, to plot all of the magnetic anomaly profiles so far collected by ships traversing the Northeast Pacific. The resulting map and its updates occupied a central spot on Bill’s huge work table for many years. Although I was not officially working with Bill, I was almost irresistibly drawn to his laboratory and to this map whenever I had a spare moment. For some years before, Bill had been mapping and writing about the great North Pacific fracture zones. With the addition of the magnetic isochrons, the sea-floor spreading story of the region unfolded before our eyes. He was like a child in a candy store; I was in heaven. It was on this chart that we identified and mapped out the isochron patterns for the region and, using their mapped geometry, we were able to explore many sea-floor spreading and plate- tectonic phenomena (e.g., Menard and Atwater, 1968, 1969; Atwater and Menard, 1970; Atwater, 1970; Menard, 1978). However, the compilation of the magnetic profiles, itself, was never published. Over the years since, many additional profiles have been measured, and various detailed surveys and partial compilations have been published. On Plates 3A and 3B we have collected and presented these, along with many of the original profiles, and superimposed them upon our updated interpretations. We present these tectonic maps in honor of H. W. Menard and in memory of his tremendous, contagious joy in science.

Abstract This paper sketches out developments in North American paleomagnetism that have markedly affected geological thought, to the exclusion of those related mostly to the past character and properties of the earth’s magnetic field or to magnetic properties of rocks. Early work (1948) at the Carnegie Institution’s Department of Terrestrial Magnetism was mainly directed, as was also that of P.M.S. Blackett in England, at elucidating the past behavior of the geomagnetic field and its origin; proving or disprov-ing continental drift was not a primary objective, and the concept of sea-floor spreading was, of course, still unheard of. The first surprise came with the discovery of numerous reversely magnetized igneous and sedimentary rocks. It took about 10 years to establish that most of these reversed rocks reflected field reversals rather than self-reversal con-tingent on mineralogical composition. The development of a magnetic time scale based on the chronology of field reversals was of paramount geological significance as it led to confirmation of the Vine-Matthews hypothesis and to plate tectonics; it is now widely applied to worldwide stratigraphie correlations. Unexpected “abnormal” directions of magnetization in pre-Tertiary rocks were first interpreted in terms of polar wander, or, in the case of deformed rocks, in terms of magnetostriction and stress-induced anisotropy; they became accepted by British paleomagnetists as evidence for continental drift a few years before their American colleagues reached the same conclusion. Paleomagnetism has further affected geological thinking by establishing, through determination of the earth’s paleoradius, the improbability of Carey’s expansionist views and, most

Abstract By 1854, there were enough deep ocean soundings to allow M. F. Maury to draw the first bathymetric chart of the North Atlantic Ocean in 1855. He defined the great shoaling “middle ground” of the Atlantic basin, later to be known as the Mid-Atlantic Ridge (MAR). It is interesting to note that during the Challenger expedition (1872–1876), Sir Wyville Thomson predicted, on the basis of water temperatures alone, that the MAR is a nearly continuous topographic barrier dividing the Atlantic Ocean. In the 1950s, continuous echogram profiles revealed the presence of a deep median valley along the axis of the MAR (Heezen and others, 1959; Hill 1960). Seismicity (Gutenburg and Richter, 1954) and the occurrence of youthful basaltic lavas on the MAR (Shand, 1949) indicated that the median valley was a geologically active rift zone. Heezen (1960) first suggested that extension across the rift valley might be responsible for accretion of basaltic oceanic crust and the drift of the continents, but his contention that this rifting resulted from expansion of the earth was incorrect. High heat flow measured on the MAR (Bullard and Day, 1961) and large axial magnetic anomalies (Heezen and others, 1959) were added to seismicity and recent volcanism as enigmas associated with the rift valley. In his seafloor spreading hypothesis Hess (1962) elegantly explained these puzzling observations in a unified model, and the bilateral symmetry of magnetic anomalies found on other midocean ridges (Vine and Matthews, 1963; Vine, 1966)