RECENT INVESTIGATIONS OF THE PENOKEAN OROGEN IN MINNESOTA: A CASE FOR INDENTER TECTONICS ALONG A PROMONTORY-REENTRANT MARGIN

The apparent southward bend of the Paleoproterozoic Penokean Orogen in central Minnesota and possible continuation southwest into Iowa along the Spirit Lake trend have been cited by previous workers as evidence for a promontory-reentrant margin in the Archean Superior Province craton, with the Minnesota River Valley subprovince forming the promontory. Testing of this hypothesis has been difficult because of very poor bedrock exposure, but recent investigations, relying on high-quality gravity and aeromagnetic data, as well as precise radiometric dating, have yielded a geologic framework that supports the promontory-reentrant hypothesis.

The Penokean terranes of northwestern Wisconsin are interpreted to extend into east-central Minnesota, but several major changes are evident. Probable equivalents to the island arc sequences of Pembine-Wausau terrane extend into east-central Minnesota, but they occur along a considerably narrowed belt that appears to be bent sharply to the south. Fold-and-thrust belts and foreland-basin deposits associated with the orogen show a similar bending to the south, before they disappear in central Minnesota in the vicinity of Minnesota River Valley rocks. Finally, post-orogenic granites, collectively referred to as the east-central Minnesota batholith, form the dominant part of the orogen in Minnesota, whereas they only occur sparingly in Wisconsin.

The changes in the orogen are consistent with the promontory-reentrant hypothesis. Promontories tend to form competent, high-standing buttresses, and similar characteristics for the inferred Minnesota River Valley promontory would have impeded development of fold-and-thrust belts and foreland basins. On the other hand, reentrants typically contain thick and relatively incompetent supracrustal sequences that accommodate thin-skinned deformation and tectonic loading onto the continental margin. Tectonic loading onto a reentrant margin in east-central Minnesota may have been further enhanced by collision with the Archean-cored Marshfield terrane of Wisconsin, which appears to have formed a continental indenter. This greatly thickened orogenic lid may have ultimately produced the mid-crustal melts for the post-orogenic granites. The relationships inferred here between reentrants and post-orogenic granites might have implications for other orogens.