Origin of Three-Dimensional Crustal Stress Over the Conterminous United States

The crustal stress field determines continental deformation, including intraplate seismicity and topographic undulations. However, the sources of observed crustal stress patterns remain debated, with proposed mechanisms including lateral variations in gravitational potential energy and mantle flow, the latter of which comprises plate boundary interactions and basal tractions. Here, we present a series of geodynamic models that simultaneously consider lithospheric and mantle dynamics in the same physical framework, based on which we investigate the sources of crustal stress over the conterminous U.S. The data-oriented nature of these models allows us to systematically explore the relative contributions of different dynamic sources to the three-dimensional crustal stress field. These models reveal that forces from the plate boundaries play a dominant role in generating the directional pattern of long-wavelength horizontal crustal stress across the conterminous U.S. In the central U.S., especially regions of high-topography, lithospheric density heterogeneities locally modify the crustal stress field. Similarly, mantle flow beneath the North American plate modulates crustal stress orientation in the eastern U.S., particularly in regions with thin lithosphere. Furthermore, we find that a denser-than-ambient lithospheric mantle beneath the central and eastern U.S. is required to match the observed continental-scale E-W topographic contrast. Plain Language Summary The origin of crustal stress within continents, such as the conterminous U.S., is a fundamental and contentious question in current geoscience research. Traditionally, the geoscience community has viewed the stress patterns within continents as due to forces applied along plate edges. However, many studies demonstrate that the forces exerted by mantle convection at the lithosphere base and the forces caused by laterally and radially varying lithospheric density structure are all critical elements of crustal stress. Here, we use numerical models to systematically investigate the relative importance of different crustal stress contributors, focusing on the conterminous U.S. Unlike previous studies, we simulate lithospheric and mantle dynamics simultaneously in one physical frame and examine horizontal principal stresses and surface elevation at the same time. Our results show that the observed crustal stress pattern mainly reflects the mantle convection effect, with modification from laterally and radially varying lithospheric density structure in regions away from plate boundaries. Also, a denser-than-ambient lithospheric mantle beneath the central and eastern U.S. is essential to reproduce the observed surface elevation over the conterminous U.S.