Bedrock mineral cooling dates in the southern Lake Superior region establish a sharp ca. 1630 Ma thermal front that separates basement having typical post-Penokean (1750–1700 Ma) cooling ages to the north from basement having thermally reset (≤ 1630 Ma) ages to the south. The thermal front coincides spatially with an apparent deformational front in overlying post-Penokean quartzites. Subhorizontal quartzite north (Barron, northwest Wisconsin) and west (Sioux, Minnesota) of the front and highly folded quartzite south of the front (Flambeau, northwest Wisconsin) all yield ion microprobe (single-grain, single-spot) 207Pb/206Pb detrital zircon ages as young as 1750 Ma (but none younger than 1714 Ma). The new detrital zircon dates and the thermal-deformational front together with other geologic information suggest that these quartzites were all deposited between 1750 Ma and 1630 Ma. This study provides the first good structural evidence that deformation accompanied the long-recognized but enigmatic 1630 Ma low-grade, thermal-metamorphic event in the southern Lake Superior region. We interpret the strong compressional deformation exhibited in the Early Proterozoic quartzite bodies throughout much of Wisconsin to reflect foreland deformation associated with the assembly of southern Laurentia during the ca. 1650 Ma Mazatzal orogeny.

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