An Integrated Approach to Understanding the Driving Forces of Deforming Western North America

We examine the relative contribution of driving forces involved in the large-scale continental deformation of western North America. Through joint modeling of lithospheric dynamics and large-scale mantle convection we quantified the total lithospheric deviatoric stress field within the deforming western North American Plate. The total deviatoric stress field is the sum of contributions from deviatoric stress fields associated with internal lithospheric buoyancy forces, basal tractions, and relative plate motions. Vertically averaged deviatoric stresses associated with gravitational potential energy (GPE) variations within the lithosphere are determined by a finite element approach using GPE estimates resulting from the ETOPO5 topographic data set, the EGM96 geoid model, and the CRUST2.0 seismic crustal thickness model. Deviatoric stresses associated with basal tractions at the base of the lithosphere are also calculated using a finite element approach where radial and shear tractions are inputs. These tractions are estimated from three-dimensional large-scale mantle circulation models, where we match the surface topography, geoid, and surface motions. Finally, in an iterative inversion we solve for the stress field boundary condition that is added to the stress fields associated with GPE and basal tractions that yields a total deviatoric stress field. The stress field boundary condition is determined such that the total deviatoric stress field is a best-fit to the surface observables (i.e., the styles of principal kinematic strain rate axes). We produce different circulation models with varying lithospheric and asthenospheric viscosities to identify geographic regions where mechanical coupling or mechanical de-coupling occurs between the asthenosphere and the lithosphere. The optimum solution for the basal tractions places constraints on the magnitude of the viscosity in the asthenosphere and the distribution of effective viscosity in the lithosphere underlying western North America.