STRAIN HETEROGENEITY IN AN EARLY PROTEROZOIC METAVOLCANIC COMPLEX, CENTRAL WISCONSIN

The Penokean orogeny (1.90-1.83 Ma) is among the earliest mountain-building events for which all the components of a ‘modern’ collisional tectonic cycle have been well documented. An early Proterozoic fold-and-thrust belt in north central Minnesota represents a passive margin sequence that was telescoped by collision with coeval magmatic arcs (Pembine-Wausau terrane) and older continental fragments (Marshfield terrane) in what is now northern and central Wisconsin.

The Dells of the Eau Claire River east of Wausau, Wisconsin provide an exceptional exposure of a lower Proterozoic (ca. 1840 Ma) metasedimentary and metavolcanic sequence within the Pembine-Wausau terrane (Penokean arc complex). The Eau Claire gorge occurs at a critical location for tectonic understanding of the region, lying just north of the boundary between the Pembine-Wausau and Marshfield terranes and just west of the Wolf River Batholith, a 1.47 Ma anorogenic granitoid pluton. The protoliths of the rocks at Eau Claire Dells appear to have been primarily pelites and felsic tuffs and/or flows. Earlier studies have resulted in two contrasting interpretations of the texture and structure of these rocks: 1) that they represent a foliated but otherwise undisturbed metavolcanic/metasedimentary sequence; and 2) that their fabric is mylonitic and that the rocks lie along a major shear zone, perhaps an important tectonic boundary.

Based on detailed petrographic and microstructural observations, we suggest that of both these interpretations are partly correct. Locally, primary volcanic textures including quartz phenocrysts and apparent lapilli, are preserved in these rocks. Within meters, however, these nearly pristine textures give way to mylonitic microstructures including feldspar porphyroclasts and recrystallized quartz ribbons (in the metavolcanic rocks) and boudinaged and rotated garnets (in the metapelites). This kind of strain localization is common in many ductile fault zones and indicates the importance of strain softening mechanisms in the deformation process.