Mantle structure, anisotropy, and dynamics of the Mendocino Triple Junction, northern California, USA

We determined a detailed 3-D model of P-wave anisotropic tomography of the crust and upper mantle beneath the Mendocino Triple Junction (northern California, USA) by using a large number of high-quality P-wave travel-time data of local and teleseismic events recorded by the dense seismic network on the North American continent. Our results show the presence of a slab window and the upwelling asthenosphere material through the slab window directly contacts the surrounding plates. A high-velocity anomaly is revealed at depths of ∼300−400 km beneath the triple junction, which reflects a fragment of the Monterey microplate associated with breaking-off of the Farallon plate. Depth-varying seismic anisotropy occurs in the upper mantle and exhibits a circular pattern, which is mainly caused by 3-D mantle flow around the southern edge of the Gorda plate. Subduction-driven corner flow appears in the mantle wedge. After the Farallon plate first contacted the North American plate, as it broke apart and a slab window formed, the Pacific plate captured the plate remnant and moved northwestward together. Then the Monterey plate fractured and sank into the mantle, where it was subjected to shearing forces from both the Pacific and North American plates and moved to its current position.