QUANTIFYING STRAIN IN VARIOUS LITHOLOGIES AND DOCUMENTING A STRAIN GRADIENT WITHIN THE SEINE RIVER BASIN, NORTHWESTERN ONTARIO, CANADA

The deformed Seine River conglomerates are located at the boundary of the Quetico and Wabigoon subprovinces in the Superior Province. They were deformed in a 2.7 Ga granite-greenstone forming orogeny. The structural fabrics indicate that deformation occurred under conditions of quasi-homogeneous transpression resulting in vertical foliations and variable oblique lineation orientations. Areas within and surrounding the conglomerates are influenced by small and large scale shear zones. While all of the conglomerates are deformed to some degree, the amount of strain varies significantly across the region.

The Seine contains clasts of varying composition, with granitoid, felsic volcanic and mafic volcanic as the three main clast lithologies. We conducted strain analysis on these clast types and matrix to determine the range of Lode's parameter (n) and strain magnitude (e_s). For granitoid clasts, n ranges from -0.69 to 0.94 with a median n of 0.218 and e_s ranges from 0.09 to 1.79 with a median e_s of 0.596. For volcanic clasts, n ranges from -0.91 to 0.90 with a median n of 0.355 and e_s ranges from 0.35 to 2.54 with a median e_s of 1.285. For the matrix, n ranges from -0.32 to 0.88 with a median n of 0.261 and e_s ranges from 0.33 to 2.50 with a median e_s of 1.234. Approximately 75% of the outcrops have n values between 0-1, indicating some degree of flattening. In general, within an outcrop the matrix has undergone more apparent flattening than the clasts. We believe this observation may be due to one or both of these reasons: 1) preferential volume loss in the matrix, or 2) the tendency of competent clasts in a viscous matrix to record a more prolate strain than was imposed.

The strain gradient and localization of shear zones within the Seine may have occurred for many reasons: 1) bulk clast composition in the deformed conglomerates controls the localization of strain at a larger scale; 2) surrounding lithological controls due to the emplacement of plutons or the preexistence of rocks of various competence adjacent to the conglomerate control the localization of strain; 3) faults that formed syndepositionally with the conglomerates control the later localization of ductile strain. Initial results suggest that the second two reasons are more likely because analysis indicates that the bulk conglomerate lithology does not notably change across the strain gradient.