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An aeromagnetic map across the eastern and central United States between lats 37°N and 38°N reflects the regional structure and lithology of the magnetic crystalline rocks. The major crustal units that are delineated are the Appalachian Mountain system, the buried southwesterly extension of the Grenville province, the prominent fault zones across Kentucky, southern Illinois, and Missouri, the igneous rocks of the St. Francois Mountains and vicinity, and the buried belt of low-grade metamorphic rocks in southeastern Kansas. The map also reveals a series of conspicuous, essentially east-trending magnetic lineaments, anomaly trends, and breaks in anomaly pattern along the entire survey area. Most of these east-trending features coincide in part with Phanerozoic surface faults, deflections in structural pattern, or the presence of intrusive rocks; some of the lineaments have no surface expression. Along one of the segments in eastern Kentucky, near the 38th parallel, magnetic and gravity anomalies associated with the Grenville front are offset more than the crest of the Cincinnati arch is. The lineaments, therefore, probably represent old zones of dislocation that are more extensively developed in the basement than in the overlying rocks. Their association with Phanerozoic structures suggests that some of the segments were reactivated during later tectonic activity.

Similar magnetic and structural dislocations have been reported from other parts of the continent. There is no apparent connection between any of these continental structures and the east-trending fracture zones in the nearby ocean basins. Their approximate parallelism suggests, however, that the oceanic fractures may be recent expressions of an east-trending lineament direction that has persisted for a long time.

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