KINEMATIC EVOLUTION OF THE GOD'S LAKE NARROWS SHEAR ZONE IN THE NORTHWESTERN SUPERIOR PROVINCE, CANADA: EVIDENCE FOR SLIP PARTITIONING AND NON-STEADY-STATE DEFORMATION

The God's Lake Narrows shear zone forms the southern margin of the Archean God's Lake greenstone belt in the NW Superior Province. Supracrustal rocks in the belt consist of, from north to south, the older Hayes River Group (mostly pillowed basalt, ca. 2830 Ma), the unconformably overlying Oxford Lake Group (conglomerate and sandstone, <ca. 2711 Ma), and a third shear zone-bounded package (mafic to felsic volcanic and sedimentary rocks, ca. 2720 Ma, referred to as VSR below).

Two major generations (G1 and G2) of shear zone deformation are distinguished. The G1 deformation is associated with a ductile shear zone centred between the VSR and the Oxford Lake Group. In most parts of the shear zone, mylonitic foliations (S-surfaces and C-surfaces) strike east-southeasterly and dip steeply to the south-southwest. Both the stretching lineations on S-surfaces (Ls) and the striations on C-surfaces (Lc) plunge steeply to down dip. Field observations and microscopic work indicate S-over-N thrust sense of shear, which is consistent with southward increase in metamorphic grade.

The G1 shear zone is folded as a result of deformation associated with the G2 shear zone, which is centred between the VSR and plutonic rocks to the south. Near the northern edge of the shear zone, the G1 foliations described above are folded. In the shear zone, they are fully transposed into new mylonitic foliations (S-surfaces and C-surfaces). The foliations strike east-southeasterly and dip steeply to the south-southwest. Striations on C-surfaces vary continuously from subhorizontal to moderately plunging, providing a good example of slip partitioning within a single shear zone. Well-developed shear sense indicators indicate dominant dextral transcurrent movement with S-over-N dip-slip component. Dextral sense of shear is also evident from the regional map pattern.

Two possible scenarios for the structural evolution of the area are being considered. In scenario 1, the G1 thrust formed before and was folded during G2 dextral transpressional shearing. In scenario 2, G1 and G2 were both kinematically related to partitioning of dextral oblique convergence. At the earlier stage of the deformation, the compressional component was accommodated by thrusting, and at the later stage it was accommodated by folding and the G1 thrust was folded.