Use of magnetic data is an important technique in the investigation and interpretation of both local and regional crustal structure. Several examples demonstrate close correspondence between discrete tectonic features and specific types of magnetic anomalies.

A large area in Wyoming is used to demonstrate application of magnetic data to the interpretation of

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regional crustal structure. Aeromagnetic and surface geologic surveys have been made of the exposed Precambrian basement rocks of the Big Horn Mountains, an area of about 1,100 square miles. Age determinations and measurements of magnetizations were made for a number of igneous and metamorphic samples. Significant relations were established between these various data. These, together with evidence of magnetic and geologic continuity from the mountains into the basins, permit an interpretation of large-order crustal structures in the Big Horn and Powder River Basins, and to a lesser extent, in the Wind River Basin.

Specifically, this regional study gives the following results:

(1) Both mountains and basins consist of one or more large crustal block units which are defined by magnetic character.

(2) The continuity of magnetic features from the Big Horn Mountains into adjacent basins, and the remarkable similarity of magnetic anomalies, indicate that the buried basement is lithologically and structurally equivalent to that studied in outcrop. The basement in the basins is interpreted as consisting of gneisses and metamorphosed granites, probably ranging in age from 2 to 3 billion years, or early Precambrian.

(3) Several long magnetic-geologic lineaments are defined. Some of these lineaments are thought to be related to fundamental crustal fracture systems similar to lineamental features recognized in the Pacific and Atlantic Oceans.

(4) Tectonic elements of Laramide age are clearly developed along structural lines established in Precambrian time in the basement rocks.

Regional magnetic and geologic data from other parts of the North American continent show a variety of magnetic characteristics, each of which suggests a particular type of tectonic origin. These supplement the Wyoming example and demonstrate the applicability of magnetics to the solution of controversial problems in crustal tectonics.

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