Mapping the Pacific Slab Edge and Toroidal Mantle Flow Beneath Kamchatka

Abstract

AbstractTo clarify the upper mantle structure and dynamics of the Kamchatka subduction zone, we conduct Rayleigh‐wave phase‐velocity azimuthal anisotropy tomography beneath Kamchatka using amplitude and phase data of teleseismic fundamental mode Rayleigh‐waves at periods of 25–150 s. With the obtained anisotropic phase‐velocity model, a 3‐D azimuthal anisotropic shear‐wave velocity (Vs) model is determined down to ∼300 km depth. Beneath Kamchatka, the subducting Pacific slab is clearly imaged as a dipping high‐Vs zone on the south of the Shiveluch volcano at ∼57°N latitude, whereas this high‐Vs zone is absent on the north of the volcano, suggesting that a slab edge exists in the region. Visible low‐Vs anomalies are revealed around the slab edge. The subducting Pacific slab beneath Kamchatka generally exhibits a fast‐velocity direction (FVD) of ∼NE‐SW in the area south of ∼54°N latitude, whereas the dominant FVD in the slab rotates to ∼N‐S or ∼NNW‐SSE in the area of ∼54°–57°N latitude. The FVDs in the subducting Pacific slab are generally parallel to the Kamchatka trench, which may result from the shape‐preferred orientation of stratified oceanic lithosphere. At shallow depths (<∼150 km), the low‐Vs mantle wedge generally exhibits ∼ NW‐SE FVDs, whereas ∼ E‐W FVDs appear on the north of the slab edge, which form semi‐toroidal FVDs around the slab edge. The semi‐toroidal FVDs may reflect a combination of corner flow in the mantle wedge and toroidal mantle flow around the slab edge beneath Kamchatka, which may deform and heat or even melt the slab near its edge.