ALBERT

Notes: Abstract We have conducted the first detailed survey of the recording of a geomagnetic reversal at an ultra-fast spreading center. The survey straddles the Brunhes/Matuyama reversal boundary at 19°30′ S on the east flank of the East Pacific Rise (EPR), which spreads at the half rate of 82 mm yr-1. In the vicinity of the reversal boundary, we performed a three-dimensional inversion of the surface magnetic field and two-dimensional inversions of several near-bottom profiles including the effects of bathymetry. The surface inversion solution shows that the polarity transition is sharp and linear, and less than 3–4 km wide. These values constitute an upper bound because the interpretation of marine magnetic anomalies observed at the sea surface is limited to wavelengths greater than 3–4 km. The polarity transition width, which represents the distance over which 90% of the change in polarity occurs, is narrow (1.5–2.1 km) as measured on individual 2-D inversion profiles of near-bottom data. This suggests a crustal zone of accretion only 3.0–4.2 km wide. Our method offers little control on accretionary processes below layer 2B because the pillow and the dike layers in young oceanic crust are by far the most significant contributors to the generation of marine magnetic anomalies. The Deep-Tow instrument package was used to determine in situ the polarity of individual volcanoes and fault scarps in the same area. We were able to make 96 in situ polarity determinations which allowed us to locate the scafloor transition boundary which separates positively and negatively magnetized lava flows. The shift between the inversion transition boundary and the seafloor transition boundary can be used to obtain an estimate of the width of the neovolcanic zone of 4–10 km. This width is significantly larger than the present width of the neovolcanic zone at 19°30′ S as documented from near-bottom bathymetric and photographic data (Bicknell et al., 1987), and also larger than the width of the neovolcanic zone at 21° N on the EPR as inferred by the three-dimensional inversion of near-bottom magnetic data (Macdonald et al., 1983). The eruption of positively magnetized lava flows over negatively magnetized crust from the numerous volcanoes present in the survey area and episodic flooding of the flanks of the ridge axis by extensive outpourings of lava erupting from a particularly robust magma chamber may result in a widened neovolcanic zone. We studied the relationship between spreading rate and polarity transition widths obtained from 2-D inversions of the near-bottom magnetic field over various spreading centers. The mean transition width corrected for the time necessary for the reversal to occur decreases with increasing spreading rate but our data set is still too sparse to draw firm conclusions from these observations. Perhaps more interesting is the fact that the range of the measured transition widths also decreases with spreading rate. In the light of these results, we propose a new model for the spreading rate dependency of polarity transition widths. At slow spreading centers, the zone of dike injection is narrow but the locus of crustal accretion is prone to small lateral shifts depending on the availability of magmatic sources, and the resulting polarity transition widths can be narrow or wide. At intermediate spreading centers, the zone of crustal accretion is narrow and does not shift laterally, which leads to narrower transition widths on the average than at slow spreading centers. An intermediate, or even a slow spreading center, may behave like a fast or hot-spot dominated ridge for short periods of time when its magmatic budget is increased due to melting events in the upper mantle. At fast spreading centers, the zone of dike injection is narrow, but the large magmatic budget of fast spreading centers may result in occasional extensive flows less than a few tens of meters thick from the axis and off-axis volcanic cones. These thin flows will not significantly contribute to the polarity transition widths, which remain narrow, but they may greatly increase the width of the neovolcanic zone. Finally the gabbro layer in the lower section of oceanic crust may also contribute to the observed polarity transition widths but this contribution will only become significant in older oceanic crust (≈50–100 m.y.).