ORIGIN OF THE BARABOO SYNCLINE AND ITS FABRICS, WISCONSIN: A TECTONIC PARADOX

The Baraboo Syncline is a regional-scale S-verging fold of the Proterozoic Baraboo Quartzite, a unit of ultra-mature quartzite interbedded with phyllite. The architecture of the fold and of associated mesoscopic structures is well documented. Published dates suggest that metamorphism of the Baraboo Quartzite took place at ~1.63 Ga, presumably in association with the Mazatzal Orogeny. The syncline has been attributed to shortening in a Mazatzal thin-skinned foreland fold-thrust belt. Previous studies demonstrate that fold intensity, intragranular strain, degree of tectonic-cleavage development, metamorphic grade, and stratigraphic thickness of strata correlative with the Baraboo Quartzite decrease progressively northwards, implying that the presumed fold-thrust belt is N-verging, overall. In this context, aspects of the syncline's structure remain paradoxical. First, vergence of the fold is opposite to that of the presumed Mazatzal fold-thrust belt. Second, since older igneous rocks underlying the Quarzite are also involved in the fold, the basal detachment must lie at depth in the basement, not at the basement-cover contact. Two solutions to these paradoxes are: (1) The syncline formed beneath a backthrust rising from a detachment in the basement (i.e., the structure is thin-skinned, but involves basement); and (2) The syncline is not associated with "thin-skinned" thrusting, but rather lies in the footwall of a Laramide-style "thick-skinned" basement-penetrating reverse fault. The existence of cleavage in the syncline indicates that the strata underwent layer-parallel shortening prior to and during folding. LPS fabrics are not typically associated with Laramide-style folds. Thus, structural features suggest a model in which the Baraboo Syncline formed in a tectonic setting comparable that of the Pyrenees, i.e. the syncline is in the footwall of a basement-penetrating thrust that initiated after closure and shortening of the basin containing the Baraboo Quartzite yielded LPS fabric. As the fold evolved, flexural-slip rotated phyllitic cleavage into an axial-planar orientation. Then, shear triggered the development of crenulation cleavage. Emplacement of overlying thrust slices produced a vertical load that rotated the crenulation cleavage so that it is now locally subhorizontal.