Internal Structure across the Emperor Seamounts from Seismic Wide-Angle Reflection-Refraction Tomography

The intraplate HawaiianEmperor Seamount Chain has long been considered a hotspot track generated by the motion of the Pacific plate over a mantle plume, and an ideal feature for studies of volcanic structure, magma supply, plume-crust interaction, flexural loading, and upper mantle rheology. Despite their importance as a major component of the chain, the Emperor Seamounts have been relatively little studied. In April-June 2019, we carried out an active-source wide-angle seismic experiment to the Emperor Seamounts during cruise MGL1902. We analyzed the seismic data collected along an OBS line oriented perpendicular to the seamount chain, crossing Jimmu guyot. A tomographic P-wave velocity model was produced using the travel times of refracted and reflected P-waves, which provides new insights into processes of volcano growth and plate loading, and the rheological properties of the upper mantle. Interpreting the velocity model in terms of lithologic structure, the edifice is composed of an internal core of high velocity (~6-7 km/s) intrusive rocks and a broad overlying region of lower velocity (~4-6 km/s) mixed extrusive, volcanoclastic sediment, and minor intrusive rocks. These mixed rocks are thickest on the eastern part of the edifice where the flanking moat infill material is thickest, suggesting that more destructive events have occurred on the eastern flank of Jimmu. The extrusive/intrusive ratio for Jimmu is estimated to be ~2.5, indicating that Jimmu is built mainly by extrusive processes. The total volume for magmatic material above the top of the oceanic crust is ~5.04×104-5.60×104 km3, and the related volume flux is ~0.91-1.01 m3/s. Under volcanic loading, the oceanic crust is depressed by ~3.4 km over a ~300-km-wide region. The tomographic image was converted to density structure via standard relationships between Vp and density, and the free-air gravity across the ridge was computed and is shown to closely match the observed ship-board gravity profile. Using the density model, the plate flexure was modeled, and indicates an effective elastic thickness (Te) of ~14 km. Finally, we found no evidence for large scale underplating within the upper mantle beneath Jimmu guyot, but we did find a small low-velocity region within the uppermost mantle just beneath the ridge that we interpret as frozen melts.