Deformation processes in a mid-crustal strike-slip shear zone: insights from the Archean Quetico Shear Zone, Superior Province, Canada

Abstract

The Quetico Shear Zone serves as an excellent analogue for active strike-slip shear zones at seismogenic depths. The Quetico Shear Zone is a right-lateral, strike-slip shear zone located at the boundary between the Wabigoon and Quetico subprovinces and has a strike length of at least 400 km. This research focuses on the eastern extent of the shear zone, north of Thunder Bay, and aims to constrain the kinematics, structures, conditions, and timing of deformation processes within the shear zone and adjacent paleo-earthquake surfaces. We found that Quetico Shear Zone deformation is characterized by increased mylonitization and brittle deformation with increasing proximity to the shear zone trace (within 500 m) where paleo-earthquake surfaces (i.e., pseudotachylite veins) were found. Mylonitization produces recrystallized quartz ribbons and a strong foliation unique to the Quetico Shear Zone (stronger than regional Quetico subprovince transpressional structures), particularly in granitic units. Non-granitic rock units within the core of the shear zone display pervasive brittle deformation with numerous faults. The recrystallized quartz grain sizes do not correlate with increased mylonitization and proximity to the shear zone. Recrystallized quartz grain sizes remained constant within error, with a median stress value of 80 MPa. The temperatures of quartz recrystallization also show no clear evolution with increasing proximity to the Quetico Shear Zone trace, with a median value of 487°C. The quartz recrystallization temperatures correlate to retrogressive temperatures from stable mineral assemblages, which are lower than estimated peak metamorphic temperatures from stable mineral assemblages (up to 700°C). The slip systems within the samples show an overprint from a higher-temperature prism <a> slip distal to the shear zone core to a lower-temperature rhomb <a> slip within the shear zone core. The calculated strain rates for the shear zone (3.9E-10 to 9.5E-12 s-1) are comparable to those in active shear zones. Apatite and titanite provided the best ages for deformation, mainly producing interpreted ages younger than the Quetico subprovince metamorphism. The interpreted Quetico Shear Zone deformation ages are 2618 to 2596 Ma. The exhumed Quetico Shear Zone appears to consist of an increasingly localized zone of deformation with proximity to the shear zone trace, though the shear zone records constant stress conditions across the shear zone core, indicating another weakening mechanism (e.g., fluid infiltration or fabric intensity) may be localizing deformation. The timing of this event is shortly after the Kenoran orogeny and may be related to the Sacred Heart Orogeny in the Minnesota River Valley.