Reconstruction Modeling of Topography and Lithosphere Dynamics using Western U.S. Strain History within the Pacific-North America Plate Boundary Zone

Alireza Bahadori, Bill E. Holt, Lucy Flesch, Lijun Liu, Troy Rasbury, Gavin Piccione, & Rubin Smith

Published August 15, 2016, SCEC Contribution #7013, 2016 SCEC Annual Meeting Poster #008

The complex deformation history of the western U.S. since 36 Ma involved a dramatic transition from a subduction-dominated to a transform-dominated margin, with widespread extension within the interior Basin and Range region. This deformation history altered the topography, drainage patterns and basins throughout the southwest. To reconstruct the topography and lithosphere dynamics we perform a comprehensive analysis of the plate boundary zone in the western U.S. since 36 Ma. Our goal is to understand the link between mantle dynamics, crustal deformation history, and topography evolution. Using position estimates from McQuarrie and Wernicke [2005], we determine lithospheric strain rates through time. Using present-day crustal thickness estimates for initial conditions, and assuming volume conservation, we integrate strain rates to determine estimates of crustal thickness changes over time. The topography and crustal thickness values provide estimates for gradients of gravity potential energy (GPE), or the body force distribution, within the lithosphere through time. We next solve the force balance equations for vertically averaged deviatoric stresses associated with the body force distribution. Our goal is to investigate the role of boundary conditions provided by plate interaction and mantle convection effects. We start by solving for stress boundary conditions, such that when boundary condition effects are added to the contribution from GPE distributions, we obtain a total stress field in accord with the kinematic solution for a given time. We also plan to incorporate the influences of mantle flow using tomography-constrained convection simulations that have been moved backward in time using adjoint methods. These convection simulations provide the lower boundary conditions in the dynamic models. Our effort on the dynamics will be combined with other independent geochemical constraints. Today we see a strong correlation between fluorine concentrations and helium isotopes in geothermal springs and wells. Elevated concentrations are found within regions of rapid transtensional strain rates within the U.S. Great Basin. We infer that these elevated concentrations of fluorine reflect a mantle fluid contribution. We plan to use U-Pb dating of fluorite deposits within fault-associated veins to provide information on the timing of mantle fluid input.

Key Words
Lithosphere, Stress, Mantle, Strain

Citation
Bahadori, A., Holt, B. E., Flesch, L., Liu, L., Rasbury, T., Piccione, G., & Smith, R. (2016, 08). Reconstruction Modeling of Topography and Lithosphere Dynamics using Western U.S. Strain History within the Pacific-North America Plate Boundary Zone . Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)