Mantle Anisotropic Structure and Dynamics Beneath the Western United States: Constraints from Shear-wave Splitting Analysis

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One of the major discoveries in the geological science made over the past 50 years is that the crustal and the upper-most mantle of the Earth are divided into tens of 'pieces', or tectonic plates. Movement of the plates against one another is responsible for the formation of ocean basins, mountain ranges, rifted valleys, volcanoes, and earthquakes. In spite of numerous studies, however, the forces that are driving the movement of the plates remain unresolved. The ongoing project is designed to provide critical information on the driving mechanism of plate motion, by measuring the strength and direction of the fabrics formed by the movement of the plates. A database of thousands of shear-wave splitting parameters, which are measures of deformation in the lithosphere and flow in the underlain asthenosphere, is being established as an updated version of an existing shear-wave splitting database (NA-SWS-1.1 which can be accessed at http://www.mst.edu/~liukh/SWS) that the PI established for North America. The new database, like its first version, will be made public as soon as possible over the course of the project. The researchers involved in the current project are collecting geological and geophysical information to interpret the shear-wave splitting observations. Major scientific issues to be addressed by the proposed study include whether the western US is underlain pervasively by two-layer anisotropy, why the Colorado Plateau and adjacent areas show complex anisotropy, and what are the characteristics of mantle structure and dynamics beneath the transitional zone between the tectonically active western US and apparently inactive Great Plains.