Collaborative Research: Understanding Lithospheric Suturing and Passive Margin Development Beneath the Southeastern U.S.

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).At the culmination of the lithospheric accretion that built the Appalachian Mountains (250-320 million years ago) the lithosphere of another continent (Gondwana) collided with proto-North America. The suture between these two lithospheres is delineated in the crust beneath southern Georgia by variations in fossils, rock types and ages, and a zone of southward dipping reflectors imaged by seismic reflection data. However, the location and geometry of the suture in the mantle is unknown.The goal of this project is to determine the position and shape of the lithospheric suture in the mantle, and to measure any contrasts in crust and mantle physical and chemical properties across the suture. 85 EarthScope USArray Flexible Array broadband stations are being deployed in two N-S linear arrays that straddle the suture. One line stretches from North Carolina to the coast of the Florida panhandle. The other extends from South Carolina into central Florida. These stations, together with the stations of the EarthScope USArray Transportable Array, will record seismic waves from around the globe. These data will be analyzed to measure three-dimensional variations in isotropic and anisotropic wave velocities, and the velocity models will be interpreted in terms of temperature, rock composition (including volatile content), and rock deformation fabrics.This research will address a series of fundamental questions. How does the crustal suture connect to mantle structures? Did mantle accretion occur on shallow dipping shear zones, or did it occur at least in part on more vertical contacts that imply horizontal shortening over a broad depth range? If dipping mantle interfaces exist, do they represent relict subducted lithosphere, or are they shear zones unrelated to prior subduction? What does anisotropy in seismic velocities imply about the distribution of deformation during accretion? What was the direction of subduction associated with the collision of Gondwana and Laurentia? Continental break-up followed the lithospheric collision, leading to the opening of the Atlantic Ocean. How was the continental lithosphere affected by this episode of extension, rifting and magmatism? Is there evidence for delamination of the lithosphere? What are the present-day patterns of flow in the weak mantle asthenosphere that underlies the lithosphere?This project is directly supporting the education and training of graduate and undergraduate students. We are developing educational materials that use this research project to build understanding of concepts such as continental collision, mountain building, and continental break-up. Experiment results will be used in our undergraduate and graduate classes, and in outreach to schools in the vicinity of our home institutions in Georgia, North Carolina, and Rhode Island.