Kinematic and tectonic implications of crenulation cleavage, kink bands, and mesoscopic folds in the Baraboo Syncline, Wisconsin (∼1.45 Ga Picuris Orogen)
The Baraboo Syncline of Wisconsin is a south-verging, regional-scale fold that formed during the ∼1.45 Ga Picuris Orogeny. It involves Mesoproterozoic quartzite and phyllite of the Baraboo Quartzite. Phyllite layers preserve two deformational assemblages. D1 includes the regional-scale flexural-slip to flexural-flow syncline and associated mesoscopic parasitic folds. It also includes spaced cleavage in quartzite layers and north-dipping phyllitic cleavage in phyllite layers. On the gently north-dipping south limb of the regional syncline, parasitic folds verge south (out-of-the-hinge). On this limb, D2 includes north-verging monoclinal kink bands as well as south-dipping asymmetric crenulation cleavage; kink bands have not been found on the north limb. Crenulation growth micro-folded phyllitic cleavage domains into asymmetric sigmoids that are shaped like an ‘S' on the Baraboo Syncline's south limb and like a ‘Z' 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, whereas others consider them to be a consequence of progressive crustal shortening that continued after D1 fabrics had developed. Our structural analysis supports the second hypothesis and therefore implies that the manifestation of the Picuris Orogeny in southern Wisconsin did not include a post-syncline extensional phase. This interpretation is compatible with a tectonic model in which the Baraboo Syncline formed during inversion of a rift or pull-apart basin during north-south crustal shortening. Specifically, we found no north-dipping normal-sense shear zones, and we demonstrate that the kink bands and asymmetric crenulation of the south limb accommodated a component of north-south shortening in the plane of phyllitic cleavage. Published analog models show that the north-verging kink bands of the south limb could have developed in the same deformation regime as did south-verging parasitic folds. The geometry of crenulation in the north limb indicates that it could be an extensional crenulation accommodating stretching of the limb once it was steep. Late-stage flattening at a high angle to bedding on the south limb locally rotated the crenulation cleavage domains to a sub-horizontal dip. Outcrops in the Baraboo Syncline emphasize that asymmetric crenulation is a fungible fabric, in that examples with similar appearance can accommodate different strains and displacements depending on local context.
Marshak, S., Wilkerson, M.S., and DeFrates, J., 2024, Kinematic and tectonic implications of crenulation cleavage, kink bands, and mesoscopic folds in the Baraboo Syncline, Wisconsin (~1.45 GA Picuris Orogen), Journal of Structural Geology, 178, doi: https://doi.org/10.1016/j.jsg.2023.105007.