Fold axes oblique to the regional plunge and Proterozoic terrane accretion in the southern Lake Superior region

The Niagara fault is believed to represent the suture between a magmatic arc terrane and a passive margin assemblage which were tectonically juxtaposed about 1.85 Ga ago during the Penokean orogeny. Accretion-related deformation is concentrated within an ～12 km wide belt that straddles the suture. A distinctive structural feature of this belt is the rotation of early fold axes toward parallelism with the direction of bulk extension, resulting in a great-circle stereonet distribution of fold axes within a vertical axial plane. Rocks of both arc and passive margin assemblages crop out in this more highly-strained belt, but there has been no material transfer between them across the fault. Structural style and metamorphic assemblages demonstrate deep-level (beneath the brittle—ductile transition) erosion of the terrane accretion boundary in the southern Lake Superior region. There is some evidence for pure shear during the accretion event, but we are unable to evaluate the amount or sense of simple shear.