Crustal Anisotropy beneath selected Pacific Ocean-Islands from Harmonic Decomposition of Receiver Functions

Abstract:

Crustal anisotropy beneath ocean islands can be attributed to preferentially aligned minerals cracks or dike structures. Stacked with harmonic weighting, receiver functions from permanent ocean-island stations display evidence of strong and distinct anisotropy parameters in the underlying crust and in an underplated layer. We conduct analysis for eleven IRIS-GSN stations in the Pacific Ocean, with 211 to 774 seismic events at each station. We observe the prevalence of two-lobed RF amplitude variations with back-azimuth, consistent with tilted-axis anisotropy. With the assumption of a slow-axis hexagonal symmetry, we observe anisotropies in the underlying crust and under-plated crust that are oppositely oriented. Synthetic modeling of representative stations indicate that the strength of anisotropy ~5-10%. The strike of the inferred symmetry axis tends to align with plate motion, with some scatter. At stations in the northwest Pacific i.e. KWAJ, TARA, and WAKE, the symmetry axis aligns with past plate motion. The anisotropy closest to the present-day Hawaiian hotspot, beneath station POHA, aligns almost orthogonal to plate spreading. We attribute the crustal anisotropy to the preferred alignment of dike structures that transported asthenospheric magma toward the seafloor volcanic edifice. Our results argue that the thermal-plume origin for ocean islands must be supplemented by tectonic-stress heterogeneities that allow magma to penetrate the lithosphere via fractures. Magma-transport fractures should align normal to the least-compressive direction, which models predict should align approximately coeval to plate motion.