Geophysical and radiometric age data now permit attempts to outline Precambrian structural provinces in the United States. There are two Archean terrains, the Superior and Wyoming provinces. The Churchill province (1,700 m.y. and older) splits around the Wyoming block. One branch extends southwest across Montana, the other south and southeast between the Wyoming and Superior cratons. An east-northeast-trending terrain probably accreted along the southern margins of the Wyoming-Churchill-Superior block beginning about 1,800 m.y. ago. The Churchill province may have been truncated by this accretionary terrain or may merge into it. Southeast geophysical trends in Missouri and Kansas suggest southwestward accretion in that region, 1,600–1,700 m.y. ago.

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