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Structure and metamorphism in the Niagara Peak area, western Cariboo Mountains, British Columbia Garwin, Stephen Lee


A more than 2000 m thick sequence of Hadrynian to Paleozoic Snowshoe Group metasedimentary rocks of the Omineca Belt (OMB) is exposed near Niagara Peak in the western Cariboo Mountains, central British Columbia. This package contains the northern extremity of the Shuswap Metamorphic Complex and lies 30 km northeast of the accretionary boundary with Intermontane Belt (IMB) Mesozoic sedimentary and volcanic rocks (Quesnellia Terrane) and Upper Paleozoic (?) ophiolitic and sedimentary rocks (Slide Mountain Terrane). Four phases of folding (D₁-D₄) are recognized. D₁ consists of isoclinal folds and transposed compositional layering. D₂ commonly forms southwest verging, open to close folds with subhorizontal axes and moderately northeast dipping axial surfaces. In the eastern part of the area, divergent fanning of D₂ axial surfaces and a reversal of vergence direction occur about a map-scale synform characterized by greater strain, bimodal fold style and a locally penetrative axial planar cleavage. D₃ and D₄ form orthogonal upright open buckles with respective northwest and northeast trending axes. Steeply dipping normal and minor reverse faults crosscut all fold structures, displaying minor offsets. Prograde regional metamorphism reached greenschist grade late in D₁. Staurolite and kyanite growth accompanied D₂, followed by postkinematic sillimanite generation under conditions of approximately 635° C and 5 kb. D₃ associated sericite-chlorite retrogression of porphyroblasts occurs in sub-sillimanite grade rocks in the western part of the area. Synmetamorphic veins represent polyepisodic hydraulic fracture development during progressive dewatering of a sedimentary pile by prograde metamorphism. Eastward obduction of Quesnellia and Slide Mountain Terranes onto theOmineca Belt took place in the Middle Jurassic. Shortly following this event, the IMB-OMB tectonic suture was deformed, forming map-scale folds of cuspate/lobate geometry.

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