Transpression in an Archean greenstone belt, northern Minnesota

Weakly metamorphosed Archean sedimentary and volcanic rocks of the Vermilion district, northern Minnesota, occupy an east-west-trending belt between gneisses of the Vermilion granitic complex to the north and the Giants Range batholith to the south. All the measured strain, a foliation, and a mineral lineation in this belt are attributed to the "main' phase of deformation (D₂). Foliation strikes parallel to the belt and dips steeply, and the mineral lineation plunges moderately to steeply east or west and is parallel to the maximum stretching direction, X, and subparallel to fold hinges. An earlier, possibly nappe-forming, event (D₁) left little evidence of fabric in the Vermilion district. A number of features indicate that the D₂ deformation involved a significant component of dextral strike-slip shear in addition to north-south compression. The S₂ cleavage is locally folded, and a new spaced cleavage developed in an orientation similar to that of the old cleavage away from the folds. We consider this the result of a process of continuous shear, with perturbations of flow resulting in folding of S₂ and the development of a new foliation axial planar to the folds. -Authors