How simple is the oceanic asthenosphere? Seismological insights for the Juan de Fuca and Gorda plates from Cascadia Initiative and Blanco Experiment data

Abstract:

‘Normal’ oceanic asthenosphere is often assumed to be homogeneous unless perturbed by spreading at mid-ocean ridges, and to flow in response to the overriding plate. Here, we show seismological data that reveal a complex asthenosphere beneath the Juan de Fuca (JdF) and Gorda plates. We present mantle tomographic images using teleseismic S phases and shear-wave splitting measurements of SKS phases using OBS data recorded during the Cascadia Initiative (CI) and Blanco Experiment. Beneath the JdF plate, Vs in the upper 200 km of the mantle is ~5% faster beneath ~10 Myr lithosphere than near the ridge. In the Jdf plate interior, fast polarization directions from shear-wave splitting broadly correlate with absolute plate motion. Beneath the Gorda plate, in contrast, Vs varies only weakly with lithospheric age. Fast polarization directions are aligned with JdF-Pacific relative plate motion over a broad region and do not vary near the Gorda ridge. We also show that low Vs regions beneath three mid-ocean ridges (JdF, Gorda, and southern East Pacific Rise) are asymmetric with respect to the ridge axis. These results indicate that the evolution of the asthenosphere differs beneath the JdF and the Gorda plates. The large increase of Vs beneath the JdF plate is much greater than the predicted ~2% change due to conductive cooling of the lithosphere, and we explore possible explanations for this large degree of heterogeneity. The weak variations in Vs near the Gorda ridge are also not well explained by conductive cooling. The broad region of low Vs could be due to melt retention or to grain size reduction in a shear-zone. Finally, models of mantle upwelling suggest that the asymmetries in Vs with respect to the ridge axes likely require an asymmetric region of upwelling in the presence of pre-existing heterogeneity in the asthenosphere. We suggest that interactions with plate boundaries and heterogeneity may create an active and complex asthenosphere.