Using Seafloor Compliance to image the Crust around Hawaii

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Hawaii has long been the textbook example of a hotspot, where Earth's convecting mantle allows hot mantle rock to ascend through a relatively narrow and isolated mantle plume. This ascending material ultimately erupts at the surface in Hawaii's volcanoes. One of the most challenging questions concerns the geometry of a plume. In laboratory tanks that are heated from below, plumes are usually vertical and straight. But in a convecting spherical Earth, plumes should have more complex geometries. A key tool to trace plumes through Earth's mantle is seismic imaging. To image the Hawaiian plume, the PLUME seismic experiment (Plume-Lithosphere Undersea Melt Experiment) deployed 80 seismometers in a 1000 km array across the Hawaiian region from 2005 to 2007 to collect earthquake data. Initial seismic tomographic images trace the plume down to 1000 km depth and reveal a complex geometry. One of the most surprising results is that the mantle plume appears to approach the surface from the west and not from the southeast as is commonly assumed. To draw more definitive conclusions however, the initial mantle images must be improved by removing the distorting effects caused by complex features in Earth's crust, which were ignored in the initial processing. This project will examine a new type of data called seafloor compliance, which is the deformation of the seafloor due to pressure variations from long period ocean waves that pass by the stations. This deformation or compliance strongly depends on the specific type of crustal rock beneath the station. By analyzing seafloor compliance, the project will catalog new and crucial details of Hawaii's crustal structure and refine the plume imaging. In terms of broader impacts, this project will support a graduate student and will make research results accessible to the wider community through teaching and public lectures. Dedicated small projects will be used as recruiting tool for undergraduate students to raise awareness in the Earth sciences and related fields. Hawaii's location far from any plate boundary provides an opportunity to test basic hypotheses regarding plume/plate interaction and related magmatism. Initial seismic tomography conducted for the Plume-Lithosphere Undersea Melt Experiment (PLUME) has revealed the first high-resolution 3-D images that support the presence of a deep-rooted mantle plume, but the complexity of findings motivate a revision of the classic plume concept. Some aspects of seismic imaging remain controversial, specifically the continuity of the plume within the mantle, its interaction with the mantle transition zone and the exact magma pathways in the very shallow mantle and crust. A potential problem with published models is that mantle images were produced assuming a relatively simple crustal architecture. Even rather modest changes in crustal structure can profoundly impact the imaging of regions beneath, and may even cascade downward into the lower mantle. Artifacts in the mantle images may lead to false conclusions about geodynamical, geochemical and geological processes that ultimately cause Hawaii's volcanism. This project focuses on combining shallow-focus seismic datasets to illuminate Hawaii's crust and shallowest mantle. The primary dataset is seafloor compliance, the transfer function between pressure exerted by passing infragravity waves and the resulting seafloor deformation. Seafloor compliance is particularly sensitive to shear velocity in the ocean sediments. These new data will be combined with other crust-sensitive datasets such as high-frequency Rayleigh wave dispersion maps, crustal receiver functions and ambient-noise Empirical Green's Functions. The resulting new crustal model will allow revision of mantle models and plume geometry with much more fidelity than is currently available.