Abstract

Correlation of an F1 recumbent fold and an upright F2 antiform across the boundary between an Archean migmatite terrane and the adjacent volcanic-plutonic belt indicates a common tectonic evolution for both terranes since the earliest stages of folding. The local boundary between amphibolite facies schists and migmatites of the southern Vermilion Granitic Complex (VGC) and low-grade metagraywackes of the adjacent Vermilion district is marked by nearly vertical dip-slip faulting along the east-trending axial trace of a regional F2 antiform. The migmatites in the southern VGC have been folded into S-symmetry folds that mark the northern limb of the major antiform. These folds are correlated with F2 folds of Z symmetry on the southern limb of the fold in adjacent downfaulted rocks of the Vermilion district. Structural facing near the boundary between the two subprovinces is downward on both limbs of the major F2 structure, which is interpreted to be part of the lower, overturned limb of a large-scale F1 recumbent fold. A change to upright-facing strata farther south indicates a crossing onto the upper limb of the structure. The F1 folding is tentatively attributed to gravitational spreading off the southern margin of the rising Lac La Croix Granite of the east-central VGC.

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