Abstract

Cleavage in graywacke, siltstone, and pelite of the Michigamme Slate appears to have been produced by tectonic dewatering. Clastic dikes, from paper-thin dikelets to zones 10 m wide, are intruded parallel to cleavage in many outcrops. In other places, liquefication and mobilization have homogenized the sediments into massive, structureless rocks. The planar cleavage which pervades all rock types is produced by pelitic folia formed during tectonic dewatering.

Ellipsoidal concretions with the same ductility as the host siltstone indicate large plane strains. Maximum shortening is perpendicular to cleavage with greater than threefold extension in the cleavage. Lengths parallel to fold axes are essentially unchanged, but stratigraphic thicknesses may be grossly exaggerated by deformation.

Tectonic dewatering of the Michigamme Slate may have occurred after tectonic dewatering of the underlying Siamo Slate.

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