[S14-1] Upper mantle and transition zone dynamics and structure I

[S14-1-02] Cold, hot mantle transition zone beneath Hawaii mapped from teleseismic Ps receiver functions

^○Matthew Agius¹, Catherine Rychert¹, Nicholas Harmon¹, Gabi Laske² (1.Ocean and Earth Science, University of Southampton, United Kingdom, 2.University of California San Diego)

Hawaii's remote geographic setting located far away from tectonic boundaries makes it an archetypal example of hotspot volcanism. Establishing whether this hotspot represents the classic plume theory of a vertical plume ascent from the core-mantle boundary to the surface, or perhaps of a plume that may have a more complex path, has proven challenging. In order to determine the exact trajectory of the Hawaiian plume seismic anomaly in the mantle we determine P-to-S (Ps) receiver functions to illuminate the 410- and 660-km depth mantle discontinuities beneath the Hawaiian Islands. The dataset is from waveforms recorded on land and ocean-bottom seismometers, with the latter having corrections for tilt and compliance noise. Our 3-D depth-migrated maps provide improved lateral resolution of the mantle transition zone (TZ) discontinuities. The 410 discontinuity is characterised by a deepened area beneath central Hawaii surrounded by an elevated shoulder. At the 660 discontinuity, a shallow topography is located to the north and far south of the islands, and a deep topographic anomaly is located far west and east. The TZ thickness varies laterally by +/-13 km depth: thin beneath north-central Hawaii and thick farther away in a horseshoe-like feature. We test for robust, large-scale characteristics of the TZ thickness, test the effect of a plume in the background velocity model, compare the topographic maps with seismic velocities, and compare temperature anomalies with those from geodynamic numerical models. We infer that at 660-km depth a broad or possibly a double region of upwelling converges into a single plume beneath central Hawaii at 410-km depth. As the plume rises farther, uppermost mantle melting and flow results in the downwelling of cold material, down to at least 410 km surrounding the plume stem. This result in the context of others supports complex plume dynamics including a possible non-vertical plume path and adjacent cold mantle downwellings.