Mineral Resources Program
New digital magnetic anomaly database of North America - processing, compilation and geologic mapping applications
A new, upgraded digital magnetic anomaly database and map for North American are the results of joint efforts of the Geological Survey of Canada (GSC), the U.S. Geological Survey (USGS), and Consejo de Recursos Minerales of Mexico (CRM). The integrated, readily accessible, modern digital database of magnetic anomaly data will be a powerful tool for evaluating the structure, geologic processes, and tectonic evolution of the continent and may also be used to help resolve societal and scientific issues that span national boundaries. The magnetic anomaly map derived from the digital database will provide a view of continental-scale trends not available in individual data sets, help link widely separated areas of outcrop, and unify disparate geologic studies.
The North American magnetic anomaly map and data base will be unveiled at the Geological Society of America's fall 2002 meeting in Denver.
Magnetic Anomaly Data
Magnetic anomalies reflect variations in the distribution and type of magnetic minerals-primarily magnetite-in the Earth's crust. Magnetic rocks can be mapped from the surface to great depths, depending on their dimensions, shape, and magnetic properties, and on the character of the local geothermal gradient. Magnetic anomaly data provide a means of "seeing through" nonmagnetic rocks and cover such as vegetation, soil, desert sands, glacial till, manmade features, and water to reveal lithologic variations and structural features such as faults, folds, and dikes. In many cases, examination of magnetic anomalies provides the most expeditious and cost-effective means to accurately map geologic features in the third dimension (depth) at a range of scales.
The new map supercedes the Magnetic Anomaly Map of North America that was produced by the Geological Society of America's Decade of North American Geology (DNAG) program in 1987 (Geological Society of America Committee for the Magnetic Anomaly Map of North America). The United States portion of the DNAG map was based on a 1982 magnetic anomaly map that was produced by Isidore Zietz (Zietz, 1982). The Canadian component of the DNAG Magnetic Anomaly Map of North America was based on a 2-kilometer grid (Dods and others, 1987) covering 70 percent of Canada (the largest data gaps over western Canada and the Arctic Islands). No data for Mexico were published in the DNAG map.
The new map differs from the DNAG map in two significant ways: (1) extent of coverage and (2) how data were treated. New surveys were added for the 2002 map. Coverage for Mexico is nearly complete, and many of the gaps for the United States and Canada are filled. Data from analog and digital surveys were merged graphically for the DNAG map. For the new map, data from analog maps were digitized, and data from both analog and digital surveys were then digitally merged. The result is a map that features almost complete coverage and much finer resolution than the 1987 DNAG map.
Utility of the compilation
Understanding the regional geology of the continent can provide information useful for a wide variety of applications such as mineral and energy resource assessments, earthquake and landslide hazards investigations, hydrologic and environmental studies. Flight-line spacing and elevation above the magnetic sources largely determine the size of the geologic feature that can be resolved with magnetic data. Only broad, regional magnetic sources such as batholiths (> 3 kilometers wide) can be resolved with coarsely-spaced (> 3 kilometers) surveys, while fine-scale features (< 1 km wide) such as shallow dikes, faults and folds can be resolved with closely-spaced (< 500 meters), low-altitude (< 500 meters above terrain) data.
Dods, S.D., Teskey, D.J., and Hood, P.J., 1987, Magnetic Anomaly Map of Canada, fifth edition: Geological Survey of Canada Map 1255A, scale 1:5,000,000.
Geological Society of America Committee for the Magnetic Anomaly Map of North America, 1987, Magnetic anomaly map of North America: Geological Society of America, continent-scale map-003, scale 1:5,000,000, 4 sheets.
Zietz, I., 1982, Composite magnetic anomaly map of the United States; Part A, Conterminous United States: U.S. Geological Survey Investigations Map GP-954-A, 59 pp., 2 sheets, scale 1:2,500,000.
For more information
Please visit the North American Magnetic web site: http://crustal.usgs.gov/projects/namad/, or contact:
Dr. Carol Finn
Dr. Mark Pilkington