Project Summary

The shallow structure of passive continental margins has been explored by numerous active-source seismic experiments, yielding much information about the thinning of the continental crust, the role of volcanism in continental breakup, and the evolution of the nascent oceanic basin. Our knowledge of the deeper structure, meaning the mantle lithosphere and asthenosphere, is poorer. However, the mantle lithosphere is important, because it is so volumetrically dominant, because its structure offers clues to the significance of processes such stretching and delamination, and because its geometry has the potential for modulating the regional asthenospheric flow. The passive seismic techniques that have worked well in contental interiors suffer from a lack of observations on the oceanic side of the margin. Nevertheless, continental-scale surface wave tomography (e.g. in eastern N. America) has yielded some significant suprises, including an unexpectedly thin (<100 km) lithosphere beneath the coast and margin-cutting structures of uncertain origin. Furthermore, land-based measurements of seismic anisotropy suggest that the rapidly changing lithospheric thickness has a sigificant impact on asthenospheric flow.

This project will use passive seismic imaging to address the following questions about the continental margin of eastern North America:

1) How thick is the lithosphere across the continent-ocean transition;

2) Are lithospheric structure present at the transition that can be related to tectonic events that shaped the margin? Such structures might include recognizably-thinned internal layers related to stretching; high-velocity/high-reflectivity layers due to melt residues or mantle depletion from syn-rift volcanism; and highly anisotropic zones that represent regions of concentrated shear or flow.

3) What is the pattern of mantle flow (as inferred from seismic anisotropy) on the oceanic side of the margin? Is that pattern consistent with the idea that the shape of the lithsophere is controling the pattern of flow?

To study these issues, we will collect a new set of seismological data with an array of broadband seismic observatories spanning the Atlantic margin of the North American continent, from the Great Lakes to the island of Bermuda. We will utilize 1) existing permanent seismic observatories on land and in Bermuda; 2) 20 ocean-bottom seismometers (OBS's) deployed on the sea floor between the shelf break and the longitude of Bermuda; and 3) four temporarily-deployed boradband seismic observatories deployed on the shore. The resulting configuration of the broad band stations will provide, for the first time, a uniformly-spaced seismometer array across the region of continent-ocean transition.

We will use techniques based on surface wave propagation across the array to study upper mantle structure beneath it, and will combine surface-wave and body-wave tools to probe the fabric of the upper mantle.

Broader Impacts. This passive onshore-offshore experiment can be viewed as a testbed for the more ambitious EarthScope project, especially as it concerns coastline-insensitive structural seismology. This project will contribute to graduate education through the training of M.S. abd Ph.D. students. As with will be conduced in the home region of the institutions, it will also provide opportunities to involve undergraduates in fieldwork and to carry out public outreach activities.