POLYPHASE DEFORMATION VS. PROGRESSIVE DEFORMATION: INTERPRETING THE ENIGMATIC KINKS AND CRENULATIONS OF THE BARABOO SYNCLINE (WISCONSIN)

Are distinct generations of fabrics and folds due to separate phases of deformation formed in response to different stress states or different orogenic events, or are they a consequence of progressive deformation during a single stress state applied to an evolving body of rock? An answer to this question can help constrain the tectonic history of a region. Our detailed kinematic analysis of mesoscopic structures in Wisconsin's Baraboo Syncline, a regional fold formed during the 1.4 Ga Picuris Orogeny, provides clues to answering the polyphase vs. progressive conundrum. The Baraboo Syncline is a south-verging fold with a gently dipping upright south limb, and a vertical to slightly overturned north limb. It involves the Baraboo Quartzite, which consists of vitreous quartzite and well-foliated phyllite. Due to the mechanical contrast between these rock types, flexural slip subjected phyllite to out-of-the-hinge shear. Monoclinal mesoscopic folds (F₂) and asymmetric crenulation cleavage (S₂) overprint parasitic folds (F₁) and phyllitic cleavage (S₁) formed during regional folding. F₂ and S₂ have been interpreted either as a post-syncline phase of extensional collapse, or as a consequence of progressive shortening during syncline evolution. Notably, on the south limb of the Baraboo Syncline, F₂ folds display down-dip vergence, opposite to that of F₁ parasitic folds. F₂ vergence has previously been attributed to displacement in normal-sense shear zones. S₂ asymmetric cleavage, has been interpreted to be an extensional crenulation cleavage (ECC). Our analysis demonstrates, in contrast, that down-dip-verging F₂ folds are flattened antithetic kink bands, and that S₂ is a flattened contractional crenulation cleavage (CCC). Both structures initiated to accommodate general shear due to simultaneous shortening, flexural slip, and limb flattening during progressive evolution of the Baraboo Syncline. Our interpretations are compatible with computer and analog models showing that monoclinal kink bands verging opposite to parasitic folds develop when pre-existing penetrative foliation dips at an angle to the direction of tectonic shortening, or is subjected to shear during shortening. F₂ and S₂ at Baraboo began to develop as soon as a suitably oriented S₁ foliation had become mechanically significant.