Megan Landman HBSc Thesis Abstract

New roadside outcrops along the Trans-Canada Highway 11/17 near Pass Lake, ON, expose the basal unconformity between Archean basement rock and the Proterozoic Gunflint Formation. Shear fractures, joints, and the Blende Lake fault damage zone seen in these two outcrops record the brittle deformation history of the area before and after the Gunflint Formation was deposited.  The unconformity represents a temporal gap in the stratigraphic sequence of at least 800 million years.  The Archean rock belongs to the Wawa subprovince of the Superior province, in close proximity to the Wawa-Quetico subprovince boundary.  Structural measurements, stereographic projections, and qualitative observations have allowed deeper insight and analysis of how structural controls have changed over an approximately 2.7 billion year-old geological history.  The Archean basement unit underneath the unconformity is a coarse-grained amphibolite that contains accessory epidote and biotite, and is homogeneous throughout the length of both high-standing outcrops.  This is interpreted to be a mafic pluton that has undergone amphibolite-facies metamorphism.  The amphibolite records a scatter in orientation of joints and shear fractures, but some trends align well with data from Mackenzie River granite plutons, including an overall east-northeast and west-southwest strike.  Later Proterozoic features, including the Blende Lake fault, have a common strike of east-northeast, which aligns with the orientation of the 1.1 Ga Mid-Continent Rift in Thunder Bay.  This similarity is further reflected by the Blende Lake fault being oriented subparallel to silver veins related to the Mid-Continent Rift.  Similarities between orientations of brittle structures in the amphibolite and Gunflint Formation suggest that the Mid-Continent Rift in Thunder Bay may have reactivated some Archean-aged orogenic-related faults and shear fractures.  Minor folding in the Gunflint Formation truncated by the Blende Lake fault, as well as reverse reactivation along the plane, may be evidence of compression during the later stage of the Mid-Continent Rift.