Age and petrographic data from the buried basement of the midcontinent region of North America, integrated with information from exposed rocks and magnetic- and gravity-anomaly maps, allow much of the Proterozoic history of the region to be assembled. The Superior craton may be traced into the subsurface on the basis of characteristic magnetic patterns and limited age data. The region between the Superior craton and the Wyoming craton to the west is evidently underlain by southerly extension of the Trans-Hudson orogen of Canada. The Penokean orogen formed on the southern margin of the Superior craton 1890–1830 Ma, but is not inferred west of northwestern Iowa in the subsurface. Between 1780 and 1720 Ma, a major orogen developed along the southern margin of the continent and is exposed in Arizona and Colorado. These rocks are volcanogenic and, for the most part, juvenile additions to the crust; they can be traced beneath the plains as far as eastern Kansas and Nebraska. Another orogen formed farther to the south about 1700–1630 Ma and is exposed in southern Arizona and New Mexico; rocks of this age and type have beer, traced as far east as central Missouri but may extend as far as central Michigan. A major geophysical feature of the midcontinent is a system of northwest-trending magnetic and gravity anomalies in Missouri, Kansas, and Nebraska; the origin of these is not currently understood.

The tectonic history of the midcontinent between 1480 and 1340 Ma was dominated by extensional formation of two widespread granite-rhyolite terranes that evidently were formed from, and overlie, the orogenic provinces. The older, formed 1450–1480 Ma, underlies the eastern midcontinent, whereas the younger, formed 1340–1400 Ma, underlies the southwestern midcontinent. The latest Proterozoic events were the formation of the midcontinent rift system and the collisional Grenville and Llano provinces about 1100 Ma.

First Page Preview

First page PDF preview
You do not currently have access to this article.