Abstract

Mountain building in the western part of the Archean Superior Province culminated with the formation of regional strike-slip faults. This paper reports on the kinematics and timing of several major faults at the juncture between the Uchi, English River, Winnipeg River, and western Wabigoon subprovinces. Sinistral-oblique mylonitization along the northeast-striking Miniss River fault occurred at forumla Ma. This involved ∼40 km of sinistral offset and a scissor-like motion whereby vertical displacement increased southwestward toward a restraining bend near Sioux Lookout. To the north, the Miniss River fault is intersected by the east-striking, dextral strike-slip Sydney Lake – Lake St. Joseph fault; the latter merges along strike with the Pashkokogan fault. Restoration of respective displacements indicates that the faults formed sequentially, not simultaneously in response to tectonic indentation. Dextral strike-slip motion along the Sydney Lake – Lake St. Joseph (– Pashkokogan) fault was instigated at ≤2670 Ma and drove greenschist-grade, dextral reactivation of the southwest segment of the Miniss River fault. U–Pb geochronology suggests that the latter coincides with an older terrane-boundary fault that juxtaposed ca. 2735 Ma juvenile, western Wabigoon arc complexes against ca. 3.05 Ga granitoid rocks of the Winnipeg River terrane. The Sydney Lake – Lake St. Joseph (– Pashkokogan) fault similarly demarcates a fundamental boundary between Uchian volcanoplutonic rocks and the English River accretionary prism. Strike-slip faults in this region therefore initiated at terrane boundaries and in some cases evolved so as to transect and displace these boundaries to accommodate further shortening during final stages of Archean orogenesis.

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