COEVAL DUCTILE AND SEISMIC BEHAVIOR IN THE PENOKEAN TWELVEFOOT FALLS SHEAR ZONE, MARINETTE COUNTY, WISCONSIN

The Twelvefoot Falls Shear Zone in NE Wisconsin is a late Penokean mylonite zone that transects rocks of the Wisconsin Magmatic Terranes south of the Niagara Fault. The shear zone is well exposed along the Pike River, where it cuts through the Twelvefoot Falls Quartz Diorite (1889 +/- 6 Ma), which lies on the southern flank of the Dunbar Gneiss Dome, a younger (1862 +/- 5 Ma) post-collisional intrusion.

Thin sections from 5 sites along the shear zone reveal a complex deformational history. The rocks have a steep, NW-striking foliation and weak down-dip lineation defined by preferred orientations of hornblende porphyroclasts and planar quartz-rich domains. The quartz is typically fine grained (<0.1 mm), with undulose extinction, irregular grain boundaries, and local ‘core and mantle’ structure, recording plastic deformation, dynamic recrystallization and partial annealing. Hornblende grains are commonly boudinaged. Narrow mylonitic to ultramylonitic zones cut the foliation at low angles. In these zones, both hornblende and quartz grains are finer than in the rest of the rock, but their microstructural character is similar: the hornblende is fragmented quasi-brittlely, while the quartz forms high-strain bands. This suggests that the foliation and mylonites developed under similar conditions, namely between 300°-600°C (thresholds for quartz and hornblende ductility), consistent with the metamorphic peak around the Dunbar Dome.

At Eightfoot and Eighteenfoot Falls, dark discordant veins 0.3-0.5 cm wide and 5-15 cm long cut the foliation. In thin section, these show a mesh of fine hornblende crystals with high aspect ratio but no preferred orientation, in a non-crystalline matrix that is dark in plane light. We interpret the veins as devitrified pseudotachylyte. Significantly, the pseudotachylyte material is in places cut by, and drawn into, the mylonitic bands, indicating that seismic failure occurred at least once while the rocks were at temperatures where plastic deformation prevailed. Such mutually cross-cutting relationships between mylonites and pseudotachylytes have been reported from several sites – mainly convergent tectonic settings -- around the world, and are interpreted as records of large earthquake ruptures that propagated downward from the brittle upper crust into the top of the ductile regime.