In this project, a team of geophysicists at Caltech comprised of a two faculty members, one in geodynamics and one in seismology, along with two PhD students and a software engineer, are building 4-D models of North America that includes the underlying mantle. The team members are refining the 4-D model and testing the predicted seismic images with USArray seismic waveform data. The team is building the models by merging together two core methods in geophysics -- plate tectonic reconstructions and mantle convection -- and then using the models to integrate seismic images (a primary outcome of the EarthScope project) and surface evolution of the continent (topographic, structural, and magmatic history). The models include recent developments (such as deforming plates in plate reconstructions and greatly improved geodynamic modeling approaches) and high-resolution tomographic images of the mantle beneath the U.S. The team is explicitly testing the images emerging from the models through detailed comparison of predicted seismic waveforms with USArray seismograms. By capitalizing on the ability to constrain the sharpness of mantle structures with seismic waveforms, they are hoping to better understand the large amplitude, but spatially small-scale changes in the properties of mantle structures. The team is addressing the following questions: (1) what is the geometry and sharpness of velocity gradients from the transition zone to the lower mantle of the putative slab below the eastern U.S.; (2) how has the geometry of the Farallon slab changed as a function of time (geological age) and how has it descended into the mantle; (3) how did the Laramide slab evolve out of the flat slab configuration; (4) how is the topographic evolution of the entire continent from the Late Cretaceous to the present related to the dynamic evolution of the mantle below the North American continent; (5) how is the topographic evolution of the eastern side of the continent related to the motion of North America over the Mezcalera/Farallon slab?
This project address one of the most significant ambiguities associated with integrating the large variety of data that scientists have made to better understand geological processes. In this project, the team at Caltech is addressing the fact that the earth is a multi-dimensional system that changes over time, but that different measurements we make only constrain part of the overall system. This is important because it relates to basic scientific questions associated with the origin of topography and landforms on the one hand and applied questions associated with the development and evolution of hydrocarbon bearing sedimentary basins that contain most of the known non-renewable energy sources, on the other hand. The ambiguity is that seismic methods give a very good "snap shot" on what the forces inside earth look like, the so-called mantle convection. But those forces are always changing, akin to the way weather patterns change on a daily basis. On the other hand, the geological observations we use to sense the evolution of the earth's surface (like the uplift and erosion of rocks) are very good in the time-domain, over millions of years, but don't directly image how the forces and processes change. The integration of the seismic images with the surface geology has been a major goal of the EarthScope project, which has deployed a large network of seismometers to map mantle convection below a continent. The Caltech project is aimed at using state-of-the-art methods to bring all of the data and information together with computational methods. In addition, the project aims at the training of two Ph.D. students in these new methods that can be used widely in the future. The students are gaining enormous experience with sophisticated numerical methods, supercomputing technologies, and the linkage of data with numerical models
