Late Cretaceous evolution of the eastern Coast Mountains, Bella Coola, British Columbia

Abstract

Structural and stratigraphic data from the eastern Coast Mountains, British Columbia, point to the presence of a Late Cretaceous thrust belt on the western margin of Stikinia. In the Bella Coola region, a fragment of thisbelt is preserved as the Sheemahant shear zone and its lower plate strata, the Early Cretaceous Monarch volcanics and Taylor Creek Group. The Sheemahant shear zone strikes northwest (∼300), dips moderately (∼55°) southwest, and verges to the northeast. Tonalitic protomylonites and mylonites within the shear zone constitute the Mt. Daunt orthogneiss. Fabrics within the orthogneiss and metamorphic patterns suggest that thrusting occurred during or soon after emplacement of the Mt. Daunt orthogneiss. U-Pb dating yields a crystallization age of 91 ′ 3 Ma for the orthogneiss, suggesting that the Sheemahant shear zone was active in Late Cretaceous time. After thrusting, the upper plate of the Sheemahant shear zone was cut by the early Tertiary Coast shear zone and intruded by the Sheemahant pluton. The Sheemahant pluton has a biotite cooling age of 54 Ma, placing a younger limit on the age of the Sheemahant shear zone. Reconstruction of the upper plate of the shear zone suggests that amphibolite facies gneiss of the Burke Channel assemblage composed the highest parts of the upper plate. This assemblage was metamorphosed and deformed prior to 82 Ma and appears to belong to a suite of Precambrian to Paleozoic volcanic-rich continental margin assemblages present in the core of the central and northern Coast Mountains. The Sheemahant shear zone is probably coeval with and kinematically linked to the eastern Mt. Waddington thrust belt and coeval thrusts near Whitesail Lake. The Monarch volcanics and Taylor Creek Group are correlated with Lower Cretaceous units in these areas and are interpreted as a coherent volcanic arc built on the western edge of Stikinia. Continuity of the thrust belt and arc strengthens the view that a northeast-vergent thrust belt formed the western margin of Stikinia in mid-Cretaceous time. This conclusion reinforces the interpretation that middle to Late Cretaceous arc magmatism in the Coast Mountains was coeval with regionally extensive contractional deformation. Existence of a coherent thrust belt along the western margin of Stikinia is difficult to reconcile with the ∼3000 km of northward transport of western British Columbia suggested by paleomagnetic data. If this interpretation of the paleomagnetic data is correct, either the thrust belt was not continuous, or parts of Stikinia had different transport histories.