ORIGIN OF SECOND-GENERATION MESOSCALE FOLDS AND CRENULATION CLEAVAGE IN THE BARABOO SYNCLINE, WISCONSIN

The Baraboo Syncline is a south-verging, doubly plunging regional-scale fold that formed at about 1.65 Ga in the foreland of the Mazatzal orogen. This structure, which has a vertical to overturned north limb and a gently north-dipping south limb, serves as a prime Midwest field-teaching site. It involves Baraboo Quartzite, a maroon quartzite with local interbeds of phyllite. Phyllitic intervals on the south limb display evidence of three generations of structures. The first generation (D1) includes S1 phyllitic foliation, which parallels the axial surface of the regional Baraboo syncline, and mesoscopic south-verging (i.e., up-dip verging) parasitic folds (F1). The second-generation (D2) includes mesoscopic north-verging (i.e., down-dip-verging) folds (F2) that affect both S0 and S1, as well as two sets of crenulation cleavage (S2) that together define a conjugate system. The antithetic crenulation cleavage set parallels the axial surface of F2 folds. Notably, up-dip-verging F1 folds and down-dip-verging F2 folds occur adjacent to one another. Local late-stage deformation (D3) flattened the steep limbs of F2 folds into M-shaped folds with horizontal axial surfaces. Debate continues about the kinematic significance of D2 structures. Are they related to formation of a normal-sense detachment during regional extensional collapse, or are they a manifestation of progressive contractional deformation during thrusting? Our structural analysis demonstrates that S2 fabric is a contractional crenulation cleavage whose formation accommodated sub-horizontal shortening, and that F2 folds originated as contractional kink bands. We propose a model in which D2 structures formed to accommodate horizontal shortening after S1 foliation had become mechanically significant and had rotated from a steep to gentle dip (in response to flexural-slip shear) to become subparallel to the regional shortening direction. Under these conditions, S1 became susceptible to kinking and crenulating. D2 structures formed during progressive thrust-related formation of the Baraboo Syncline, and are not associated with a discrete extensional detachment—i.e., normal-sense vergence can form during thrust-sense shear. We suggest that D3 deformation represents late vertical shortening that developed in response to thrust-stack overburden.