North Atlantic magnetic smooth zones

Model studies show that the low anomaly amplitudes in the 400-km-wide magnetic smooth zones bordering the North Atlantic can be explained by sea-floor spreading during the Early Triassic. At that time the northern mid-Atlantic ridge was crossed at a fairly high angle by the magnetic equator, and low anomaly amplitudes should result in these circumstances. The 100-km-wide transition between the low-amplitude zones and correctable 200-gamma anomalies toward the ridge crest can be explained in terms of rapid polar wandering (25 cm/yr), independently established for the Late Triassic, and simultaneous sea-floor spreading of 1 cm/yr. An alternate theory for the smooth zone follows from the low anomaly amplitudes being created by the Tertiary arctic extension of the mid-Atlantic Ridge. While the rift is still close to terrestrial sediment sources, new sea floor is kept inundated by sediment, which gains access to the rift valley by utilizing transverse fracture valleys. In place of highly magnetized, rapidly quenched pillow basalts, the magma forms less magnetic dolerite sills at the basement-sediment interface. Any magnetized pillows buried by sediment may be heated while still in the axial zone.