Articles techniques et présentations

The Basin and Range province in western North America is known primarily for its unique topography with sharp changes in elevation and complex deformational history. There is general agreement that the structure of the Basin and Range is the result of large-scale extension and crustal thinning which resulted in the exposure of metamorphic core complexes to the surface (Bahadori et al., 2018). These core complexes provide information on the direction of lower crustal flow during formation from their orientation and are evidence of crustal extension and lower crustal exhumation (Wernicke et al., 1992). During a period of extension in the Basin and Range at 17 Ma, the Colorado Plateau experienced uplift and tilting without any internal deformation; this late Cenozoic rotation of the Colorado Plateau has been linked to extensional behavior in the Rio Grande Rift, playing a role in its formation (Chapin & Cather, 1994). We investigate the role of gravitational collapse in generating the observed geologic evolution of the western US from 17-14 Ma using a 3-D time-dependent finite element model implemented with COMSOL Multiphysics. The instantaneous physics is defined using COMSOL’s creeping flow functionality (Calvelage 2021). Starting with paleotopography, crustal thickness, and 3-D viscosity and density distributions (Bahadori et al., 2019), we then allow our model to deform in response to deviatoric stresses associated with gravity acting on high topography. This, in combination with the presence of a comparatively weak lower crustal layer, allows us to examine the resulting propagation of lower crustal flow and its impact on distribution of surface deformation. This analysis is done without including plate tectonic boundary conditions in order to explore the impact of gravity on metamorphic core complex formation in isolation. The forward modeling is done using COMSOL’s deforming domain moving mesh. By seeding the lower crust with a particle field, we can utilize COMSOL’s particle tracing module in order to quantify crustal thinning and relate that to locations of known metamorphic core complexes.

We find that the inclusion of a laterally varying viscosity in the lower crust produces models that support the high topography of the Colorado Plateau, while simultaneously allowing for collapse and extension in the Nevadaplano region. Flow of the lower crust in these models is consistent with the rotation of the Colorado Plateau and the opening of the Rio Grande Rift; in addition, we observe both surface extension and crustal thinning in areas consistent with known active core complexes during this time period. With this model we are able to reconstruct conditions favorable for metamorphic core complex formation in the American southwest with only the influence of gravity acting on high topography and a weak lower crust of laterally varying strength, and without considering velocity boundary conditions from tectonic plate motion.