High-resolution studies of the oceanic magnetic anomaly pattern have become increasingly popular as a tools for investigating the structure and dynamic evolution of the mid-ocean ridge system and as a means to understand the magmatic accretion process. Two-dimensional magnetic anomalies have been interpreted as reflecting chemical variations in the oceanic crust, as first suggested by Peter Vogt and G. Leonard Johnson in 1973 with the concept of ``magnetic telechemistry''. Although questions remain concerning the character and stability of the source layer of marine magnetic anomalies, the anomaly pattern still offers a powerful means of studying accretionary processes through time.
Several key observations have been made recently that illuminate the underlying processes that control accretion. First, magnetization intensity has been shown to correlate with tectonic segmentation of ridge axes: higher anomaly amplitudes and magnetization intensities (obtained in inversions) have been observed at first- and second-order discontinuities (transform faults and overlapping spreading centers, respectively) on the southern mid-Atlantic Ridge (MAR) [ Carbotte et al., 1991], the East Pacific Rise [ Sempéré, 1991; Carbotte and Macdonald, 1992; Perram et al., 1993] the Juan de Fuca Ridge [ Tivey, 1994], and at the Galapagos spreading center [ Perram and Macdonald, 1994]. These higher-than-average magnetizations have been attributed to more highly fractionated (Fe-Ti) basalts at the segment tips. The larger degree of fractionation likely results from lower degrees of partial melting in smaller, deeper, and/or more disrupted magmatic sources at segment tips. An alternative hypothesis is that the total iron content of the erupted basalts between the segment centers and tips is controlled by variations in heat and magma supply. The magnetic signature of the segment tips can be traced off-axis to map the propagation history of the discontinuities [ Carbotte and Macdonald, 1992; Perram et al., 1993] and/or the waxing and waning of the magma supply [ Grindlay et al., 1992; Carbotte and Macdonald, 1992].
A recent study related to the above observations but with much broader
scope is that of Johnson and Pariso [1993]. These
authors presented a new analysis of selected Deep Sea Drilling
Project/Ocean Drilling Program rock magnetic results for crustal
samples ranging in age from Cretaceous to present. They found a
systematic variation in intrinsic magnetic properties with age that
correlates with variations in oceanic crustal magnetization revealed by
inversion of gridded magnetic anomalies [e.g., Sayanagi
and Tamaki, 1992]. The magnetizations decrease to a low at
30 Ma, then increase again. Johnson and
Pariso [1993] concluded, on the basis of several lines of evidence,
that these trends are the result of gross variations in the amount of
magnetic minerals in the crust. Magnetic minerals in Cretaceous crust,
for example, are twice as abundant as in crust from certain other time
periods. This intriguing conclusion, considered in concert with the
above suggestion that magnetization variations within an individual
segment are controlled by the magma flux rate and temperature, may
suggest that either magma supply or temperature or both were unusually
high during a period of 40 million years that roughly coincides with
the Cretaceous Long Normal Period of constant geomagnetic polarity.
The second important observation to emerge from fine-scale anomaly
studies is that detailed spreading rate histories, derived from
inversion results, reveal the episodicity of the accretion process
within and between segments, and its variability with time.
Carbotte et al. [1991] documented a 20 percent
variation in spreading rates across segment boundaries at the southern
MAR indicating significant non-rigid plate behavior on the time scale
of individual anomalies (
0.8 million years). This
observation should have important implications for assessing the
instantaneous state of stress on faults, such as the San Andreas, for
which a relative plate motion model averaged over several million years
is commonly used.
Third, the response of the plate boundary to changes in plate motion can be quantified by the magnetically-determined evolution of discontinuities that develop to accommodate the new geometry [ Macdonald et al., 1991]. The above studies all support the notion of the independence of individual spreading cells and the longevity of first- and second-order discontinuities. The magnetization variations associated with the discontinuities appear to be fixed with respect to the segments they bound (i.e., magnetic highs always occur at segment tips). Thus, mapping highly magnetized zones in older crust may reveal spatial and temporal variations in magma supply and variations in near- and far-field stresses at the boundary.
In another development, the spreading rate dependence of anomalous skewness (anomaly distortion that is unrelated to geomagnetic latitude) was documented in two studies [ Roest et al., 1992; Dyment et al., 1994]. The latter study confirmed the negative correlation between anomalous skewness and spreading rate seen in the former study and further found a transition to virtually no anomalous skewness at spreading rates above 50 mm/yr. These findings have important implications for the character of the magnetic source layer and the roles of faulting and magmatism in creating the oceanic crust and shaping its magnetic anomalies. One important implication is that paleopoles derived from anomaly skewness, especially for single plates, should be more reliable for faster spreading episodes.