FORMATION OF CRENULATION CLEAVAGE AND KINK BANDS IN THE BARABOO SYNCLINE, WISCONSIN: A MECHANISM TO ACCOMMODATE SUBSIMPLE SHEAR DURING REGIONAL FOLDING

The Baraboo Syncline of Wisconsin is a doubly-plunging, south-vergent, regional-scale fold with a slightly overturned north limb and a gently north-dipping south limb. This Proterozoic-age structure involves Baraboo Quartzite, consisting of greenschist-grade, ultramature quartzite interlayered with phyllite. Mesoscopic structures developed during regional folding include F₁ parasitic folds related to regional flexural slip, and S₁ cleavage whose intensity and orientation correlates with the concentration of phylllosilicates. In phyllitic layers, S₁ is roughly axial planar to F₁. At several localities, especially in thicker phyllitic layers of the south limb, S₁ cleavage is deformed by S₂ asymmetric crenulation cleavage and by F₂ kink bands. F₂ kinks become folds with rounded hinges where thin quartzite layers are involved. S₂ cleavage dips in a direction opposite to S₁ (i.e., is antithetic), and F₂ folds verge in a direction opposite to the parasitic mesoscopic F₁ folds formed by out-of-the-syncline shear (e.g., F₂ verge down-dip on the south limb; local exceptions represent conjugate F₂ kinks). High-resolution digital photography allows us to trace S₁ microlithons across several S₂ crenulation domains, and to trace S₂ domains into hinge surfaces of F₂ kink bands/folds. These observations emphasize that the overall sigmoidal shape of crenulated S₁ microlithons is due to the intersection of the microlithon with four micro-scale kink hinges. We interpret S₂ and F₂ to be manifestations of late-stage subsimple shear within phyllitic layers during progressive development of the Baraboo Syncline. Specifically, out-of-the-syncline flexural-slip causes S₁ cleavage in the phyllite to rotate into an orientation that allows slip on S₁ domains to initiate. Local resistance to this slip, along with lateral confinement of the phyllitic layers, triggers initiation of contractional antithetic kink bands. Larger ones become F₂ folds, and smaller ones become S₂ crenulations. Progressive deformation locally overturns F₂ limbs. Shortening across subhorizontal S₂ domains accommodates overall thinning of phyllitic layers, thereby amplifying the effects of subsimple shear in these layers.