STRUCTURE AND KINEMATICS OF THE BARABOO SYNCLINE: A THREE-DIMENSIONAL ANALYSIS

The Baraboo Syncline is a doubly-plunging, tight, asymmetric syncline in the Proterozoic-aged Baraboo Quartzite, in south-central Wisconsin. Approximately 40 kilometers long by 15 kilometers wide, the syncline is strikingly three-dimensional. The northern limb is subvertical to slightly overturned; the eastern termination plunges approximately 35 degrees westward; the southern limb dips around 35 degrees northward; and the western termination plunges approximately 25 degrees eastward.

On the southern limb, where quartzite beds are sandwiched within phyllitic layers, three-dimensional pinch-and-swell structures ("chocolate tablet boudinage") show extension parallel to layering, both along strike and down dip. This extension is clearly NOT related to fold curvature, as it is within the straight, long limb of the syncline.

Both the fold geometry and the extension within the gently dipping limb of the syncline are suggestive of formation in a non-plane strain, top-to-the-south simple shear environment. In such an environment, non-cylindrical fold trains would verge southward. The longer, north-dipping fold limbs would be favorably oriented for localized layer-parallel extension, while the shorter, steeply dipping limbs would not. Moreover, extension along the fold axis would also be predicted. This model therefore could explain both the strong change in plunge along trend and the three-dimensional boudinage within the southern limb of the Baraboo Syncline.