S. Allerton et al., Extremely asymmetric magmatic accretion of oceanic crust at the ends of slow-spreading ridge segments, GEOLOGY, 28(2), 2000, pp. 179-182
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.