USGS - science for a changing world

Open-File Report 02-99


Detailed correlation of the aeromagnetic survey data with geologic maps at several scales has produced a wealth of insight into the geologic history and structure of this geologically very complex area. At least three distinct periods of reversed geomagnetic polarity from Jurassic to Cenozoic times are required to explain the observed magnetic anomaly field. Measurements of magnetic susceptibility and remanent polarity of outcrops were invaluable in sorting out which rocks could explain observed magnetic anomalies and which are due to buried sources. In several cases the magnetic anomaly data are able to confidently predict rock unit identities in areas where they are undifferentiated in the geologic mapping. Several areas of mineral resource potential have been identified and several areas of interest for further geological and geophysical studies have been found. For example, the Gringo Gulch pluton has been shown to both a normally polarized and a reversely polarized phase and detailed dating studies might accurately date the geomagnetic field reversal. Detailed study of the Laramide intrusives using the techniques of magnetic petrology (Clark, 1999) holds promise for evaluation of these rocks as hosts or sources of ore deposits.

Analysis of trends in the magnetic anomaly data correlated with mapped faults and boundaries have quantified major structural directions associated with the major tectonic events in the geologic history of the study area. In particular, penetrative structures probably associated with original formation of the crust in the Triassic-Jurassic magmatic arc event have been identified. These structures are important because they control loci of subsequent magmatism and mineralization. Further study of these trends and associations is merited and would yield further insight into geologic development of this mineral-rich area.

The use of textural measures in the analysis of the aeromagnetic survey data to infer structure is a useful procedure because it emphasizes properties that are difficult to see in the data with traditional representations such as shaded relief images and contour maps. The measures of number of peaks and troughs per km and Euclidean length provide filters which allow comparison of texture in areas of large anomalies with areas of small ones; in traditional displays the large amplitudes dominate and obscure the areas of smaller anomalies. The process of ranking incorporates both "roughness" ( number of peaks and troughs/km) and amplitude (Euclidean length/km) into one image. More research needs to be done on the display of the ranked values to better show all the information contained in the ranking. A principal component analysis type of approach and better display schemes (such as peak/troughs in blue channel and Euclidean length in red channel) should be investigated in future studies. In addition, it would be desirable to incorporate some kind of downward continuation correction for deeper sources to make this a really valuable tool for mapping buried lithology.

By using a window size that defines the minimum size feature of interest together with the textural measues, the shallow trends and dominant features in the data are removed and textures and features of larger areas are displayed. For example, the centers of igneous activity (mainly Laramide and Tertiary) are well delineated (pl. 7). Trends and textures of underlying older crust that are subtle in the data because of the dominance of larger features are revealed. Areas of similar magnetic texture, inferred to represent similar lithology, are mapped until buried too deeply, even though overlain by younger units of varying magnetic character and polarity.

Finally, although emphasizing certain aspects of the data, like any other derivative procedure, the textural measures help one to identify the features in the original data itself. If features in the derivative maps cannot be seen in the original data, they may be artifacts of computation rather than expressions of geologic structure.

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