Abstract

An intensive paleomagnetic and rock magnetic study has been carried out on the basalt samples from the FAMOUS area of the Mid-Atlantic Ridge at lat 37 °N. In addition to the samples obtained by the submersible Alvin from the floor and walls of the central rift valley, dredge and rock-drill samples were taken on a line perpendicular to the axis of the ridge out to a distance of 30 km from the central valley. Although the rocks from the floor of the central valley were roughly uniformly magnetized, a strong decrease in remanence intensity occurred at the valley walls. This progressive decrease continued until the remanent intensity was reduced by a factor of 5 at a distance 10 km and farther from the ridge axis. The other magnetic parameters of weak-field susceptibility, median demagnetizing field, Q, and Curie temperature were also roughly constant in samples from the valley floor. However, the rise in Curie temperature, which began at the valley walls, was well correlated with the decrease in remanent intensity and Q, an increase in the median demagnetizing field, and a sharp increase and then slow decrease in susceptibility; this rise can be attributed to the low-temperature oxidation of titanomagnetite to titanomaghemite. Of the 9 vertically oriented samples that were obtained from the valley floor and inner west wall, seven were normally magnetized and two were reversely magnetized. The two reversely magnetized units from the valley floor and east wall may be from blocks of older, pre-Brunhes crust that have not yet moved out of the rift valley. Comparison of the magnetic results of the surface rocks with models of the associated magnetic anomalies suggests that the oceanic magnetic layer can be represented by a permeable zone of pillow basalts roughly one km thick and that the oxidation state of these pillows progressively increases with time.

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