The continent of North America is geologically and tectonically diverse. The western half of the continent includes a major plate boundary and has been host to mountain-building events, volcanism, and extension within the recent geological past. The eastern portion of North America is located far from any plate boundary and has not experienced significant tectonic activity in hundreds of millions of years. Understanding how the dynamic processes occurring in Earth's interior have given rise to this complex terrain is fundamental to understanding the growth and evolution of continents in general and North America in particular. Imaging variations in the speed of seismic waves traveling through the crust and mantle provides important constraints on the temperature and composition of those regions as well as the presence of partially molten rock. However, since each of these factors, and any combination of them, can affect velocity, additional data sets are needed to achieve a robust interpretation. The absorption of seismic-wave energy is also controlled by temperature, composition, and melt content, and thus jointly interpreting variations in seismic-wave velocity and attenuation allows for improved constraints on the properties of Earth's interior. To date, there have been very few studies of seismic attenuation beneath North America, largely because of the difficulties involved with measuring and analyzing the amplitudes of seismic waves.
The PIs will develop 3-D models of shear attenuation and radially anisotropic shear velocity for the North American upper mantle using surface waves recorded by USArray. Surface-wave phase and amplitude are measured using a new interstation cross-correlation approach that exploits waveform similarity at nearby stations, and the phase and amplitude data will be utilized together to take advantage of the dependence of both data sets on seismic velocity and attenuation. From these models, variations in temperature, composition, and partial melt across the continent will be estimated, and a more complete picture of how structures like the Colorado Plateau, the Basin and Range, the Mid-Continent Rift, and the North American craton are controlled by the processes in and properties of the underlying mantle will emerge.
