SHEAR-WAVE SPLITTING AND UPPER MANTLE FLOW OF THE PACIFIC BASIN AND MARGINS

I present a synthesis of the relationship between upper mantle flow and lithosphere tectonics around the Pacific Basin margins. The primary data for this study derive from observations of shear-wave splitting, a manifestation of seismic anisotropy in the upper mantle, which is closely related to upper mantle deformation fabrics, both ancient and frozen in the lithosphere and actively forming in the asthenosphere. By using observations of splitting of core shear phases (SKS, SKKS) and the teleseismic S phase traveling between earthquakes and seismographs, one can isolate splitting and anisotropic flow fabrics from asthenospheric regions. Comparison of lithospheric tectonics and asthenospheric flow at disparate regions of the Pacific margin reveals a set of distinct lithosphere-asthenosphere interactions.Data from the unsubducted portions of the Pacific plate indicate that plate motion towards western Pacific subduction zones shears the underlying asthenosphere, decoupling the plate from the deeper upper mantle. Thick lithospheric structures like the Ontong-Java Plateau, which protrudes into the asthenosphere, divert flow systematically. Similarly, the thick cratonic roots of Australia also affect flow, especially where the craton converges with subduction zones along its northern, leading edge. The relationship between upper mantle flow and subducted oceanic lithosphere can also be examined along western South America, where slab rollback enhanced by the current rapid westward motion of South America induces flow beneath the Nazca slab around South America. A similar trench-parallel upper mantle flow around the edge of Pacific oceanic lithosphere subducting beneath Kamchatka is also observed. These observations imply that asthenospheric upper mantle flow is strongly related to both lithospheric shape and motions, which serve to guide and divert the asthenosphere while imparting deformation fabrics which can be observed at teleseismic distances.