Date of Award

December 2018

Degree Type

Thesis

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dyanna M Czeck

Committee Members

Lindsay J McHenry, Barry I Cameron

Keywords

Archean, Rainy Lake, Shear Zones, Structural Geology, Superior Province

Abstract

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 20 km long and 1-2 km 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 accommodated the bulk of the deformation within the largely competent sill. The orientations of the networks vary along the length of the sill, apparently related to strain variations, as the inferred shortening directions of the shear zone networks matches those assumed from regional foliations and calculated from previous work on deformed dikes and veins. At all locations, both steeply dipping dextral and sinistral sets of shear zones formed, presumably simultaneously as evidenced by mutual cross-cutting relationships. The shear zones are curviplanar and dip more shallowly near some of their intersections. The style of deformation varies within the gabbro based on grain size. Fine grained lithologies are deformed via a pervasive foliation. Coarser grained lithologies generally contain discrete (mm-cm scale) shear zones, often without a pervasive foliation. There is no relationship between mineralogy and shear zone or foliation formation. Microstructural analysis of shear zone bearing samples indicated that dislocation creep served as the primary deformation mechanism throughout the GPS, but showed no pattern regionally, or with grain size. 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.

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