Evolution of Penokean foldbelt, Lake Superior region, and its tectonic environment
The Penokean foldbelt is a northeast-trending zone of deformed and metamorphosed Archean and early Proterozoic rocks, as much as 250 km wide in the Lake Superior region, along the southern margin of the Superior province. The rocks within the foldbelt were deformed 1,880–1,770 m.y. ago during the Penokean tectonothermal event.
Evolution of the foldbelt began with rift-faulting that was localized along a preexisting zone of weakness in Archean rocks, the boundary between the two crustal segments previously recognized in the region: a greenstone-granite terrane (~2,700 m.y. old) to the north (Superior province) and a mostly older (in part 3,500 m.y. old) gneiss terrane to the south. The faulting and later broad foundering provided sites for deposition of detritus shed mainly from the craton to the north and for chemical sediments, including the vast iron-formations for which the region is famed. The central part of the basin also received mafic volcanic rocks intimately intercalated with the sedimentary rocks, and similar rocks dominate in the southern part. The terminal, compressional stage ended deposition. It involved folding of the supracrustal rocks, penetrative deformation of the basement rocks, and emplacement of diapiric gneiss domes with accompanying appression of intervening supracrustal rocks. Deformation was complex and prolonged, and because of basement participation it differed from place to place in style and orientation of folds and in intensity and nature of metamorphism. Granite-tonalite plutons locally were emplaced in the southern part of the foldbelt near and after the end of deformation.
The rifting and terminal compression occurred in an intracratonic or continental-margin environment; possibly the compression was caused by forces transmitted from a remote distance to the southeast, in an area undergoing rifting of a continental margin and subsequent continent-continent collision.