Abstract

The Southern Province in the Lake Huron area of Ontario, and the Penokean Fold Belt of Minnesota, Wisconsin, and Michigan have the earmarks of a collisional orogen. The Huronian Supergroup, a southerly facing passive margin sequence, was deposited during early Aphebian (Early Proterozoic) crustal stretching (and ocean basin formation?) along the southern margin of the Superior Province Archean craton. It accumulated as four unconformity-bounded, northerly tapering, onlapping clastic wedges (megacycles). The thickness and facies variations reflect syndepositional down-to-the-basin (south) normal faulting that controlled the accumulation and preservation of the lower three Huronian megacycles. These are overlapped northward by the youngest megacycle, an extensive sheet of clastic sediments recording post-stretching regional subsidence of the cratonic margin due to cooling and thermal contraction. Soft-sediment folds in the rocks of the youngest megaycycle to the extreme south probably indicate southward slumping into an adjacent basin. Nipissing diabase instrusions, 2100 Ma old, cut Huronian strata and soft-sediment folds.An enigmatic episode of compressional deformation is dated by 2333 Ma old plutons. During the first ductile (penetrative cleavage and peak metamorphism) phase of the Penokean Orogeny (ca. 1900 Ma), the thicker part of the Huronian supracrustal wedge was overridden and depressed to mid-crustal levels, presumably because it was overridden by an allochthonous terrane, the remainder of which is postulated to lie south of the Manitoulin Island Discontinuity. The early down-to-the-basin normal faults were reactivated and became the loci of north-verging, listric reverse faults. The thick wedge of Huronian rocks south of the Murray Fault Zone, which was deformed and metamorphosed as it was carried down to mid-crustal levels, subsequently was thrust northward over a thinner sequence of Huronian rocks that apparently was never deeply buried. Concurrently, the edge of the Superior craton and its cover of Huronian deposits were compressed as thrusting propagated northward and eventually deformed the 1850 Ma old Sudbury Nickel Irruptive. Structures that formed in the thick part of the wedge during the ductile deformation were rotated during the thrusting to their present near-vertical position. Late or post-Penokean brittle deformation marked by conjugate small strike-slip faults reflects continued or renewed north–south compresssion in an isostatically rising foldbelt, and post-Penokean right-lateral strike slip along the Murray Fault Zone is probably related to northwestward compression during the Grenville (approx. 1000 Ma) Orogeny.

You do not currently have access to this article.