The Structural geology and strain history of the northern Selwyn Range, Rocky Mountains near Valemont, British Columbia
Creators: name: McDonough, Michael R.
Publisher: name: University of Calgary
Year: 1989
Subjects: subject: QE 621.5 C36 M166 1989
subject: Geology, Structural
subject: Geology - Rocky Mountains, Canadian (B.C. and Alta.)
subject: Geology - British Columbia - Valemount region
subject: Geology - British Columbia - Selwyn Range

description: Bibliography: p. 257-270.
description: Three informal units of the Late Proterozoic Miette Group in the northern Selwyn Range of the Rocky Mountain belt, the lower, middle, and upper Miette, are conformable. New stratigraphic data indicate that the middle Miette is "glued" to the lower Miette. A distinctive marker unit near the middle of the middle Miette, the Old Fort Point member is a regional stratigraphic marker. Numerous local stratigraphic markers have served to clarify the structural geometry of the study area. The Valemount strain zone (VSZ), a narrow zone of high orogen-parallel (OR) strain in pebble conglomerate of the Late Proterozoic Miette Group, is the footwall expression of a thrust fault on the west side of the northern Selwyn Range, marking the eastern limit of a wide zone of OP fabrics distributed through the Omineca crystalline and western Rocky Mountain belts of the southeastern Canadian Cordillera. Kinematic indicators from the VSZ and the adjacent Bear Foot thrust zone indicate that both thrust and dextral displacement are associated with folding and thrust motion in the western Rocky Mountains, thereby linking the southern Rocky Mountain belt to the Omineca belt by an oblique-slip thrust regime that is tectonically unrelated to the Southern Rocky Mountain Trench. Transverse shortening of thrust sheets and subsequent distribution of OP shear are invoked to explain the parallelism of stretching lineations and fold axes. Kinematic and finite strain data, and the thrust belt geometry of the VSZ, suggest OP lineations are a product of a large amount of transverse shortening during slightly oblique A-type subduction. Thus, OP lineations are not representative of relative plate motions between North America and accreted terranes, but probably are a function of footwall buttressing of thrust sheets, a mechanism which may be widely applicable to the internal zones of collisional orogens. Numerical finite strain modelling of deformed and undeformed Miette conglomerate suggests that large OP strains may be related to parallelism of strain and initial fabric ellipses, which may be responsible for a grain size sensitive deformation mechanism operative in XZ planes in conglomerate. Pressure solution is also proposed as a possible mechanism to explain the grain size partitioning of OP strain in the Valemount strain zone. The physical conditions of deformation in the Valemount strain zone were estimated to be about 450°C and 4.5 kb (450 MPa) by garnet-chlorite and garnet-biotite geothermometry, and by projection of isobars from the hanging wall of the Bear Foot thrust. Two garnet zones were recognized: a biotite-absent spessartine zone in the east, and a biotite-present almandine zone in the west.