This Quadrennial Report departs in format from its predecessors. Earlier reports provided brief descriptions of the entire range of topics on ``crustal magnetic anomalies.'' Following guidelines established by the AGU, we instead focus our discussion on only a few selected topics. Our abbreviated list of references reflects this restricted focus. A more comprehensive bibliography can be obtained from the authors as noted subsequently.
The past quadrennium was distinguished by a significant change in the scale and scope of crustal magnetic anomaly research. Advances in instrumentation, navigation, and data processing techniques resulted in increasing use of two-dimensional anomaly maps rather than profile data. Although maps are commonly used in aeromagnetic studies of continental areas, their use in the ocean basins has been limited until recently. The other important change has been a trend towards increased integration of magnetic anomaly data with gravity, seismic, geochemical, remote sensing, and geologic data to enhance the interpretation and reduce the ambiguity of magnetic field data.
With most of the magnetic anomalies in the ocean basins mapped to first-order, current studies are focusing on very detailed magnetic observations, mostly near the mid-ocean ridge axes. Detailed, three-dimensional magnetization distributions derived from marine magnetic anomalies are providing fundamental constraints on spatial and temporal variability in the physical and chemical characteristics of crustal magmatic accretion. Inferences concerning the accretion process obtained in these studies prove to be complementary to the larger, regional picture deduced from systematic variations in anomaly amplitude and from petromagnetic properties of basalt.
The traditional role of aeromagnetic studies over continental areas has been in establishing geologic and tectonic frameworks and in exploring for minerals and other commodities. The techniques established in these traditional investigations were applied during the past quadrennium to the identification and understanding of several critical geologic hazards. Some geologic hazards, such as the San Andreas fault in California, are clearly expressed at the surface and can be studied by geologic mapping and other surface-based methods. Most geologic hazards are not so easily accessible. The New Madrid seismic zone , for example, has no expression at the surface but nevertheless poses a continuing and potentially devastating threat to major population areas. The magnetic method is a relatively inexpensive way to learn about geologic hazards, such as seismically active faults, shallow magma chambers, and volcanic centers. Indeed, it may be the only way to study hazardous structures in places where they are concealed beneath young deposits, water, vegetation, and urban development.
Much attention has been directed recently toward removing residual external field contamination from the Magsat anomaly field. These efforts represent an important step toward realizing globally continuous, high-accuracy lithospheric anomaly fields that will undoubtedly furnish valuable constraints on understanding the genesis and evolution of the lower crust and large scale regional crustal processes. The satellite anomaly field continues to be a valuable resource in deciphering regional tectonic fabric and compositional variations.