Tectonic implications of finite strain variations in Baraboo-interval quartzites (ca. 1700 Ma), Mazatzal orogen, Wisconsin and Minnesota, USA

Variably deformed Baraboo-interval quartzites (1750–1630 Ma) are now correlated by detrital zircon ages [Holm, D., Schneider, D., Coath, C.D., 1998. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: implications for extent of foreland deformation during final assembly of Laurentia. Geology 26, 907–910] and were analyzed using finite strain techniques which document a SE–NW tectonic shortening orientation from the Baraboo syncline (south) to the Flambeau syncline (north). Fabric interpretations using bedding and the principal strain axis orientations indicate there was a mix of layer-parallel and layer-normal (synfolding) shortening in the plane of tectonic transport (SE–NW) in the thrust belt. Finite strain magnitudes increase from the foreland Barron–Sioux quartzites south toward the Baraboo syncline. Electron backscatter device (EBSD) fabric diagrams (pole figures) of quartz optic axes also demonstrate an increasingly penetrative strain to the south. Anisotropy of [low-field ac] magnetic susceptibility (AMS), as a strain proxy, preserve Kmin measurements in the Baraboo syncline quartzites that are sub-horizontal, E–W and intersect bedding. AMS measurements in underlying rhyolite on opposite limbs of the Baraboo syncline are clean, appear to be primary (magmatic) as Kmax intersects bedding, but require a complex folding interpretation that is inconsistent with field kinematic indicators. The flat-lying foreland Barron and Sioux Quartzites preserve a mix of results: quartzites with no strain (finite strain and EBSD measurements) or small strains that preserve vertical shortening. AMS measurements in the foreland preserve Kmin axes that are sub-horizontal and parallel to the regional shortening orientation in the adjacent thrust belt. The Mazatzal orogen produced a thrust belt that is orders of magnitude wider (∼500 km) and of higher metamorphic grade, than most thrust belts, where both the thrust belt and foreland are quartzites.