REVISITING KINK BANDS AND CRENULATION CLEAVAGE IN THE PRECAMBRIAN BARABOO SYNCLINE: KINEMATICS OF FABRIC EVOLUTION DURING PROGRESSIVE SUB-SIMPLE SHEAR ACCOMPANYING FOLDING

The Baraboo Syncline in southern Wisconsin is a regional-scale, south-verging, asymmetric fold involving Proterozoic-age Baraboo Quartzite. Dalziel and Dott, in their influential 1970 work on Baraboo, describe how flexural slip during folding produces an S₁phyllitic cleavage, in pelitic layers, that parallels the axial plane of the regional syncline and associated parasitic F₁folds. They noted that these structures have locally been deformed by mesoscopic F₂folds and by an S₂asymmetric crenulation cleavage. The F₂folds appear to have a downdip vergence relative to bedding, opposite to that of F₁folds, so in exposures on the north-dipping upright south limb of the syncline, F₂axial planes dip south (i.e., they are antithetic to S₁). S₂crenulation domains are parallel to F₂axial planes. In the S₂cleavage domains, relict S₁foliation has a sigmoidal form that implies a top-down-to-the-south shear sense on domain surfaces, as would occur in ECC fabrics. These enigmatic F₂and S₂structures have previously been attributed to a second phase of deformation that accommodated normal-sense extensional shear. Our study of F₂and S₂ using high-resolution digital photographs emphasizes, in contrast, that F₂folds are monoclinic contractional kink bands; shear associated with kinking is top-up-to-the-north (reverse sense). Closer examination of S₂cleavage domains reveals that they are not a normal-sense ECC fabric, but rather are manifestations of the same reverse-sense kinking process that produced F₂ folds, but on a finer scale. Based on published theoretical and experimental studies of antithetic kink-band formation, we suggest that F₂and S₂initiated when S₁planes in phyllite rotated to a low angle (< 15°) relative to the regional shortening direction, so that slip could initiate on S₁planes. This slip generated kink instabilities that evolved into antithetic F₂kink bands and into S₂crenulation cleavage that dipped in the opposite direction to orientations of earlier S₁/F₁features. Such a model explains how apparent down-dip-verging kink bands/folds and crenulation cleavage domains actually develop during progressive sub-simple shear strain associated with up-dip shear on the limbs of a regional fold during compression, and that the presence of these structures does not require regional extension.