Collaborative Research: Understanding lithospheric structure and deformation in Alaska via integration of seismic imaging and geodynamic modeling

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In the state of Alaska, the Earth's surface is moving at rates of 10's of millimeters per year with respect to the rest of the North American plate. This motion indicates that the Earth's lithosphere (the more rigid outer layer of the Earth that makes up the tectonic plates) is deforming. In southern Alaska, where the Pacific plate lithosphere is subducting beneath the North American plate, the North American plate is being compressed, for example creating the mountains of the Alaska Range. However, the forces that drive surface motion and North American plate deformation in the interior of Alaska are much less clear. The goal of this project is to measure the structure of the North American plate, in particular its thickness and its internal strength, and to model how forces acting on the edges and base of the plate are transmitted to the surface. The properties of the North American plate and the underlying mantle will be measured using surface and body waves that emanate from distant earthquakes and are recorded at sensors in Alaska, in particular the stations of the NSF Earthscope Transportable Array. The measured North American plate properties will be incorporated in numerical models that will explain motions observed at the surface in Alaska with the forces exerted on the North American plate by other plates and the motion of the deeper mantle. This work will improve understanding of the forces that drive motion and earthquakes on the Denali fault and other faults and that have created the mountains in Alaska. This project will contribute to the education of graduate and undergraduate students at Brown University and Purdue University.Deformation in Alaska manifests dramatic variations, from convergence and uplift in southern Alaska associated with subduction to more enigmatic deformation in interior Alaska that includes southward surface velocities. The drivers of this deformation and their relationship to underlying crust and mantle structure are debated and not yet understood. This project will address these questions by obtaining new models of the seismic structure of North American lithosphere and underlying asthenosphere beneath Alaska (using data from the NSF EarthScope Transportable Array and other networks). These constraints will be incorporated into 3D geodynamic modeling of the driving forces of upper plate deformation that explain observed surface deformation (as constrained by GPS, geologic and seismicity data). To accomplish this the project team will: (1) Carry out individual and joint analyses and inversions of converted body waves (Sp and Ps) and Rayleigh surface waves to obtain detailed models of crust and mantle shear velocity that robustly image mantle discontinuities. Attenuation and azimuthal anisotropy will also be determined; (2) Use shear velocity and attenuation to place bounds on temperature, bulk composition, grain size, water content and partial melt, and use these parameter ranges to place bounds on viscosity and density, incorporating viscosity constraints from dynamic modeling of observed surface motion; (3) Integrate models of viscosity and density with 3D geodynamic modeling of observed surface deformation and test geodynamic models with observations of azimuthal anisotropy in Rayleigh wave phase velocities and SKS splitting. The proposed work will improve understanding of the seismic structure of North American lithosphere and underlying asthenosphere; its implications for crust and mantle rheology and density; and the impact of upper plate and asthenosphere density and rheology on the rates and dynamics of upper plate deformation. A range of key questions will be addressed, including: (1) What processes enable the high rates of deformation and uplift observed in Alaska, and what is the role of upper plate rheology? How coupled are the lithosphere and asthenosphere and what are the impacts of tractions from asthenospheric flow at the base of the upper plate? (2) How has subduction modified the upper plate lithosphere? What are the effects of Yakutat terrane subduction, and does Yakutat mantle have distinct thermal or compositional properties that make it anomalous in terms of viscosity or buoyancy? What is the relationship of Yakutat terrane subduction and the Denali volcanic gap or the Wrangell volcanic field? (3) How do upper plate structure and tectonics relate north of the subduction zone? Where do offsets in upper plate properties occur across terrane boundaries and strike-slip faults? The project will contribute to the education and career development of graduate students at Brown and Purdue. At least one undergraduate will work on this project at Brown through the Leadership Alliance, a program that recruits students from groups underrepresented in STEM fields; one to two Brown undergraduates will also participate. The project will also reach a broader group of students and postdocs at Brown and Purdue through research group meetings and courses, and will be featured in outreach with elementary schools in Providence, RI.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.