Abstract

A concentrically zoned circular gravity high about 2,800 km in diameter is recognized in the gravity map of northeastern North America. The high consists of three major zones: a central region of high average gravity encircled by two belts of progressively lower average gravity outward from the high. Changes in surface geology occur across some of the zone boundaries, but these cannot fully explain the changes in background level of gravity between each zone.

The major part of the Superior province of the Canadian Shield lies within the inner two zones of the concentric pattern, and the tectonic fabric cuts across the pattern. Rocks older than 3.1 b.y. have not yet been found within the Superior province, but rocks as old as 3.8 b.y. occur around the Superior province in the outer two zones and beyond. Proterozoic foldbelts tend to be parallel to and mimic the concentric pattern. Except for the Cape Smith–Belcher Island part, gravity anomalies associated with the Proterozoic foldbelts are strongly developed where the foldbelts are parallel to the gravity zone boundaries but are weak and discontinuous where they cut across the gravity zones. The parallelism of Proterozoic foldbelts to the boundaries of the gravity zones suggests that the gravity pattern is at least 2.0 b.y. old, but it may be much older. The feature may be the vestige of a large-scale meteorite impact that occurred early in Earth history.

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