We report the results of a deep-towed magnetic survey of part of a Mid-Atlantic Ridge spreading segment. Analysis of the magnetic reversals indicates that for the past 0.7 m.y., magmatic accretion at the end of the segment has been effectively one sided, with new crust being added only to the outside corner of the ridge offset (eastern flank), and not to the inside corner (western flank). Spreading on the inside corner was accommodated by significant displacement on a single, large fault. The area between the fault and the axial volcanic ridge was effectively a thin static sliver at the plate boundary during this process. In the short term, asymmetric magmatic accretion was probably accomplished by progressively shifting the axial volcanic ridge to a new location at the inside corner (western) edge of the old one. Asymmetric spreading is unlikely to be sustainable as a steady-state process. The termination of a period of asymmetric spreading may be achieved either by establishing a new axial volcanic ridge to a position on the outside corner (eastern) plate (thus isolating the old axial volcanic ridge on the inside-corner plate), or by simply arresting movement on the large fault, and reverting to symmetric spreading at the axial volcanic ridge. Highly asymmetric accretion may be a common process at slow-spreading segments, particularly near discontinuities. This asymmetry cannot be maintained for long periods, and may be directly linked to intervals of spreading by tectonic extension.

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