POLYPHASE DEFORMATION AND MYLONITIC SHEAR IN THE GRAND FORKS COMPLEX, SOUTHEASTERN B.C

The Grand Forks metamorphic core complex in southeastern British Columbia exposes upper amphibolite to granulite facies Proterozoic metasediments of North American affinity in a broad structural dome bounded by Eocene detachment faults. There are at least five deformational episodes within the complex that postdate Late Cretaceous peak metamorphism at 84±3 Ma. The first episode (D1) is evidenced by a mylonitic, syn-to-post-metamorphic gneissic fabric developed throughout the complex. A less deformed “core zone,” as described in the southern extension of the GFC in northeastern Washington State by Cheney (1980) and others, is not present in the GFC in SE BC. The gneissic fabric is in turn deformed by two episodes of non-coaxial buckle folding (D2 and D3), which precede the emplacement of Paleocene biotite leucogranites dated at 56±1 Ma (U-Pb zircon) by Parrish (1992).

Leucogranite emplacement appears to have been controlled in part by the D2 axial planes, as evidenced on the map-scale by parallel dykes of undeformed leucogranite trending 340-160 through all structural levels of the core complex. Crenulations and mineral lineations linked to the dominant D3 buckle folding event have been refolded by a late D4 deformation, characterized by broad, gentle folds that also appear to weakly fold the Paleocene leucogranite. Post-51 Ma brittle deformation related to the Eocene Kettle River and Granby detachment faults (D5) crosscuts the earlier deformational fabrics and Paleocene leucogranite, but timing relationships between D4 folding and post-51 Ma faulting are unknown. Argon fission track data shows overlapping (2σ) ages between the core complex and adjacent hanging walls at a mean age of 34.5±1.9 Ma, suggesting there has been no subsequent tectonism in the area.

On the eastern margin of the core complex, a previously undocumented mylonitic shear zone in the hanging wall to the Kettle River fault is characterized by strong top-to-the east shear in granitoids of the Jurassic Nelson suite and metasediments of the Pennsylvanian-Permian Mollie Creek assemblage. This shear zone does not extend south into the Jurassic Rossland Group and Eocene Sanpoil Volcanics, consistent with southward-increasing displacement on the late Kettle River fault that juxtaposes higher stratigraphic levels against the Grand Forks Complex.