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The Humboldt Bay High Strain Zone (HBHSZ) is the most prominent Archean shear zone within the Onaman-Tashota greenstone belt of the eastern Wabigoon subprovince. The HBHSZ was initiated as a terrane boundary during assembly of the greenstone belt (D1) and displays several periods of reactivation (D2–D4) during post-assembly transpression. Each deformation phase was restricted to successively narrow zones and transcurrent shear sense changed with each phase. The boundary of the North Wind Pluton adjacent to the shear zone exhibits similar D2–D4 fabric and kinematics to the HBHSZ and the relations of plutonic phases to the boundary fabrics demonstrate that it was likely emplaced during D2 transpression. The occurrence of plutonism during transpression indicates the lithospheric scale of post-assembly phase D2. The best-defined 40Ar/39Ar plateau ages come from texturally simple, extensively recrystallized amphibolite that formed locally during D2 in the HBHSZ and throughout the margins of the North Wind Pluton. The average of these is ca. 2667 Ma, similar to the maximum mica age, and gives a minimum age for D2. Texturally complex amphibolite, formed by partial recrystallization during D2, contains D2 foliation-forming amphibole, and porphyroclasts of actinolite. These amphibolites yield discordant spectra with age gradients and, together with variably lower mica ages, may record later reactivation of the shear zone (D3–D4 and later events). By analogy with the linkage between the late, deep crustal flow and upper-crustal shear zones in the Wawa subprovince the structural and thermal reactivation may be the indicator in a stiff, old superstructure of deep-crustal flow beneath the eastern Wabigoon. |
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