Geometry and folding history of the Baraboo Syncline: Implications for the Mazatzal Orogeny in the north-central U.S.

The Baraboo Syncline in south-central Wisconsin formed during the Mazatzal Orogeny. It is a doubly plunging, asymmetric syncline. Internal structures, such as chocolate-tablet style boudinage and tension gashes, indicate stretching parallel to the fold axis, and preclude folding by plane-strain models. Both the fold geometry and the internal deformation within the fold limbs suggest a history of (1) buckling that varied along the fold axis resulting in doubly plunging folds and stretching along the fold axis followed by (2) continued buckling combined with top-to-the-south simple shearing that may have varied along the fold axis. The second stage of deformation could be the result of the onset of an overriding thrust package. The deformation history is consistent with a typical south-verging fold forming in a fold and thrust belt that inherited its geometry from a north-dipping subduction zone.