Structural analysis of a ductile shear zone within the Marquette iron range, Upper Peninsula, Michigan

The Marquette Synclinorium is interpreted as an asymmetric rift-related basin; truncated stratigraphy on the southern margin and a rollover structure to the north provides evidence for this interpretation. The Palmer Gneiss (PG) marks the southern boundary of the trough. Mining operations have exposed the PG revealing mega-scale shear bands; foliation measurements indicate a reverse sense of shear. The formation of the Palmer Gneiss was previously interpreted as the alteration of the Archean gneiss. Whole rock chemical analysis suggests that the PG rocks are not granitic, but basaltic indicating a Proterozoic age. The folds measured in the adjacent iron formation consistently plunge gently towards the WNW, reflecting that they were formed under the same strain conditions as the shear zone. The NNE-SSW compressive direction is similar to those measured in previous studies of the area. The Penokean Orogeny caused closure of the Marquette trough that reactivated preexisting normal faults and resulted in a reverse dip-slip. Strain was concentrated in a portion of a metadiabase sill that was rotated into the shear zona causing alteration and shearing of the rock to form the PG.