Investigating the effect of a weak lower crust on Basin and Range extensional history

The deformation mechanisms responsible for the extension and rifting in Basin and Range extension over the past ~36 Ma, and their relative importance remain debated. Slab rollback, lithospheric body forces, and relative plate motions have all been shown to contribute, but the relative importance of each mechanism is not fully understood. Here, we build three-dimensional (3D) steady state geodynamic models to simulate the full tectonic reconstruction of Basin and Range extension and compare these results with known geologic field observations and other detailed reconstructions of surface deformation. Our modeling approximates lithospheric deformation through Stokes flow in a spherical cap of variable viscosities. By applying reconstructed boundary conditions, crustal thickness, and surface elevation at 17 Ma, and varying lithospheric viscosity we map out the predicted response of the surface motions and lower crustal flow for different assumed lithospheric viscosity contrasts and investigate the origin of core complex formation. Comparisons between predicted model deformation and geologic field observations from metamorphic core complexes and exposed fluorite deposits indicate: (1) The primary driving force of the formation of geologic features in the western US is regional gravitational collapse focused in the lower crust. Plate motions are second order by comparison at this time period and act to rotate velocities near the plate boundary. (2) A weak lower crust facilitates metamorphic core complex formation and extension in the Nevadaplano. Lateral extrusion of the lower crust serves as a mechanism for both core complex formation and the flattening of the Moho that is observed at present day. (3) Lower crustal flow is a contributes to the rotation and tilt of the Colorado Plateau and formation of the Rio Grande Rift.