Deformation in the Archean Knife Lake Group, Vermilion District, Northeastern Minnesota

Abstract

Finite strain analysis and detailed mapping of structures in Archean volcanic and sedimentary rocks in the eastern Vermilion District of northern Minnesota reveal three deformational events, the first of which appears to be responsible for the measured strain. D1 produced regional isoclinal folds with approximately E-W trending, gently plunging axes, and approximately vertical axial planes. A strong penetrative foliation occurs parallel to the axial planes of these folds and clasts in conglomerates and volcanic rocks have been strongly flattened in the plane of the foliation. D2 was a dextral simple shear event with the shear plane oriented approximately E-W which produced sigmoidal tension gashes and rotational microstructures. Kink bands indicative of EW shortening may also have been produced in an EW compressive component of this event. D3 was another simple shear event with the shear plane oriented E-W, dipping 60S with a sinistral sense of movement reflected on east-facing vertical planes or a north-side-up movement in plan view. Rotational microstructures and a local SC foliation were produced in the D3 event. Finite strain analysis reveals flattening strains consistent with D1 kinematics. Variations in measured strain are interpreted to be due to competence contrasts between strain markers and matrix. Analysis of the effects of competence contrast on apparent strain recorded by various lithologies reveals significant variations. Strain analysis using conglomerate clasts of several lithologies as well as lithic lapilli as markers thus gives an approximation of finite strain but is not sufficiently sensitive to reflect D2 and D3 strains in the study area.