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