Structure and deformation across the Quesnellia-Omineca terrane boundary, Mt. Perseus area, east-central British Columbia
Creators: name: Elsby, Darren C.
Publisher: name: University of British Columbia
Year: 2010

description: Detailed structural mapping near Mt. Perseus, British Columbia, provides an overview of the nature of deformation across a portion of the Quesnellia-Omineca terrance boundary. Rocks within the Omineca Belt are comprised of the Hadrym'an to mid-Paleozoic Snowshoe Group. These rocks are structurally overlain by and act as basement to accreted rocks of the Intermontane Belt (Quesnellia): the Upper Paleozoic Slide Mountain Group (Antler Formation), Upper Triassic Black Phyllite (unnamed), and Jurassic volcanic rocks of Takla Group equivalence. Within the Snowshoe Group, four phases of regionally significant deformation have been established. Both basement and cover have common phases of deformation wherein the first phase of deformation present within the cover sequence is equivalent to the second phase within the basement. In general, deformation within the cover is less well developed with respect to the basement. Earliest structures, only observed within the Snowshoe Group are east-verging rootless isoclinal folds accompanied by a transposed foliation of a regional nature. Associated with this event is the intrusion of a large tabular granitic body, later metamorphosed into the Mt. Perseus Gneiss. Second phase structures are easterly verging and comprise large recumbent nappe structures. Third phase westerly verging folds dip moderately to the northeast. It is these large scale structures which control the present regional map pattern and local configuration of the Omineca-Quesnellia boundary, which in this study, is manifest in the Mt. Perseus antiformal culmination. Small scale crenulations and easterly verging buckle folds comprise the fourth deformational phase and do not appreciably affect earlier geometries. Second phase deformation marks the obduction of the easterly converging Quesnellia accretionary package onto the Omineca terrane. This tectonic contact is flanked by narrow longitudinal ductile shear zones containing mylonites, which in Snowshoe rocks are often associated with isolated fault bounded slivers of oceanic cover rock (ophiolite). These tectonic slivers are thought to be related to geometry resulting from the eastward subduction of oceanic Quesnellia rocks beneath the Omineca craton during the third deformational phase. The development of the late crenulation cleavage is likely a consequence of late eastward thrusting of early Jurassic volcanics during the later deformation stages of the underlying phyllites. Mineral assemblages describe a Barrovian metamorphic sequence which ranges from the middle to upper greenschist facies in cover rocks to the lower amphibolite in the Snowshoe basement. The earliest recorded metamorphism is associated with phase 1 deformation but details regarding this event remain ambiguous as most textures have been destroyed by successive metamorphism. Microscopic textures indicate that the peak of metamorphism is synchronous with phase 2 deformation followed by a reduction to the middle greenschist facies during the third deformational phase. Both obduction and subduction processes and their associated deformation and metamorphism were most likely the result of mid Mesozoic tectonics related to the Columbian Orogeny.