THE ROLE OF HETEROGENEITY IN CONTROLLING PATTERNS OF STRIKE-SLIP DEFORMATION, VERMILION GRANITIC COMPLEX, NORTHERN MINNESOTA

The Vermilion fault, Haley fault, and Burntside Lake fault compose the local boundary in northeastern Minnesota between the Archean Quetico Metasedimentary subprovince to the north and the Wawa Volcano-plutonic subprovince to the south. The Wakemup Bay pluton and Burntside trondhjemite are small, elliptical, Archean granites, which lie immediately adjacent to this fault system. These plutons acted as rigid bodies (strength heterogeneities) and locally controlled the orientation and style of faulting along this tectonic boundary. The Wakemup Bay pluton, a biotite-tonalite that intrudes biotite schist, is contained within a lozenge shaped fault-bounded block, which consists of the Vermilion fault to the north, and the Haley fault to the south. Gravity analyses indicate that the Wakemup Bay pluton is a thin, 0.5 km slab, with two root zones on its southern end. The Haley fault lies immediately adjacent to the thickest of the southern root zones, and the fault lozenge geometry is hypothesized to develop in response to the Haley fault curving around the thickest, and therefore the strongest, section of the pluton. The Burntside trondhjemite intrudes biotite schist to the north of the Burntside Lake fault, which is presently structurally continuous with the Vermilion fault to the west. Gravity inversions indicate that the pluton is 2-3 km thick, and that the southern margin of the pluton is juxtaposed against the Burntside Lake fault. As the Burntside Lake fault is oriented parallel to the south boundary of the Burntside pluton, this pluton presumably locally controlled the fault orientation. A two-dimensional forward gravity model, performed on the Quetico/Wawa subprovince boundary in the vicinity of the Burntside pluton, constrains the dip of the Burntside Lake fault to be 70 degrees N to vertical. This gravity data is not consistent with previous interpretations of south-side down movement on an originally continuous Burntside Lake-Haley normal fault, prior to strike-slip motion on the Vermilion fault system. Our alternative explanation is that the present juxtaposition of metamorphic grade across the Burntside Lake, Haley, and Vermilion faults, was a result of differential exhumation of the Quetico metasedimentary subprovince along these steeply-dipping strike-slip faults.