Magnetic fabrics and anisotropy-controlled thrusting in the Kapuskasing Structural Zone, Canada

Abstract

Magnetic fabrics of upthrust Archean lower crustal rocks in the Kapuskasing Structural Zone (KSZ), northern Ontario, reveal previously unrecognized, consistently oriented mineral orientation–distributions. These define a schistosity and extension lineation that cuts across the heterogeneously strained contorted gneissic layering. The cryptic, magnetically determined schistosity is subparallel to diffuse but consistently oriented seismic reflectors in granulite and upper amphibolite facies gneisses. The seismic reflections were previously unexplained because their consistent orientation did not conform to lithological layering but we can now attribute them to preferred mineral orientations instead. The magnetic lineation lies within the plane of the seismic reflectors. The bounding thrust fault postdates the magnetic fabrics recorded by anisotropy of low-field magnetic susceptibility (AMS) that are primarily dictated by the subtle orientation distribution of silicate minerals and their magnetite inclusions. Anisotropy of anhysteretic remanence (AARM) isolates the component of magnetic fabric due to magnetite. This differs slightly from the `whole-rock' AMS signature because the magnetite is susceptible to late recrystallization or domain-rearrangement that changes its anisotropy in response to later phases of a noncoaxial stress history. Both AMS and AARM magnetic lineations are subhorizontal, trending 075° and 060°, respectively. These are incompatible with uplift kinematics and must reflect the ancient extension direction at depth. However, the similarity of the lineation trends and the strike of the fault suggest that the boundary thrust propagated upwards across the foliation using the path of least resistance afforded by the lineation.