The Conundrum of Using Kink Bands and Asymmetric Crenulation Cleavage to Interpret Shear Sense in Tectonic Analyses: A Case Study from the Baraboo Syncline (Wisconsin)

Stephen Marshak, University of Illinois
M. Scott Wilkerson, DePauw University

Abstract

Interpreting the kinematic and tectonic significance of kink bands and asymmetric crenulation cleavage remains challenging. In fact, the same assemblage of these mesoscopic structures has been attributed to regional thrust-sense shear by some researchers, and to regional normal-sense shear by others. Such conflicting interpretations have been put forth concerning the kink bands and crenulation of cleavage of Wisconsin's regional-scale Baraboo Syncline. This south-verging asymmetric fold (with a near-vertical north limb and a gently north-dipping south limb) involves the Baraboo Quartzite (a Proterozoic unit composed of quartzite locally interlayered with phyllite). Recent dating attributes folding to the Picuris Orogeny. Phyllite outcrops on both limbs contain two generations of structures. These outcrops provide an excellent opportunity to study the kinematic relationship of kink bands and asymmetric crenulation cleavage to different shear senses, because the shear sense of interbed displacement developed during out-of-the-hinge flexural slip on one limb of the Baraboo Syncline is, of course, opposite to that on the other. The regional syncline together with mesoscopic thrusts, parasitic folds, and an axial-planar phyllitic cleavage are first-generation structures (S1 and F1) formed during crustal shortening. Second-generation structures include F2 kink bands (whose axial surfaces are antithetic to those of F1) and S2 asymmetric crenulation cleavage (whose domains are antithetic to those of S1 and are parallel to F2), both of which deform S1. The local down-dip vergence of F2 folds on the south limb has been attributed to a normal-sense shear zone, and the S2 cleavage has been described as an extensional crenulation cleavage (ECC). Our analysis demonstrates, however, that down-dip-verging F2 folds are flattened antithetic kink bands, and that S2 is a flattened contractional crenulation cleavage (CCC). It follows that both structures initiated to accommodate general shear during progressive shortening, flexural slip, and fold tightening as the Baraboo Syncline evolved, and did not develop during a post-shortening extensional-collapse event. Our interpretations are compatible with results of analog and computational models showing that antithetic monoclinic kink bands and asymmetric crenulation cleavage develop when a preexisting penetrative foliation dips at an angle to the direction of tectonic shortening, or is subjected to shear during shortening. The Baraboo Syncline case study implies that the tectonic significance of enigmatic down-dip-verging folds and ECC fabrics in other orogens may warrant kinematic reexamination.

Session

Session 1: Fault Zones from Top to Bottom