Jordan Baird Honours Thesis Abstract

Deformation in the Animikie Group near Thunder Bay is characterized by intense, localized fold-and-thrust belt deformation within units that are otherwise relatively undeformed and flat-lying.  This deformation is interpreted to be the result of regional stress during Proterozoic tectonism.  Stereographic analyses of structural measurements and outcrop observations are used to determine beta-axes (mean fold axes) for fold populations, as well as to determine structural relationships within the region.  Two main fold populations are observable in the data.  The primary population exhibits a north-south trending beta-axis, indicating east-west compressive stress.  The secondary population exhibits an east-west trending beta-axis, indicating either north-south compressive stress or the presence of lateral ramps.  Slickenlines are also observed to trend north-south and east-west, depending on outcrop location.  East-west trending slickenlines tend to be in areas of more intense folding, indicating they may be older than the north-south trending slickenlines.   Older slickenlines may have been destroyed when fault surfaces were reactivated in areas of less intense folding replaced by the slickenlines associated with the most recent deformation.  Additional observations include the presence of fold-hinge breccia associated with non-cylindrical folding, which may indicate lateral ramp formation, as well as the presence of a possible cleavage duplex structure, which may indicate repetitive east-verging thrusting.
These observations suggest that the deformation of the Animikie Group began with regional east-west compression resulting in the development of an east-verging thrust belt.  Stresses associated with this thrust belt are interpreted to have resulted in fault-bend folding on frontal and lateral ramps in the region.  Most of these folds are included in the population with north-south trending axes, although lateral ramps on this thrust sheet likely resulted in the development of folds with east-west trending axes, as well as non-cylindrical folds and fold-hinge breccia.  East-west trending slickenlines on thrust faults were also developed during this compressive deformation.  Subsequently, north-south compression may have overprinted this deformation, developing folds with east-west trending axes and north-south trending slickenlines on fault surfaces, though not in areas of intense folding (indicated by preservation of east-west trending slickenlines).  Finally, north-south directed extensional deformation deformed the area, resulting in normal faulting and the reactivation of fault surfaces in areas not intensely folded.  This caused the development of north-south trending slickenlines on new and reactivated fault surfaces.  A specific tectonic history for the Animikie Group is suggested based on these observations and interpretations.  It is proposed here that primary east-verging thrusting was associated with the Trans-Hudson orogeny in the Paleoproterozoic.  Following this, there may or may not have been a secondary compressional phase due to the Yavapai-Mazatzal orogenies during the early to middle Proterozoic; the effects of these orogenies remain unclear.  Deformation likely culminated in the late Proterozoic with north-south extension related to the Mid-Continent Rift.