Systematic low-level aeromagnetic sarveys 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 85.3øN, 13øE to 86øN, 50øE, 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 varia- tions 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 040øT to 065øT, subparallel to the Alpha Cordillera. If these lineations reflect sea-floor spreading and geomag- netic 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. 1.
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