Paper No. 5
Presentation Time: 10:00 AM
STRUCTURAL AND KINEMATIC ANALYSIS OF THE SHAGAWA LAKE SHEAR ZONE AND SNOWBANK LAKE STOCK, VERMILION DISTRICT, NE MINNESOTA
The Vermillion District, southern Superior Province, is a Neoarchean (2.8-2.5 Ga) granite-greenstone terrane dominated by a series of NE-striking subvertical shear zones with ovoid granitic bodies scattered throughout. Shear zones have been interpreted as both primarily dextral strike-slip and as dominantly oblique to dip-slip. Differing interpretations invoke different assumptions about displacement direction during non-coaxial shear. Displacement direction is genetically tied to foliation and elongation lineation orientation. We conducted a structural and kinematic analysis of the Shagawa Lake shear zone and adjacent Snowbank Lake stock, NE Minnesota, in three phases: 1) analysis of regional tectonic fabrics through LiDAR altimetry data; 2) structural analysis of outcrop-scale structures and detailed field mapping; and 3) analysis of shear-sense indicators through kinematic analysis of thinsections. The Shagawa Lake shear zone contains a regional subvertical metamorphic foliation with a mean strike of 065 varying locally from 065 to 100. Near the Snowbank Lake stock foliation is roughly parallel to the stock boundary. We recognize two types of elongation lineation, ridge-in-groove striations on C-foliation surfaces (Lc) and stretching lineations on S-surfaces (Ls). The Shagawa Lake shear zone hosts unimodal Lc and Ls ranging from dominantly steep plunge to moderate plunge. Asymmetric fabrics occur in foliation-perpendicular, lineation-parallel planes and symmetric fabrics occur in foliation-perpendicular, lineation-perpendicular planes, consistent with lineation-parallel non-coaxial shear displacement. Therefore, in high pitch domains, displacement was vertical to oblique, and microstructures record both north-side-up and south-side-up displacement in different samples. Samples with oblique lineation record an apparent dextral strike-slip shear-sense despite varying lineation orientation. Our data indicate the Shagawa Lake shear zone experienced both N-side-up and S-side-up dip- to oblique-slip with relatively minor apparent dextral strike-slip and does not record significant unidirectional strike-slip displacement.