THREE-DIMENSIONAL GEOMETRY AND STRAIN OF THE BARABOO SYNCLINE: KINEMATIC IMPLICATIONS
ORMAND, Carol J., Department of Geology, Wittenberg University, Springfield OH 45501, firstname.lastname@example.org and CZECK, Dyanna M. Department of Geosciences, University of Wisconsin - Milwaukee, P.O. Box 413, Milwaukee, WI 53201
Paleoproterozoic sedimentary rocks of the "Baraboo interval" were deposited on a stable cratonic margin with subdued topography, in a warm, humid climate, as evidenced by their physical and chemical maturity (e.g. Dott, 1983; Medaris et al., 2003). The southern deposits, including the Baraboo Quartzite and associated rocks, subsequently underwent both low-grade thermal metamorphism and simultaneous deformation during the Mazatzal orogeny, ~1650-1630 Ma (Holm et al., 1998; Romano et al., 2000). During this collisional event, the flat-lying marginal sediments were crumpled into tight, asymmetric, southward-verging folds. The foreland fold-and-thrust belt of this collision is preserved in isolated outcrops in southern Wisconsin including the Baraboo hills (LaBerge and Klasner, 1986).
Approximately 40 kilometers long by 15 kilometers wide, the geometry of the Baraboo Syncline is strikingly three-dimensional. The fold axis trends approximately N80E. The northern limb of the fold is subvertical to slightly overturned; the southern limb dips around 35 degrees northward; the eas,tern termination plunges approximately 35 degrees westward; and the western termination plunges approximately 25 degrees eastward.
Strain on the limbs of the Baraboo Syncline is as three-dimensional as the fold itself. Both limbs of the fold have axial planar phyllitic cleavage, refracted into quartzitic strata. Within the southern limb, however, 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. Strain data from quartz grain shapes also indicate three-dimensional strain, with extension either layer-parallel or layer-normal (McKiernan , 2002; Craddock, pers. comm.). In addition, slickensides on the southern limb of the fold indicate one paleostress direction, while slickensides on the northern limb show multiple paleostress solutions (Kirschner et al., 1989).
Both the fold geometry and the extension within the gently dipping limb of the syncline are consistent with formation in a top-to-the-south simple shear environment (Cambray, 1987). In such an environment, non-cylindrical fold trains would verge southward. In this model, the longer, north-dipping fold limbs are favorably oriented for localized layer-parallel extension, while the shorter, steeply dipping limbs rotate and shorten during deformation. The location of boudinage exclusively on the south limb is consistent with this model. The multiple paleostress directions on the north limb, inferred from slickensides (Kirschner et al., 1989), is also consistent with the rotation of the north limb explicit in the model. Therefore, this simple shear model is consistent with the majority of the field data. However, it is a two-dimensional model that does not account for the strongly three-dimensional fold geometry. To account for the three-dimensional shape of the syncline, we invoke a component of non-plane strain. We envision a variation in degree of shearing along strike, resulting in extension along the fold axis. 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. We are analyzing microstructural data on both limbs of the fold to further evaluate this kinematic model.
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