Abstract

A detailed study of the magnetic anomalies of the Mid-Atlantic Ridge crest near lat 37 °N (FAMOUS) was conducted using a deeply towed instrument package. The most recent expression of the accreting plate boundary in rift valley 2 is an alternating series of linear central volcanoes and depressions that are marked by a sharp maximum in crustal magnetization only 2 to 3 km wide. Spreading in the FAMOUS area is highly asymmetric, with rates of 13.4 mm/yr to the east and 7.0 mm/yr to the west. At 1.7 m.y. B.P., the sense of asymmetry reverses in direction, with spreading faster to the west; this results in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Spreading in the FAMOUS area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate-boundary system. Spreading has been stably oblique for at least 8 m.y., even through a change in spreading direction. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been observed and may be due to old crust left behind or to recording of a geomagnetic field event. Crustal magnetization decays to 1/e its initial value in less than 0.6 m.y. The rapid decay may be facilitated by very intense crustal fracturing observed in the inner floor. Crustal magnetic sources may be approximated (mathematically) by a uniformly magnetized layer 700 m thick.

Magnetic studies indicate that over 90 percent of the extrusive volcanism occurs within the rift inner floor and is extremely rare in the rift mountains. Magnetic anomaly transition widths vary from 1 to 8 km with time and appear to reflect a time-varying median-valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths), or it has a narrow inner floor and wide terraces — the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present rift valley 2 has a narrow inner floor and volcanic zone (1 to 3 km), whereas rift valley 3 is at the opposite end of the cycle with a wide inner floor and volcanic zone (9 to 11 km).

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