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Open-File Report 02-99


High resolution aeromagnetic survey data flown at 250 m above the terrain and 250 m line spacing over the Santa Cruz Valley and the surrounding Tumacacori, Patagonia, and Santa Rita Mountains has been interpreted by correlation of the magnetic anomaly field and various derivative maps with geologic maps. Measurements of in-situ magnetic properties of several of the map units determined whether or not mapped lithologies were responsible for observed anomalies. Correlation of the magnetic anomaly field with mapped geology shows that numerous map units of volcanic and intrusive rocks from Jurassic Middle Tertiary in age are reversely polarized, some of which have not been previously reported. Trends derived from the magnetic anomaly data correlate closely with structures from major tectonic events in the geologic history of the area including Triassic-Jurassic crustal accretion and magmatism, Laramide magmatism and tectonism, northeast-southwest Mid-Tertiary extension, and east-west Basin and Range extension. Application of two textural measures to the magnetic anomaly data, number of peaks and troughs per km (a measure of roughness) and Euclidean length per km (a measure of amplitude), delineated areas of consistent magnetic anomaly texture. These measures were successful at the delineation of areas of consistent magnetic lithology both on the surface and in the subsurface beneath basin fill. Several areas of basement prospective for mineral resources beneath basin fill were identified.



A high resolution aeromagnetic survey was flown over the Patagonia Mountains, Santa Rita Mountains, upper Santa Cruz Valley, and Tumacacori Mountains (figure 1) in late 1996 and early 1997 under contract for the U.S. Geological Survey. The data are available to the public via the Worldwide Web (Sweeney, 2000). Phillips (2001) describes the processing of this data into a final corrected flight line dataset used for the work described here. This dataset was used to prepare maps of the aeromagnetic anomaly field and derivative maps of textural measures that were correlated with published geological maps to yield the interpretation described in this report. Measurements of in-situ magnetic properties at a number of sites in the survey area were completed to help constrain interpretations and provide model susceptibility values.

Previous studies of the aeromagnetic field in the area include the aeromagnetic map for the state of Arizona (Sauck and Sumner, 1970), Ponce (1990), and Gettings (1996). Proprietary aeromagnetic data flown by a consortium of mineral exploration companies includes the eastern 20% of the survey area. Primary sources of geologic data for this study include Drewes (1971, 1980, 1996), Simons (1974), and Gettings and Houser (1997).
This report introduces several new techniques for characterizing the texture of magnetic field data in ways that can be used for correlation with lithology and geologic structure. In order to focus on the techniques, the author has chosen to organize the report by subject rather than by geographic area or geologic features. Thus, some areas will be discussed several times at different places in the report as the results of the application of the various techniques are discussed. For each technique, a catalog style keyed to the map plates was used and in general, the order of discussion is the order of the numbered areas or anomalies on the map plates. This organization allows the reader to ascertain from which technique a particular interpretive feature was derived and to judge the relative effectiveness of techniques. Not all anomalies or magnetic field features have been interpreted or discussed; the dataset contains a great richness of detail that is beyond the scope of this study to interpret in its entirety. It is the author’s hope that the most important features have been discussed.

After the presentation of the field measurements of magnetic property data, a comparison of the aeromagnetic anomaly data with the geologic map data is given. This is followed by a trend or lineament analysis of the aeromagnetic anomaly data. Next comes a discussion of the techniques of quantifying two textural measures of the aeromagnetic data and an analysis of the textural measures applied to the aeromagnetic dataset of this study. The report concludes with some generalizations and suggestions for further work.

index map showing location of study area
Figure 1. Index map showing location of study area (in pink).


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