CZECK, Dyanna M. and HUDLESTON, Peter J., Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, firstname.lastname@example.org
The Superior Province consists of approximately east-west trending subprovinces defined by lithological differences, metamorphic grade, and structural boundaries (Card & Ciesielski 1986). This study concentrates on the boundary between the Quetico metasedimentary subprovince and the Wabigoon metavolcanic subprovince. The Seine River Metasedimentary Group, including the Seine River conglomerates, are distinctive rocks found within a wedge along this boundary.
Structural work along the Quetico and Wabigoon describes subvertical foliation overprinting large lithological folds (Poulsen 1986, Tabor & Hudleston 1991). These structures indicate a strain history consisting of an early recumbent nappe deformation followed by a more ubiquitous overprinting of a subvertical flattening fabric and shear zones attributed to dextral transpression. Strain is pervasive throughout the boundary, but also localized into more discrete shear zones including two main, connecting shear zones coincident with the Rainy Lake-Seine River and Quetico Faults. These large shear zones are interconnected with smaller shear zones forming an anastomosing pattern. The Seine River conglomerates are located in a wedge between these two shear zones west of their intersection. Poulsen (1986) interpreted the Seine River conglomerates to have formed syn-kinematically during transpression, possibly in structures similar to pull-apart basins.
This preliminary structural work made it clear that the Wabigoon- Quetico subprovince boundary is not merely a discrete suture between two tectonic microplates, but is instead a complex network of shear zones which together accommodated oblique collision. The goal of this study is to describe how this complex shear zone geometry is reflected in spatially variable strain magnitudes and structural fabrics.
Foliations in the field show a consistent regional orientation with strike of approximately 080 and subvertical dips. This strike is consistent with the dextral transpression model. One exception to this is in the area of Shoal Lake where the strike changes to approximately 045, creating an overall S-like geometry with a map pattern analogous to a dextral shear sense indicator. The same shear sense is observed at a smaller scale within the conglomerate clasts that often form sigma and delta dextral shear sense indicators on the sub-horizontal plane.
Mineral lineations in the field are predominantly defined by chlorite and amphiboles. General transpression theory predicts either vertical or horizontal lineations (Fossen & Tikoff 1993). However, lineation orientations in the field vary remarkably within the foliation plane. Typically, the Seine Group lineations are spatially similar on the scale of 0.5-1 km. Therefore, the lineation variations are not caused by small-scale phenomena such as clast interaction, but by some regional phenomenon. Rather than reflecting different bulk kinematic conditions along the boundary (such as partitioned triclinic transpression), the lineation variations may reflect anastomosing shear zone patterns and variable pressure gradients within the rock.
Taken together, foliation and lineation match the variations in the finite strain ellipsoid orientation. The foliation pole aligns with the minimum stretching direction, and the lineations parallel the maximum stretching direction.
The Seine River conglomerates are useful strain recorders. Qualitative observations and quantitative strain analyses indicate heterogeneous strain distributions within the Seine River conglomerates. Analysis confirms a flattening style strain that could only be a result of a three-dimensional deformational phenomenon such as transpression rather than the two-dimensional strain produced by wrench or thrust movements.
Results to date show that the longest principal axis corresponds to the mineral lineation measured in the field and the shortest principal axis corresponds well with the foliation pole. This supports the idea that structural fabric measured in the field may be used to represent the orientation of the finite strain ellipsoid.
The variations in strain ellipsoid orientations and magnitudes could be used to argue for multiple deformation events or complex and variable kinematic boundary conditions. However, the geometry of the anastomosing shear zone pattern can be used to explain the variations in the orientation of the strain ellipsoid and the heterogeneous strain magnitudes within the Seine Group within a simple dextral transpressional setting.
Poles to foliation
Card, K. D. & Ciesielski, A. 1986. DNAG Subdivisions of the Superior
Province of the Canadian Shield. Geoscience Canada 13, 5-13.
Fossen, H. & Tikoff, B. 1993. The deformation matrix for simultaneous simple shearing, pure shearing and volume change, and its application to transpression- transtension tectonics. Journal of Structural Geology 15, 413-422.
Poulsen, K. H. 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: Tectonic evolution of greenstone belts Technical Report (edited by de Wit, M. J. & Ashwal, L. D.) 86-10. Houston TX, Lunar and planetary Inst., 177-179.
Tabor, J. R. & Hudleston, P. J. 1991. Deformation at an Archean subprovince boundary, northern Minnesota. Canadian Journal of Earth Sciences 28, 292-307.