Abstract
In central British Columbia, the boundary is well exposed between parautochthonous Hadrynian-Paleozoic Snowshoe Group rocks of the craton and Upper Triassic — Lower Jurassic rocks of the allochthonous Quesnellia terrane. The boundary is marked by mylonitic mafic and ultramafic rocks, and is observed to cut across part of a 10 km thick, structurally continuous succession of Snowshoe Group rocks. Above the boundary is a locally broken succession, at least 3 km in present thickness, of phyllites and volcanics of the Quesnellia terrane.
The parautochthonous Snowshoe Group is dominated by a regionally developed transposed foliation. The earliest structures common to both parautochthon and allochthon are syn-metamorphic east-verging folds concentrated immediately adjacent to the boundary, and these are believed related to initial convergence. These early folds, the boundary, and metamorphic isograds are refolded by west-verging folds that control the present map pattern. The latter folds are buckles with constant wavelength and amplitude at the boundary. Strain intensity decreases structurally downwards from the suture zone, indicating the suture ramped to the east. The present configuration of the suture zone indicates a minimum of 70 to 80 kilometers of thrusting associated with initial convergence. All deformation was accomplished under semi-brittle conditions dominated by mechanisms of dislocation glide, grain boundary solution, and hydraulic fracturing.
Prograde metamorph ism within the two packages varies from chlorite through sillimanite grade. Metamorphic peak conditions of 600° and 6kb were attained in three distinct thermal nodes just prior to the formation of the latest west-verging folds. The asymmetry of isograds with respect to these nodes, the disposition of syn-metamorphic veins, and the presence of zones of hydrothermal retrogression indicate that heat flow was by advection.
Simplistic tectonic indications are that initial convergence was related to short-lived easterly obduction of Quesnellia and oceanic crust onto the craton, followed by easterly subduction of an oceanic Quesnellia below the craton, thus producing the observed temporal changes in style and directions of vergence. All evidence of transport direction(s) points to a high angle convergence with no transport parallel to the strike of the zone in the region examined.
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© 1992 Springer Science+Business Media Dordrecht
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Ross, J.V., Garwin, S.L., Lewis, P.D. (1992). Geology of the Quesnel Lake Region, Central British Columbia: Geometry and Implications. In: Mason, R. (eds) Basement Tectonics 7. Proceedings of the International Conferences on Basement Tectonics, vol 1. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-0833-3_22
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DOI: https://doi.org/10.1007/978-94-017-0833-3_22
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