Metamorphism and Normal Faulting in the Grand Forks Complex, Southeastern British Columbia

The Grand Forks Complex is a fault-bounded metamorphic core complex situated in the extensional Shuswap domain of the Omineca Crystalline Belt, southeastern British Columbia. The eastern edge of this complex is bounded by the Kettle River Fault (KRF), an Eocene east-dipping normal fault. Where observed to date, the KRF brittlely deforms both hanging wall and footwall lithologies, with cataclastic metasediments found locally up to 300m from the mapped fault trace.

The footwall to the KRF consists of a Proterozoic sedimentary package of North American affinity, metamorphosed to upper-amphibolite to granulite facies, whereas the hanging wall consists of Paleozoic to Mesozoic metasediments of exotic Quesnel affinity, metamorphosed to greenschist to lower-amphibolite facies. The presence of garnet and more rarely orthopyroxene in mafic gneisses in the footwall indicates upper-amphibolite to locally granulite facies conditions. Migmatitic samples from the basal sillimanite gneiss display biotite+sillimanite+cordierite+K-feldspar+plagioclase+quartz±garnet±spinel assemblages in which altered cordierite forms coronas around sillimanite, with spinel inclusions contained within the cordierite. The textures suggest decompression and reequilibration of the core complex following peak metamorphism.

Hanging-wall metapelitic hornfelses near the contact with Jurassic-aged intrusions (Nelson suite) contain low pressure (2-4 kbar), amphibolite-facies biotite+cordierite+plagioclase+quartz±sillimanite ±garnet±muscovite±andalusite? assemblages. In localized areas, there is extensive migmatization of semipelites up to 600m from intrusive contacts. Further from the intrusions, the presence of coexisting hornblende and actinolite in metavolcanic rocks indicates transitional greenschist-amphibolite facies conditions. These results bear on a number of larger Cordilleran issues, including the importance of normal faulting in orogen collapse and the potential for channel flow and vertical tectonics in core complex formation.