Systematic low‐level aeromagnetic surveys conducted during 1974–1975 reveal details of the magnetic fabric in two parts of the Arctic Basin. These profiles extend coverage of the Nansen (Gakkel; Mid‐Arctic) Ridge from 85.3°N, 13°E to 86°N, 50°E, where these new data overlap previous Soviet aeromagnetic coverage. Prominent magnetic lineations can be identified despite spreading half‐rates as low as 0.3 cm/yr about 25–35 m.y. B.P. The separation of Lomonosov Ridge from Eurasia occurred at or before anomaly 24 time (55 m.y. B.P.). Although there is ‘room’ for anomalies 25–27 between 24 and the continental margin, a broad magnetic negative exists in their place. Either anomalies 25–27 were suppressed or erased by thick sediment fill or some other process associated with initial rifting, or the associated crust is subsided continental material. All anomalies, particularly the central anomaly, exhibit dramatic variations of amplitude along their strike. It is the low values of amplitude that are anomalous with respect to the Mid‐Atlantic Ridge to the south. Bathymetric data demonstrate that the high central anomaly amplitudes correlate with shallower rift valley floors (3500–4000 m) and higher rift mountains. We propose that the pillow basalt layer (2a) is thicker in the magnetic high amplitude zones. There is no bathymetric evidence for sediment gaining access to the valley floor in the area examined. A second survey was flown across the northern Canada Basin and Alpha Ridge. Complex, but lineated, anomalies of 1500 to 2500 nT relief parallel the crest of Alpha Ridge. A crestal valley, 2500 m deep and 30 km wide, flanked by ridges with crests 1200–1500 m deep, correlates well with magnetic and gravity anomalies; this suggests that prominent Alpha Ridge magnetic anomalies are caused by basement topography of high magnetization (20–30 A/M) and normal polarity. If the Alpha Ridge is floored by an anomalous type of oceanic crust, it probably originated during the long mid‐Cretaceous period of normal polarity. In the northern Canada Basin, several prominent linear anomalies cross the surveyed swath on strikes of 040°T to 065°T, subparallel to the Alpha Cordillera. If these lineations reflect sea‐floor spreading and geomagnetic reversals (possibly during the lower Cretaceous to upper Jurassic), the spreading axis cannot have paralleled either the Canadian continental margin or the Alpha Ridge as has been proposed, but instead, lay parallel to the Northwind Escarpment.

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Over 150,000 line‐kilometers of low‐level (152 m) aeromagnetic data were recorded in the western Arctic Basin by the U.S. Navy during four field seasons (1975–1978); data from the first 2 years were presented by Vogt et al. (1979a), while the data from the last two years are described in this paper. These data (1977–1978) cover a swath from the north slope of Alaska to the north geographic pole. Flight lines were spaced between 10 and 24 km. The east‐west oriented aeromagnetic profiles across the Canada Basin and the Beaufort Sea suggest that Alaska was moving away from the Queen Elizabeth Islands of the Canadian Arctic from 153 m.y. B.P. (anomaly M‐25 time) to 127 m.y. B.P. (M‐12), at an opening rate of 2.6 cm/yr. An extinct spreading center is defined by a positive free‐air gravity anomaly, with the relic spreading axis generally paralleling the 150°W meridian. We are unable to recognize a coherent pattern of lineated anomalies over the Alpha Ridge; therefore its origin remains uncertain. A series of seafloor spreading type anomalies have been tentatively identified in the Fletcher (Makarov) Basin. Spreading began in the Upper Cretaceous (anomaly 34; 80 m.y. B.P.) and continued until mid‐Eocene (anomaly 21; 53 m.y. B.P.); total opening rate was about 1.7 cm/yr. During the opening of the Fletcher Basin, rifting began in Baffin Bay and the Norwegian and Labrador seas and the Eurasian Basin. Our results suggest a tectonic coupling between these areas at this time, with the Nares Strait acting as a transform fault and serving as a connector with the Baffin Bay/Labrador Sea spreading centers.

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Transform faults offset the pipe‐shaped region of partial melting and magma generation below the Midoceanic ridge. Hence if there is flow along the pipe, it will be blocked or at least impeded at major transform faults. There is evidence on the Reykjanes ridge that minor transform faults, with offsets of a few tens of kilometers, may be converted to oblique spreading axes by asthenosphere flow. Quantitative estimates of the extent of blocking are derived from a Parker‐Oldenburg law of plate thickness increasing as the square root of crustal age. There are several kinds of evidence that the subcrustal partial melts generally moving away from long‐wavelength topographic and gravity highs on the Midoceanic ridge (hot spots) are partly blocked at transform faults: (1) elevations of particular isochrons, including the present spreading axis, frequently jump discontinuously across major transform faults (the best examples of this are in the northeast Atlantic); (2) morphology and seismicity change abruptly across the Blanco fracture zone, which separates a ridge influenced by a hot spot (the Juan de Fuca ridge) from one not so influenced (the Gorda ridge); (3) a zone of relatively high magnetic amplitudes is associated with the hot spot influenced Juan de Fuca and central Galapagos ridges (this zone may delineate how much crust was produced by the Fe/Ti‐rich melts of hot spot origin (whether due to distinctive source composition or subsequent fractionation); on both ridges the high‐amplitude magnetic zones are terminated on both ends at transform faults, again suggesting blockage of the hot spot melts); (4) prominent ridges, such as the Mendocino and Charlie Gibbs ridges, may form on the ‘upstream’ side of some fracture zones at certain times during their evolution (such fracture ridges seem to be constructional volcanic piles formed continuously at or near the fracture‐ridge crest junction; they are here interpreted as being due to excess basalt melts, produced from the partial melts ponded on the upstream side of the transform fault; fracture ridges are proposed to record the past intensity of pipe flow and hence hot spot activity); and (5) recent studies by Solomon (1973) show a region of high S wave attenuation and low Q at the southern end of the Reykjanes ridge, below the prominent fracture ridge just north of the Charlie Gibbs fracture zone. Taken together, the various observations support the existence of pipelike flow at shallow depths (up to a few tens of kilometers) below the Midoceanic ridge. The interaction between mantle hot spots and nearby plate boundaries discussed in this paper may produce features, such as the Ninetyeast and Broken ridges, that exhibit a strong plate tectonic fabric (structures parallel or perpendicular to transform faults) but nevertheless attest to special convective processes in the mantle.