Collaborative Research: The Effects of Water and Lithology on the Strength of the North American Lithosphere

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Nontechnical SummaryA full understanding of mountain-building and earthquake processes requires knowledge of three-dimensional variations in flow strength of continents. Measurements of flow strength are among the major goals of EarthScope, because they are key to understanding the timing and spatial distribution of earthquakes and volcanism, as well as the evolution of North America through time. Characterizing flow strength is challenging, however, because strength is very sensitive to three independent physical properties -- temperature, rock-type, and water content -- whose effects are not easily distinguished using commonly-used geophysical imaging techniques. This project identifies several diverse geophysical measurements that can be combined to separate the effects of temperature, rock type and water in the Earth's crust and upper mantle. In addition, this project is supporting the education of three graduate students and the development of software tools for use in graduate classrooms. Finally, this project is providing maps and other data products that are widely useful to scientists working to understand such diverse topics as likelihood of future earthquakes, volcanic eruptions, and the potential for economic mineral deposits at depth.Technical Description This project seeks to determine the relative effects of temperature, water and lithology on strength of the North American continental lithosphere. The investigators are estimating temperature variations using uppermost mantle seismic velocities and surface heat flow measurements; characterizing crustal lithology from a combination of gravity and seismic receiver function data; and combining these with measurements of lithospheric flexural rigidity (an integral measure of bending strength) to assess the remainder effects of water on flow rheology. This novel approach to characterizing physical property variations enables in situ estimation of fully three-dimensional flow rheology of the lithosphere, and lends new insight into the dynamical processes of heat and fluid flux that determine mobility versus stability of continental lithosphere. When combined with new constraints on lithospheric mass density variations, these properties promise to revolutionize the Earth Science community's understanding of lithospheric evolution, deformation and dynamical processes. The grant supports research efforts by three graduate students. The Colorado State University PI and two M.S. students are estimating geothermal variations and associated uncertainties, and also developing a series of teaching tools to demonstrate concepts of lithospheric flexure and rheology to undergraduate Earth science students. The Utah State University PI and student are estimating anisotropic flexural rigidity, crustal lithology and thickness, and using the combined measurements to model the water content and rheology of the lithosphere.