Investigating Mantle Dynamics in the Pacific Northwest Using 3D Anisotropic Velocity Models from Surface Wave Tomography

University Of Houston, Houston TX

Investigators

Abstract

The Pacific Northwest (PNW) is in a unique geological setting, where the oceanic Juan De Fuca (JDF) plate is subducting under the North American plate. It is believed that the JDF slab has interacted with the Yellowstone hotspot since at least 20 million years ago, generating pervasive volcanism in the northwestern United States. The plate subduction and the slab-hotspot interaction are the driving forces for the surface geology in the PNW, including volcanoes and earthquakes. The alignment of age-progressive volcanos (hotspot track) in the Snake River Plain (SRP) is SW-NE, agreeing with the plate motion of the North American plate, suggesting the Yellowstone hotspot is probably a plume that originates from the deep mantle, like the Hawaii hotspot. However, the volcanos in the High Lava Plains (HLP) form a hotspot track in the NW direction, against the plume hypothesis. The mechanisms for the bimodal hotspot tracks and the voluminous magmatism in the PNW are still under debate. Previous seismic velocity models beneath the northwestern United States have improved the understanding of the complex geology in the area. The velocity of a seismic wave also depends on its propagation direction, called seismic anisotropy, which is associated with lithospheric deformation and mantle flow. Therefore, imaging seismic anisotropy is critical to understanding the dynamic mantle process of the earth. The investigator will produce a high-resolution, 3D seismic anisotropy model in the Pacific Northwest to address the slab-hotspot interaction and its contribution to the extensive volcanism. The model is also helpful in understanding and monitoring seismicity and volcano eruptions in the area. The project will train one graduate and one undergraduate student in seismic data processing and tomography methods. The research results will be shared with the scientific community through conferences, journals, and websites and with students in the investigator’s undergraduate and graduate courses at the University of Houston, a minority serving institution. Oceanic plate subduction, back-arc volcanism, and hotspot activity have shaped the geological history of the Pacific Northwest. Many high-resolution seismic models have been developed using data from the USArray Transportable Array (TA), improving our understanding of tectonics and geodynamics in the region. However, whether the source of the widespread magmatism along the SRP and HLP is from a mantle plume or intruding hot oceanic asthenosphere is not resolved. Many local velocity anomalies imply complex small-scale mantle convection in the region, which has not been verified in current anisotropic models with limited lateral or vertical resolution. The investigator will construct a high-resolution anisotropic model using Rayleigh and Love wave data from earthquakes recorded at the TA and other coexisting stations in the northwestern United States. The resolution for radial and azimuthal anisotropy in these models is similar to that for isotropic velocity, helping reveal detailed mantle dynamic processes and test possible mechanisms for geophysical observations in the Pacific Northwest. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.