Magnetic fabrics and anisotropy-controlled thrusting in the Kapuskasing structural zone, Canada

Magnetic fabrics of upthrust Archean lower crustal rocks in the Kapuskasing Structural Zone (KSZ), northern Ontario, reveal previously unrecognized, c onsistently oriented mineral orientation-distributions. These define a schi stosity and extension lineation that cuts across the heterogeneously strain ed contorted gneissic layering. The cryptic, magnetically determined schist osity is subparallel to diffuse but consistently oriented seismic reflector s in granulite and upper amphibolite facies gneisses. The seismic reflectio ns were previously unexplained because their consistent orientation did not conform to Lithological layering but we can now attribute them to preferre d mineral orientations instead. The magnetic lineation lies within the plan e of the seismic reflectors. The bounding thrust fault postdates the magnet ic fabrics recorded by anisotropy of low-field magnetic susceptibility (AMS ) that are primarily dictated by the subtle orientation distribution of sil icate minerals and their magnetite inclusions. Anisotropy of anhysteretic r emanence (AARM) isolates the component of magnetic fabric due to magnetite. This differs slightly from the 'whole-rock' AMS signature because the magn etite 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 degrees and 060 degrees, respectively These are incompatible with uplif t kinematics and must reflect the ancient extension direction at depth. How ever, the similarity of the lineation trends and the strike of the fault su ggest that the boundary thrust propagated upwards across the foliation usin g the path of least resistance afforded by the Lineation. (C) 1999 Elsevier Science B.V. All rights reserved.