Structure and metamorphism along the Burntside Lake Shear Zone near Ely, Minnesota

University of Minnesota M.S. thesis. August 2013. Major: Geological Sciences. Advisors: John Goodge, Vicki Hansen. 1 computer file (PDF); vi, 92 pages + 1 map (PDF).Archean granite-greenstone terrains (GGTs) occur in the stable interior cratons of Australia, India, South Africa, and North America. GGTs represent the best remaining evidence of the tectonic regime that reigned in the Archean. For this reason, GGTs continue to be thoroughly studied to better understand the tectonic evolution of Archean cratons. GGTs consist of ovoid granitoid bodies surrounded by deformed, metamorphosed volcanic sequences of intermediate to mafic composition. There are two principal opposing hypotheses on the genesis of GGTs: 1) sagduction/diapirism, whereby density instabilities lead to the subsiding of greenstone layers, and the rising of granitoid bodies; and 2) modern-style island-arc accretionary tectonics. The island arc-accretion hypothesis requires a rheologically strong, brittle lithosphere in order for subduction to occur. In contrast, the sagduction hypothesis requires a rheologically ductile weak lithosphere to allow for the diapiric mechanism of GGT genesis. It is assumed that the Archean geotherm was much steeper compared to the modern. A higher geotherm would have affected the lithosphere¡¦s rheology, making it more ductile than present lithosphere. If this scenario holds true, the ability of the lithosphere to act in a brittle manner, thus allowing subduction, becomes questionable. Many workers have classically interpreted the Superior Province of northern Minnesota and Canada as the prime example of modern-style accretionary tectonics occurring during the Archean. The northeast-trending Burntside Lake shear zone (BLSZ) lies at the boundary of the Archean Vermilion Granitic Complex and The Newton Lake Formation. The position of the BLSZ between a granitoid body and a greenstone belt allows for the study of the interaction between these two units, offering an opportunity to study how Archean tectonic processes may have occurred. Through examination of tectonic fabrics in the field and microstructures in the lab, this shear zone appears to be a vertical zone of dip-slip deformation. The vertical, dip-slip, north-side-up orientation of this zone is also supported by geothermometric calculations indicating a thermal gradient across the zone of relatively warmer temperatures to the north and cooler conditions to the south of the shear zone. Foliation throughout the BLSZ strikes generally NE-SW, at an angle slightly oblique to the topographic expression of the zone itself, and has a nearly vertical dip. Lineations, primarily elongation lineations, throughout the area are steep, mostly plunging 80-90,,a. On the thin section scale, foliations vary from planar to anastomosing. Foliation is defined primarily by the alignment of biotite, chlorite and hornblende, and elongate needles of these minerals define lineations. S-C¡¦ fabrics composed of biotite and chlorite were used to determine sense of shear, along with sigma grains when applicable. These microfabrics are poorly preserved and in some thin sections completely ambiguous, resulting in few samples having definitive petrofabrics with which to interpret shear sense. Throughout this paper, confidence in shear sense is assigned a number from one to five, with five being the most confident and one being the least. North-side-up shear sense dominates rocks in the field area with three thin section samples giving evidence with a confidence rating of 5, one at 3, and one at 2. Two samples indicated south-side-up kinematics with a confidence level of 1 and 3. The evidence from observations in the field (changes in grain size, metamorphic grade) and microstructural analysis supports the idea of a vertical zone of deformation with dominant sense of north-side-up displacement in addition to local south-side-up displacement. North-side-up displacement is supported by geothermometry completed on garnet-biotite pairs, chlorite compositions, and hornblende-plagioclase pairs from metamorphic rocks. Geothermometry indicates amphibolite-facies metamorphism north of the BLSZ and lower greenschist-facies metamorphism south of the BLSZ. In the study area, the BLSZ does not show evidence of strike-slip displacement, as predicted by plate tectonic hypotheses. The vertical motion recorded by fabric orientations, fabric kinematics and thermometry along this portion of the BLSZ fits the hypothesis of sagduction/diapirism. Further study will allow for a greater understanding of the complex history recorded in the rocks defining the Burntside Lake shear zone. Specifically, study could be done to further constrain pressure/temperature conditions, kinematics, and the timing between the conflicting senses of shear apparent in some of the samples utilized in this study. Last, a comparison of the many shear zones in the area will help to better understand timing and kinematic relations between units of the broader Superior Province