Interpreting Enigmatic Mesoscopic Structures of the Baraboo Syncline (Picuris-Baraboo Orogen), Wisconsin

Stephen Marshak, University of Illinois Urbana-Champaign
M. Scott Wilkerson, DePauw University
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Abstract

The Baraboo Syncline of Wisconsin is a south-verging, regional-scale fold. This doubly-plunging structure, which involves Mesoproterozoic quartzite and phyllite, has a roughly east-northeast-trending hinge, a near-vertical to overturned north limb, and a gently north-dipping south limb. It is the only exposed example of an east-northeast-trending belt of folds visible in magnetic-anomaly maps; other examples are covered by Paleozoic strata. Recently published dates indicates that this belt formed at about 1.45 Ga, and therefore is a manifestation of the Picuris-Baraboo Orogeny. Phyllite layers preserve two mesoscopic deformational assemblages. D1 includes south-verging mesoscopic flexural-slip to flexural-flow parasitic folds that developed on limbs of the regional-scale south-verging syncline. D1 also includes spaced cleavage in quartzite layers and north-dipping axial-planar phyllitic cleavage in phyllite layers. D2 structures, found only on the south limb of the Baraboo Syncline, include north-verging monoclinal kink bands and mesoscopic folds. In phyllite layers, D2 also includes a south-dipping asymmetric crenulation cleavage. Cleavage domains of this fabric are S-shaped on the Baraboo Syncline's south limb and Z-shaped on the fold's north limb (as viewed looking east). The origin of D2 structures has long been controversial. Some authors attribute them to a post-syncline phase of extensional tectonism on north-dipping shear zones, for the vergence of D2 folds hints that they developed during top-down-to-the-north shear. Other authors consider them to be a consequence of progressive crustal shortening that continued after D1 fabrics had developed. Our structural analysis supports the second hypothesis. Specifically, we demonstrate that the kink bands and crenulation could develop during north-south shortening once the D1 phyllitic cleavage had rotated to a low angle relative to regional compression. This interpretation is compatible with classic laboratory models of kink-band generation. Locally, however, the strain significance of the asymmetric crenulation varies, in that in some localities it accommodates stretching and in others, it accommodates shortening, depending on the relative amounts of rotation of different segments of microlithon sigmoids. The tectonic significance of the Syncline's south vergence has also been puzzling. Some authors place the syncline in a north-verging fold-thrust belt, to explain why regionally, deformation in correlative quartzites die out northwards, and others in a south-verging belt because of the fold geometry. We suggest that it formed by inversion of a rift or pull-apart basin during north-south crustal shortening. The Baraboo Syncline is on the south-verging margin of the resulting bivergent wedge.

Session

Deformation in the upper crust