Extremely asymmetric magmatic accretion of oceanic crust at the ends of slow-spreading ridge segments

We report the results of a deep-towed magnetic survey of part of a Mid-Atla ntic 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 ha s been effectively one sided, with new crust being added only to the outsid e corner of the ridge offset (eastern flank), and not to the inside corner (western flank). Spreading on the inside corner was accommodated by signifi cant 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 accret ion 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 proce ss. The termination of a period of asymmetric spreading may be achieved eit her 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 as ymmetric accretion may be a common process at slow-spreading segments, part icularly near discontinuities. This asymmetry cannot be maintained for long periods, and may be directly linked to intervals of spreading by tectonic extension.