Mylonite genesis: Columbia River fault zone, British Columbia

The Columbia River Fault of southeastern British Columbia separates the high grade gneisses of the Shuswap Metamorphic Complex, in the footwall, from a hanging wall composed of low grade Precambrian and Paleozoic rocks of the Northern Selkirk Mountains stratigraphic sequence, low grade Paleozoic and Mesozoic rocks of the Kootenay Arc assemblage, and plutonic and high grade metamorphic rocks of the Clachnacudainn Salient. The fault zone presently conforms to the shape of the Shuswap domes, dipping to the west on the western flank of the domes, to the east on the eastern flank, and horizontally near the culmination of the domes; this geometry may reflect late arching, post-dating major faulting.

The fault zone is marked by a zone of mylonites that may be as thick as one kilometer. A structural analysis of part of the mylonite zone has shown that the mylonitic strain resulted from penetrative inhomogeneous simple shearing. The sense of rotation displayed by both mesoscopic and microscopic kinematic indicators is compatible with southeastwardly directed translation of the hanging wall rocks with respect to the footwall.

The stable synkinematic mineral assemblages in mylonitic rocks of various compositions are characteristic of a lower greenschist facies metamorphism. Textural evidence suggests that these minerals formed in the presence of a fluid phase enriched in silica, iron, and possibly, potassium and calcium. Temperatures of deformation estimated from this data were used in conjunction with published deformation mechanism maps to estimate differential stress for the deformation on the order of one kilobar.