Teleseismic data recorded across the western U.S. provide information on the structural nature of the mantle beneath North America, a volume of Earth's interior that has been shaped by a complex history of subduction of oceanic lithosphere. Tomography images high-velocity structures that are commonly attributed to subducted slabs but which are not as simple as expected from our current understanding of the geologic history of subduction. Hence, such images hold key information on the actual subduction history, the fate of young subducted slabs, and convective flow in the upper mantle and across the transition zone to the lower mantle. The area affected by the Laramide orogeny is now experiencing small-scale convection beneath most of the western U.S., and a presumed Yellowstone plume is (probably) interacting with subducted slab and small-scale downwellings. Understanding this richness of geological processes and resolving the individual structures from one another requires careful and accurate seismic imaging. The goal of this project is to test specific hypotheses about the complex history of the western U.S. by creating a comprehensive 3-D P-wave velocity model extending from the base of the crust to ~1000 km depth. The seismic modeling will combine all available local, regional, and global travel-time datasets with newly available constraints on crust and transition zone structure. In addition, it uses finite frequency techniques (where useful) and 3-D wave propagation techniques. This work supports two graduate students.
The main task that the MIT group accomplished under this grant was the production of 3D tomographic models of mantle structure to a depth of about 1000 km beneath North America. We used data from a global earthquake catalog as well as arrival times of seismic waves recorded at a total of about 2000 seismograph stations of USArray, the seismometry component of the NSF-funded Earthscope project. These phase arrivals were measured at the Array National Facility (ANF). Each installment of USArray involved some 400 recorders, but over time this roving array moved eastward to synthesize a continental array of over 2000 recording sites. Data from these sensors enabled us to delineate the 3D structure of the mantle in unprecedented detail. In order for these results to be of optimal value for other researchers working on the Earthscope project we have been making annual updates of our models (along with computer code to read and display our models) openly available to the community at large. The publication of the model updates occurred in several installments, following the eastward progression of the roving arrays. The tomographic images clearly reveal the complex structure of the fragments of the Pacific lithospheric plate that have subducted during the past many tens of millions of years, suggesting that the history of subduction, the associated mantle processes, or both, are more complicated than hitherto thought. The results also show that the spatial scales of upper mantle heterogeneity in the western part of the country (which continues to be shaped by ongoing tectonic processes) are much smaller than in the geologically older and tectonically more stable central and eastern parts that form the core of the North American continent. Another task that we completed was the determination of the depths to upper mantle discontinuities beneath western USA. The depth to these discontinuities depends on temperature, composition, and the presence of volatiles and partial melt, and our results suggest that high temperature anomalies beneath Yellowstone extend across the mantle transition zone, thus suggesting that one or more thermal upwellings feeding into Yellowstone may have their origin very deep in Earthâ€™s mantle. The tomographic maps are also suggestive of the presence of melt, or water, in the mantle transition zone, at depths between 410 and 660 km beneath Earthâ€™s surface. The results from tomographic and imaging with converted waves are largely consistent with the outcomes of other groups working with the same seismological data, thus demonstrating a remarkable convergence of understanding of the complicated structure of the mantle beneath USA. Our ongoing research focuses on the extraction of new types of information from the seismic wavefields recorded by USArray, and results from these studies are expected in 2015.