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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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Abstract. A new Chile ridge tectonic framework is developed based on satellite altimetry data, shipboard geophysical data and, primarily, 38,500 km of magnetic data gathered on a joint U.S.-Chilean aeromagnetic survey. Eighteen active transforms with fossil fracture zones (FZs), including two complex systems (the Chile FZ and Valdivia FZ systems), have been mapped between the northern end of the Antarctic-Nazca plate boundary (Chile ridge) at 35øS and the Chile margin triple junction at 47øS. Chile ridge spreading rates from 23 Ma to Present have been determined and show slowdowns in spreading rates that correspond to times of ridge-trench collisions. The Valdivia FZ system, previously mapped as two FZs with an uncharted seismically active region between them, is now recognized to be a multiple-offset FZ system composed of six FZs separated by short ridge segments 22 to 27 km long. At chron 5A (-12 Ma), the Chile ridge propagated from the Valdivia FZ system northward into the Nazca plate through crust formed 5 Myr earlier at the Pacific-Nazca ridge. Evidence for this propagation event includes the Friday and Crusoe troughs, located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults. This propagation event led to the formation of the Friday microplate, which resulted in the transferal of crust from the Nazca plate to the Antarctic plate, and in a 500-km northward stepwise migration of the Pacific-Antarctic-Nazca triple junction. Rift propagation, microplate formation, microplate extinction, and stepwise triple junction migration are found to occur during large-scale plate motion changes and plate boundary changes in the southeast Pacific.

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.