THE PENOKEAN OROGENY IN THE LAKE SUPERIOR REGION

The term “Penokean” was introduced in 1914 for an orogeny believed to have deformed both Paleoproterozoic and Mesoproterozoic rocks in the Lake Superior region. Since about 1960 the term has been restricted to events affecting only Paleoproterozoic strata. By the 1970s radiometric ages indicated that the Penokean, as expressed in Michigan and Wisconsin, was confined to the period 1890-1830 Ma, the end of the orogeny being marked by 1830 Ma post-orogenic granite plutons that cut strongly deformed strata. Hence, for many years the Penokean orogeny was assumed to be responsible for all Paleoproterozoic structural and metamorphic events throughout the Lake Superior region and in much of the buried mid-continent basement. During the past decade, a growing body of geochronological and geophysical data suggests a far different interpretation. The aerial extent of the Penokean orogen is smaller than previously inferred and the “type” Penokean orogen in Michigan and Wisconsin is overprinted by eastern extensions of both the Yavapai (Geon 17) and Mazatzal (Geon 16) orogens and contains many post-Penokean features. The Penokean orogen in the Lake Superior region records a 40 m.y. period of arc-continent collisions and development and deformation of a foreland basin along the continental margin north of the accreting arc. The orogen consists of a southern internal domain of island arc volcanics and related mafic to felsic intrusions, and a northern external domain, the foreland basin. The sedimentary sequence within the foreland basin contains the classic Lake Superior iron-formations, which are succeeded by a thick sequence of turbidites. The age of turbidite deposition overlaps the age of folding and faulting during which the sediments were incorporated into a north-verging fold-and-thrust belt. Many features within the orogen, such as basement-cored domes, nodes of regional metamorphism, and granite plutons, previously ascribed to the Penokean orogeny, are overprints of the Yavapai and Mazatzal orogenies and formed as much as 200 m.y. after Penokean deformation.