Petrologic History of Late Cretaceous Nanaimo Group in Nanaimo Basin, Western Washington and British Columbia: Implications for Cretaceous Tectonics: ABSTRACT

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The tectonic development of the Late Cretaceous Nanaimo basin is an important factor in determining plate interactions along the northwest Pacific Coast where the basin was formed within an orogenic collage, surrounded by the North Cascades, coastal plutonic belt, insular belt, and San Juan Island terranes. The Nanaimo basin may have been an intramassif fore-arc basin or a pull-apart basin, which developed within a proto-Queen Charlotte transform zone. Paleocurrent and petrographic data from the Nanaimo basin indicate derivation of plutonic debris from the coastal plutonic belt and intermediate volcanic and low-grade metamorphic rock fragments from the North Cascades. Plutonic debris and silicic to basic volcanic rock fragments were derived from the insular belt. Chert and subordinate intermediate-volcanic and argillaceous rock fragments were contributed by terranes of the San Juan Islands. Nanaimo sandstones contrast greatly with Late Jurassic to middle Cretaceous fore-arc basin sandstones in this region. Contemporaneous volcanic rock fragments are conspiciously absent in Nanaimo rocks and the dominance of plutonic over volcanic debris from the coastal plutonic belt suggests deep dissection of the massif. Subduction along the continental margin of British Columbia and Washington may have greatly slowed or ceased during the Late Cretaceous. The tectonic setting was probably characterized by a broad zone of right-lateral transcurrent faulting. Cretaceous and early Tertiary structures of southern Vancouver Island are similar to structures observed along the San Andreas transform zone. Additionally, a magmatic gap is noted in the southern coastal plutonic belt during Nanaimo basin development. End_of_Article - Last_Page 968------------