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Understanding Continental Collisions

SESAME (The Southeastern Suture of the Appalachian Margin Experiment): At the culmination of Appalachian mountain building, the lithospheres of proto-North America and proto-Africa/South America collided. The SESAME project seeks to image the suture between these continents in the crust and mantle with an array of stations in Georgia and Florida.

• PI's :
Karen Fischer, Brown University
Rob Hawman, University of Georgia
Lara Wagner, University of North Carolina, Chapel Hill
Don Forsyth, Brown University

• 85 broadband EarthScope USArray Flexible Array seismometers in Georgia and Florida
• Stations deployed in three stages: July 2010, May 2011, and May 2012
• In field until 2014 to coincide with the EarthScope Transportable Array

• Funded by the NSF EarthScope program under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)

• More information can be found at

The broad goal of this project is to better understand lithospheric accretion processes and the role of magmatism in continental rifting. The Alleghenian suture between Laurentia (proto-North America) and Gondwana (proto-Africa/South America) represents the culmination of Paleozoic lithospheric accretion in eastern 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 (magenta line segments in Figure 1). However, the location and geometry of the suture in the mantle is unknown. The adjacent passive margin experienced rifting in the Mesozoic, and active source studies have shown that large volumes of magma were added to the crust during rifting. However, fundamental questions remain about how the mantle lithosphere was altered by extension and magmatic processes.

85 EarthScope USArray Flexible Array broadband stations are being deployed in two N-S arrays that cross the suture; both lines have their densest station spacing directly over the location of the suture in the crust (Figure 1). A third leg is oriented roughly normal to Appalachian crustal terranes in northern Georgia. 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.

These results should help to constrain models for how continental lithosphere deforms during continental collision and how it is altered by later continental rifting. Questions of particular interest include: How does the crustal suture connect to mantle structures? What was the direction of subduction associated with the collision of Gondwana and Laurentia? Did mantle accretion occur on shallow dipping shear zones or through horizontal shortening over a broad depth range? Or was the suture a largely strike-slip boundary? Do the bulk properties (composition, fabric, etc.) of the two lithospheres differ? How was the continental lithosphere altered by Mesozoic extension, rifting and magmatism, and what role did the suture play? Is there evidence for delamination of the lithosphere? What are the present-day patterns of flow in the underlying mantle asthenosphere?

Brown faculty collaborators:

Don Forsyth

Other project collaborators:

Rob Hawman, University of Georgia
Lara Wagner, University of North Carolina, Chapel Hill

Map of SESAME stations and other broadband seismic stations in the region, superimposed on topography. SESAME stations were deployed in July, 2010 (green dots), May and October, 2011 (blue dots), and May, 2012 with four more planned (black dots). The stations are densest across the location of the continental suture in the crust (magenta line segments), as seen on COCORP reflection lines (e.g. McBride and Nelson, GSAB, 1988).

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