The role of rheological evolution on active deformation of Southwestern North America within the Pacific-North America Plate Boundary Zone since the Oligocene

Alireza Bahadori, William E. Holt, Jeonghyeop Kim, Troy Rasbury, Weisen Shen, & Julia Grossman

Submitted August 15, 2018, SCEC Contribution #8747, 2018 SCEC Annual Meeting Poster #163 (PDF)

Poster Image: 
Through a subduction to a transform dominated margin transition in western North America, highlands and thick crustal welts were dramatically altered to Basin and Range style topography and thin crustal structure. We incorporate the finite strain, paleotopography and crustal thickness models of Bahadori et al. (2018) to quantify vertically averaged deviatoric stress fields in relation to body forces that drove deformation. We recover the laterally varying viscosity with respect to velocity and associated strain rates to provide associated deviatoric stresses. We calculate the average present-day upper mantle temperature using constraints from seismic shear velocities. We use NAVDAT database to calculate temperature variations at 100 km depth from present-day to 36 Ma. With these parameters we solve for effective water content variation within the lithosphere through time. Based on our results, as the East Pacific Rise contacts and evolves along the trench margin, and slab rollback progresses, boundary-sourced compressional stresses drop significantly within Nevada and Arizona, and gravitational potential energy associated stresses dominate in driving the extensional collapse of topography. Our results indicate that fluids must have had a central control on the effective viscosity of the lithosphere and weakening mechanisms for middle or lower crustal zones in dictating the timing of the gravitational collapse of topography (Nevadaplano and Mogollon highlands). Our inference on the role of fluids arises from several lines of evidence: (1) We observe a strong correlation between fluorine (F) concentrations in geothermal springs within the Great Basin and present-day transtensional strain rates. These high F concentrations also correlate with high He-3/He-4 ratios in geothermal fluids, indicating that F may be a powerful proxy for mantle fluid input. (2) Our time-dependent estimates of relative fluid content in our dynamic models correlate in space and time with the collapse of topography and active volcanism. (3)Position reconstructions of fluorite deposits within the southwestern Cordillera place them along linear belts that align with the paleotopography positions at 36 Ma, which subsequently experienced gravitational collapse. U-Pb dating of fluorite samples from these localities holds promise for testing the hypothesis that the timing of crustal weakening, and gravitational collapse of topography, resulted from significant volumes of crustal fluid input.

Key Words
Nevadaplano, Mogollon highlands, Rheology, Collapse, Deformation

Citation
Bahadori, A., Holt, W. E., Kim, J., Rasbury, T., Shen, W., & Grossman, J. (2018, 08). The role of rheological evolution on active deformation of Southwestern North America within the Pacific-North America Plate Boundary Zone since the Oligocene. Poster Presentation at 2018 SCEC Annual Meeting.


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