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Wei Leads $5.2M NSF Grant into Mantle Convection System

A fundamental goal of solid earth science is to improve our understanding of the Earth’s interior evolution. For EES Seismologist Dr. Shawn Wei, this means looking at mantle convection in one of the most remote and historic parts of the Southern Pacific Ocean.

Wei is the Lead PI on a $1.1 million National Science Foundation grant to investigate mantle flow in a slab-plume setting known as the Tonga-Lau-Samoa system. The grant will also provide $4.1 million worth of ship time and instrument usage for two seagoing expeditions. The project seeks to understand how the distinctive compositional and thermal characteristics of plume materials are distributed.

Cartton of Mantle Convection System
An overview of the mantle convection system in the Tonga-Lau-Samoa region.

There are three primary components of the mantle convection system: 1) subduction where one tectonic plate (slab) descends into the Earth’s interior along an oceanic trench, 2) plate spreading at mid-ocean ridges where the majority of seafloor is created, and 3) buoyant upwelling, or mantle plumes, where hot and less dense materials rise from the deep earth to the surface to create volcanoes like the Hawaiian islands.

Wei described the Tonga-Lau-Samoa system as, “One of the few places in the world that we can see such a close interaction between the down going material and the up going materials”

The project will feature a combination of seismic imaging, geochemical analysis of submarine lavas, and geodynamic modeling to examine this interaction. As Wei explained, “The question is how does the plume bring all the materials up and do some of these materials end up being trained by this down going subduction to go back into the deep earth?”

Google Earth shot of Tonga-Lau-Samoa region
The Tonga-Lau-Samoa region

To answer those questions, Wei’s team will deploy 30 Ocean Bottom Seismographs (OBSs) and 5 land seismometers in the northern Tonga-Lau and Samoa regions for 18 months. Several different seismic imaging techniques will provide 3-D velocity models of the upper mantle, precise earthquake locations will image the slab tear and associated deformation, and the variation of seismic velocity with direction will provide constraints on mantle flow.

They will also dredge lava samples at 10 seamount locations east of the Tonga Trench to better understand the subduction inputs in the trench’s northern region. The samples will be analyzed for isotopic compositions to identify possible sources of non-Samoan hotspot materials in the Lau Basin, which is critical for geochemical interpretations of mantle flow.

Weather is a large factor in this project. In order to avoid hurricane season, the project will have to take place in the summer months, affording the team milder seas in which to deploy the OBSs. Deployment is tricky too. Each OBS is the size of an average human being and the water is too deep to rely on submersibles for placement. Therefore, each OBS will have to descend on its own with the help of gravity. Going to all the proposed locations will take about 30 days and the average deployment time for each unit is about 3 hours.

“This is a big and expensive project and it's very difficult to get it funded, so I'm grateful that after much persistence, we succeed.” Wei exclaimed.

This project is a multi-institution and multi-disciplinary project with participants from Washington University in St. Louis, UC Santa Barbara, and UC San Diego. The ship will be provided by the University-National Oceanographic Laboratory System (UNOLS), OBS equipment and technical help will be provided by the Woods Hole Oceanographic Institution.

Though he’s never been there in person, Wei has studied this region for over 10 years. “That subduction zone has been my main focus, so I really want to go there. This is where plate tectonics and subduction was discovered by scientists in 1960s. Before that, there was no clue of plate tectonics. To me this is kind of a historic place and very exciting.”








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