Abstract
Calc-alkaline arc magmatism has been documented throughout NW Washington and SW British Columbia at around 95–85 Ma. Migmatites, orthogneisses, directionless plutons, and volcanic flows of mid-Cretaceous age are exposed in the region, and represent the major constituents of the arc. In the Ross Lake area of northern Washington, upper to middle crustal levels of the arc have been uplifted and exposed by large-scale Paleogene folding as a relatively intact vertical crustal section. Thus, the vertical variation in deformation, metamorphism, and magma behavior in a magmatic arc may be directly observed. Based on stratigraphic, structural, and petrologic data, the arc-section consists of a brittlely deformed upper crust containing a few plutons, and a ductilely deformed and migmatized middle crust containing numerous large plutons. Ductility in the largely quartzose country rocks and plutons is observed only below the uppermost limit of voluminous middle crustal plutons at about 12–14 km depth in the crustal section. Assuming the onset of ductility in quartz represents a high strength gradient in the crust, the high strength “lid” at the brittle-ductile transition is suggested to have caused large-scale ponding of magma. Ambient country rock viscosity thus appears to be a major control on the mechanics of magma ascent in this region.
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Kriens, B., Wernicke, B. (1990). Characteristics of a Continental Margin Magmatic Arc as a Function of Depth: The Skagit-Methow Crustal Section. In: Salisbury, M.H., Fountain, D.M. (eds) Exposed Cross-Sections of the Continental Crust. NATO ASI Series, vol 317. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0675-4_7
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DOI: https://doi.org/10.1007/978-94-009-0675-4_7
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