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UBC Theses and Dissertations

Polyphase deformation and metamorphism in the western Cariboo Mountains near Ogden Park, British Columbia Lewis, Peter D.


The boundary between the Omineca Belt and the Intermontane Belt in Central British Columbia represents the suture between autochthonous North America (Barkerville Terrane) and several allochthonous terranes accreted from the west In the Quesnel Lake region allochthonous sedimentary and volcanic rocks of Quesnellia Terrane, accreted in the Jurassic, are in sharp tectonic contact with underlying siliclastic and carbonate metasedimentary rocks. The Ogden Peak study area is located 10 km east of and structurally below this suture zone and is thus well situated for observing deformational styles within the autochthonous package. Rocks exposed near Ogden Peak comprise the Hadrynian(?) to Paleozoic(?) Snowshoe Group and local diabasic intrusions. These rocks record a deformational history involving four phases of folding (D₁-D₄) and later brittle faulting (D₅). Earliest recognizable structures consist of recumbant isoclinal folds with a well-developed transposed foliation. This foliation is tightly folded about northwest trending, southwest verging second phase structures. Northwest trending third phase structures and northeast trending fourth phase structures occur as both crenulations and open buckles. Southeast dipping faults cut all earlier structures with tens of meters of normal offset Phase 1 and Phase 2 fold styles are compatible with a flattened buckle fold mechanism of formation, associated with elevated temperatures and reduced viscosity contrasts across layering. Later fold styles are controlled by higher viscosity contrasts and detachment along layering. All phases of deformation are dominated by semi-brittle mechanisms of dislocation slip and glide, mechanical twinning, and microcracking. Temperature activated diffusional creep is only locally active and does not contribute appreciably to total strain. The mineral assemblage garnet-staurolite-kyanite defines a metamorphic peak late in D₂. Metamorphic temperatures of approximately 530° C at 6.0 kb have been determined using garnet/biotite geothermometry. Extensive retrograde metamorphism spans D₃ and D₄, overprinting prograde assemblages and providing evidence for abundant fluids late in deformation. Late phase 1 diabase dykes locally intruded an area to the southeast of Ogden Peak. Major and trace element analyses of samples from these intrusions suggest a calc-alkaline, volcanic arc affinity.

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