The northern limit of the subducted Juan de Fuca Plate system

Abstract

Analysis of data recorded at an array of three‐component broadband seismograph stations deployed on northern Vancouver Island and the adjacent British Columbia mainland, at the northern end of the Cascadia subduction zone, provides the first constraints on the S wave velocity structure of this region and permits us to define the northern limit of the subducted Juan de Fuca plate system. During a 2‐year period, more than 80 teleseisms were recorded at our five stations. The method of receiver function analysis was used to constrain the S velocity structure to upper mantle depths. Beneath the northern three stations, a relatively simple continental crust is interpreted with a well‐defined Moho near 37–39 km depth. An upper crustal S velocity discontinuity at these stations is interpreted as the top of the high‐velocity rocks of the Wrangellia terrane. In contrast, more complicated structure dominated by pronounced low‐velocity zones dipping to the NE are interpreted beneath 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 within 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 beneath Vancouver Island. This change coincides with significant changes in topography, heat flow, gravity, and geochemistry.