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Smith Lecture: Reconstruction of Topography and Lithosphere Dynamics Within the Basin and Range of Western North America Since 36 Ma

Bill Holt, Stony Brook University
Friday, March 24, 2017
3:30-4:30 PM
1528 C.C. Little Building Map
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 and drainage patterns and basins throughout the southwest. We perform a comprehensive analysis of the dynamics of the plate boundary zone in the western U.S. since 36 Ma, focusing on the U.S. Basin and Range region, with the goal of understanding 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 and integrate these to determine estimates of crustal thickness evolution. Final estimates of crustal thickness and topography show prominent ‘Nevadaplano’ and Mogollon highland regions that would have resulted from Sevier through Laramide aged shortening. The locations of Cordilleran metamorphic core complexes lie within these zones of reconstructed thick crustal welts, in agreement with Coney and Harms [1984]. Using reconstructions of topography and crustal thickness we investigate solutions to vertically integrated force-balance equations for the lithosphere through time. Results show that deviatoric stresses associated with gravitational collapse of topography are consistent with early crustal stretch directions along the belt of metamorphic core complexes within the southwestern Cordillera. We also solve for stress field boundary conditions through time, such that the contributions from crustal body forces, together with boundary condition effects, can provide a total stress field that is consistent with the orientations of crustal strain through time. The boundary conditions suggest the need for additional extension imposed, first, on the Great Basin (between 36 – 20 Ma) and then on the southern Basin and Range (25 – 15 Ma). These calculations are consistent with the proposed slab rollback history of the Farallon slab [Dickinson, 2002]. Between 15 Ma and 5 Ma the boundary conditions are dominated by stresses associated with the development of the strike-slip plate boundary zone in California. Our calculations for the dynamics over time within the southwestern Cordillera suggest that following Sevier and Laramide shortening events, the lithosphere must have undergone dramatic weakening. This weakening, together with the changing boundary conditions, led to a dramatic collapse of a mountain belt that rivaled the present-day Andes in elevation. The most likely weakening mechanism is associated with the introduction of heat, fluids and volcanism, which can be linked to the slab rollback history of the Farallon plate.
Building: C.C. Little Building
Event Type: Lecture / Discussion
Tags: Lecture
Source: Happening @ Michigan from Earth and Environmental Sciences