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MAGNETIC ANOMALY MAP

A magnetic anomaly map of the region surrounding the San Andreas fault system is shown in figure 9.3. This map is based on the magnetic anomaly map of the Western United States by Bond and Zietz (1987), which was compiled from hundreds of magnetic surveys with widely differing flight heights, flightline spacings, and sensor types..

In contrast to the lengthy series of reduction steps that were required to convert the gravity observations to the form shown in figure 9.2, very little was done to the observed magnetic data to prepare them for compilation. Although the original data were collected at many different heights, no analytic procedures were used to continue them to a common elevation. Instead, the various surveys were referenced to the International Geomagnetic Reference Field (IGRF) adjusted for the date of the survey and an arbitrary zero datum, and then combined manually by inspection. Long profiles of magnetic data collected under the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy and by the U.S. Naval Oceanographic Office (NOO) served as guides for determining the zero datum for the various surveys. The resulting data are presented at a color contour interval of 100 nT (gammas) in figure 9.3.

Our magnetic anomaly map (fig. 9.3) is the most complete compilation available for the San Andreas fault system and is useful for qualitatively determining the location, shape, and regional setting of large magnetic bodies. However, because of the compilation methods used to construct this map and because the contour interval is relatively coarse (100 nT), it will not, in general, be adequate for detailed qualitative or quantitative examination of individual anomalies. Where detailed information is required, the reader is referred to the original sources from which our map was compiled; a comprehensive listing of these sources is given by Bond and Zietz (1987).

A particularly valuable source of aeromagnetic data over the San Andreas fault system is the profile data collected under the NURE program. In general, these data were collected along long profiles oriented east-west at a nominal height of 120 m above terrain and spaced about 5 km apart. The wide flightline spacing and low altitude of the survey lines preclude constructing realistic contour maps from these data in most places, but the long profiles are well suited for quantitative modeling. These data are available in the form of atlas folios or digital tapes for individual 10 by 20 quadrangles from the U.S. Department of Energy, Grand Junction, Colo.

When interpreting magnetic data, the inclination of the Earth's magnetic field must be taken into account because the magnetization induced in the magnetic source rocks by this field will have a similar inclination. Along the San Andreas fault, this inclination ranges from 58� to 64� downward toward magnetic north. Contoured magnetic anomalies over inductively magnetized or normally magnetized sources at these field inclinations will commonly display a dipole response, namely, magnetic lows associated with the north sides of magnetic highs. Inspection of our magnetic-anomaly map (fig. 9.3) indeed identifies numerous such magnetic lows on the north or northeast sides of major magnetic highs. In general, each magnetic low is located directly beyond the north or northeast contact of the causative magnetic mass.