ANALYZING DUCTILE SHEAR ZONE NETWORK GEOMETRIES IN THE GRASSY PORTAGE SILL, RAINY LAKE REGION, NORTHWESTERN ONTARIO, CANADA

The Grassy Portage Sill (GPS) is a ~2.7 Ga metagabbroic sill located in the Rainy Lake region of northwestern Ontario. The Rainy Lake region is located in the Superior Province between the metavolcanic Wabigoon subprovince to the north and the metasedimentary Quetico subprovince to the south. Two regional faults bound the region and intersect to the east, forming a wedge which defines the Rainy Lake zone. This area was regionally deformed due to oblique transpression, resulting from the Kenoran Orogeny (~2.7 Ga). The GPS is approximately 20km long and 1-2km wide, and has undergone heterogeneous strain along its length. This strain variation is a function of the competence contrast between the GPS, the gneissic Rice Bay Dome to the west, and the metavolcanic and metasedimentary units between the two. The GPS has a higher competence than the adjacent metavolcanic and metasedimentary units, but all have a lower competence than the Rice Bay Dome. Within the GPS, anastomosing ductile shear zone networks apparently accommodated the bulk of the deformation within the largely competent sill. The geometries of the networks vary along the length of the sill, apparently related to strain variations. At all locations, both steeply dipping dextral and sinistral sets of shear zones formed, presumably simultaneously. The shear zones are curviplanar and dip more shallowly near some of their intersections. At the lowest strain sites, the gabbro has a pervasive foliation , but few, if any, shear zones. At low-medium strain sites, the sinistral and dextral shear zone sets have fairly consistent orientations, approximately 65-75° apart from one another. As strain increases, the orientations of both sets become increasingly more variable and the average angle between the two sets decreases. We hypothesize that the shear zone sets formed at relatively high angle to one another and rotated to a lower relative angle with increasing strain. The newer strands in the higher strained sites formed at high angle, causing the orientations of each shear zone set to become more diffuse at higher strain.