Variations in layer 2A thickness and the origin of the central anomaly magnetic high

The seismically determined off-axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high t.ypic. ally associated with the ridge crest. The positive magnenzauon contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales (2-3 km) comparable to that of the proposed Layer 2A thickening. We present magnetic rema- nence data from basalts dredged on and near the East Pacific Rise axis at 12oN which are compatible with a rapid magnetization reduction (.-.20 k.y. to decay to l/e). Together with near bottom magnetic profiles from the ultra-fast- spreading East Pacific Rise at 19.5oS, these data suggest that previous estimates of the time constant of alteration inferred from slow-spreading ridges (0.5 m.y.) may be more than an order of magnitude too high. Seismic studies of the fast-spreading East Pacific Rise (EPR) indicate a rapid thickening of seismic Layer 2A within a few kilometers of the ridge axis (Vera et al., 1990; Toomey et aI., 1990; Detrick et al, 1993). These seismic data have been interpreted to reflect an approximate doubling of the extrusive layer thickness, from -100-200 m to 300-500 m, through addition of flows whi.ch may extend 1-2 km off-axis (Christeson et al., !992; Vera and Diebold, in press). Such thickening of the extrusive source layer had been argued on the basis of the widths of magnetic reversal transitions (e.g., Macdonald et al., 1983). However, this geometry for a uniformly magnetized source layer should produce a marked magnetic anomaly minimum over the ridge axis and is therefore difficult to reconcile with the pronounced anomaly high typically associated with the ridge crest (Klitgord, 1976). Sea surface magnetic profiles from the ultra-fast-spreading (77 mrn/yr half rate) EPR near 19.5oS (Figure 1) illustrate the short wavelength axial anomaly high within the ~120 km wide Central Anomaly which has been termed the Central Anomaly magnetic high (CAMH; Klitgord, 1976). The CAMH typically has a wavelength of 8-10 km in sea surface profiles. At in- termediate-spreading ridges (15-45 mm/yr) the CAMH occupies a larger proportion of the Central Anomaly, and at slow-spreading ridges it is not readily distinguishable in sea surface profiles from the Central Anomaly because the wavelengths become similar. Near bottom magnetic profiles (Figure 1) typically have higher amplitude and shorter wavelength (2-4 kin) axial anomalies as expected from the closer proximity to the source (Klitg6rd et al., 1975; Miller, 1977; Perram et al., 1990) and even allow distinction of the CAMH from the Central Anomaly at some slow-spreading ridges (Macdonald, 1977). The higher temporal resolution of faster spreading ridges therefore provides the best opportunity to evaluate the processes responsible for the CAMH. Thickness variations in the extraslye layer at fast-spreading fi. dges have a profound effect on the axial magnetic anomaly, since the presence of an axial magma chamber and the low re- rnanent intensity of intrusive rocks (e.g. Smith and Banerice, 1986; Pariso et al., 1991) minimize deeper source contributions in the near-ridge environment. Based on seismic