THE ROLE OF LITHOSPHERE STRUCTURE AND LARGE-SCALE MANTLE FLOW IN THE FORCES THAT AFFECT NORTH AMERICA

In this paper we present an overview of the origin of stresses that impact the North American plate. The USArray portion of EarthScope has provided researchers the opportunity to determine details regarding crust and upper mantle structure. These model results are now providing new vital constraints on the distribution of body forces within the lithosphere. New details are also emerging from seismic tomography studies. Tomography results can be used to infer density distributions within the mantle, which drive mantle flow. This mantle flow gives rise to radial and tangential tractions at the base of the lithosphere, which impacts the stress field within the lithosphere. We use a finite element approach to calculate lithosphere stresses, strain rates, and surface motions arising from the effects of (1) topography and lithosphere structure, and (2) coupling with 3-D mantle flow. Models possess both radial and lateral (lithosphere) viscosity variations. Model results are compared with plate motions, stress orientations from the World Stress Map (WSM), and surface deformation patterns in western North America, inferred primarily from the EarthScope Plate Boundary Observatory (PBO) Global Positioning System (GPS) observations. We test a variety of recent tomography models, and also a range of radial viscosity profiles. The dominant mantle flow pattern that has a profound influence on stresses and deformation patterns within North America is associated with the history of subduction of the Farallon Plate. The mantle flow pattern from this Farallon subduction history has a dominant influence on stresses within eastern North America, but stresses there are also strongly modulated by the crustal structure variations from the Appalachians into the continental margin setting. Within western North America lithospheric structure (gravitational potential energy), or GPE differences, likely dominate over the influence from mantle flow. There is significant evidence that the long-wavelength pattern of mantle flow, associated with the deeper, older Farallon system, is disrupted or canceled, in part, by patterns of smaller-scale convection beneath western North America. This smaller-scale convection is most likely associated with the more recent subduction history, but lithosphere delamination may also play a role.