Abstract

A COCORP (Consortium for Continental Reflection Profiling) seismic-reflection survey across the boundary zone between the late Archean Superior Province granite-greenstone terrane and older Archean gneiss and granulite terrane of the Minnesota River Valley shows numerous north-to-northwest–dipping reflection events throughout the crust. A particularly prominent and continuous series of such events projects to the surface near the trace of a fault zone that had been previously identified as the boundary between these two terranes, within the Great Lakes tectonic zone. The moderate dips of these events (about 30°), their correlation with presumed faults near the surface, their continuity over tens of kilometres, and the persistence of these and other reflection zones throughout the crust suggest that these events correspond to fault zones, probably thrusts. They may have originated during late Archean collision between the Superior Province crust and older continental crust to the south.

Reflections are sparser and more concordant in the gneiss and granulite terrane than in the Superior Province. There are several prominent reflection groups with gentle northerly dips in the middle and upper crust of the granulite and gneiss terrane. Their distribution appears to correlate with the sources of aeromagnetic and gravity anomalies that are part of a belt that extends west to central South Dakota and that apparently reveal the position of the postulated Archean suture. The axes of deposition and deformation of Proterozoic metasedimentary rocks are approximately parallel to and apparently coincide with the boundary between two Archean terranes farther east in Minnesota, Wisconsin, and Michigan, but Proterozoic (Penokean) deformation has not been distinguished either in the study area or in the seismic sections.

The Minnesota COCORP survey has demonstrated the capability of this seismic-reflection method to reveal important but previously unsuspected structures in complexly deformed Archean terranes. This has provided structural evidence that the separate subprovinces of the North American Archean crust might have been assembled by subduction-related collision.

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