THE NORTHERN LIMIT OF THE SUBDUCTED JUAN-DE-FUCA PLATE SYSTEM

Analysis of data recorded at an array of three-component broadband sei smograph stations deployed on northern Vancouver Island and the adjace nt British Columbia mainland, at the northern end of the Cascadia subd uction zone, provides the first constraints on the S wave velocity str ucture of this region and permits us to define the northern limit of t he subducted Juan de Fuca plate system. During a 2-year period, more t han 80 teleseisms were recorded at our five stations. The method of re ceiver function analysis was used to constrain the S velocity structur e to upper mantle depths. Beneath the northern three stations, a relat ively simple continental crust is interpreted with a well-defined Moho near 37-39 km depth. An upper crustal S velocity discontinuity at the se stations is interpreted as the top of the high-velocity rocks of th e Wrangellia terrane. In contrast, more complicated structure dominate d by pronounced low-velocity zones dipping to the NE are interpreted b eneath our southern two stations. The shallower low-velocity zone is 6 -8 km thick, has an S velocity contrast of 0.6-1.1 km/s, and lies with in the continental crust. This feature is similar to a pronounced low- velocity layer (the E zone) imaged beneath southern Vancouver Island. The deeper low-velocity. zone is interpreted as the subducted oceanic crust. We interpret the pronounced change in S velocity structure that we observe as the northern limit of the subducted oceanic plate benea th Vancouver Island. This change coincides with significant changes in topography, heat flow, gravity, and geochemistry.