Characteristics of a continental margin magmatic arc as a function of depth: the Skagit Methow crustal section

Abstract

Calc-alkaline arc magmatism has been documented throughout NW Washington and SW British Columbia at around 95-85 Ma. 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. 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. -from Authors