Collaborative Research: Mapping Upper-Mantle Anisotropy in Western U.S.: Constraints on Crust-Mantle Coupling

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0545777GahertyA principal goal of the EarthScope program is to develop a better understanding of the processes that control the assembly and evolution of the continents. It is likely that many of these processes are associated with upper-mantle structure and dynamics -- mantle buoyancy variations drive surface deformation, for example. The character of the resulting tectonism is dependent not only on the nature of the buoyancy forces, but also on the strength of the crust and mantle lithosphere that transmit those forces. Seismic anisotropy produced by fabric in mantle rocks provides a means to map deformation in the mantle, and geology and geodetic observations provide a picture of surface deformation. Using numerical models to combine the two, scientists can derive new constraints on mantle flow, lithospheric strength, and the mechanisms that control surface deformation. In this project, an integrated seismic-geodynamic analysis is being developed and applied to several key questions regarding the coupling between mantle flow and crustal deformation in the western US, including: (1) Can variations in mantle fabric along the San Andreas Fault in California be explained by changes in the strength of the crust and/or mantle from north to south? (2) Is complex mantle fabric inferred beneath the central Basin and Range produced by mantle flow around a thickened lithosphere, or is it more suggestive of buoyancy-driven upwelling beneath this region? Specifically, seismologists are measuring finite-frequency travel-times from seismic waves recorded at USArray and existing broad-band seismic stations, and inverting these travel-times for 3-D upper-mantle models in which mantle fabric is constrained using realistic sensitivity kernels for generally anisotropic structure. Simultaneously, geodynamicists are developing regional numerical flow models that are derived from geological constraints and observations of surface kinematics. The kinematic flow models provide estimates of mantle fabric development for a variety of deformation scenarios, and these fabric estimates are then quantitatively evaluated for compatibility with the seismic observations. In addition to constraining the coupling between mantle flow and crustal deformation along the western US plate boundary, the project will provide a general methodology for regional-scale anisotropy modeling for continents.