VARYING LITHOSPHERIC THICKNESS CONTROLS MANTLE AND CRUSTAL DEFORMATION IN WESTERN US

The western U.S. (WUS) is one of the most tectonically active regions on Earth, with a complex crustal deformation pattern. The Basin and Range (B&R) has been extending in the northwest-southeast orientation, while the Great Valley and Colorado Plateau remain relatively undeformed. Along the east boundary of B&R, a prominent intermountain seismic belt (ISB) forms. In addition, geodetic studies reveal a clockwise rotation of crustal motion in the northwestern U.S. relative to the stable central and eastern U.S. Both the driving force for this complex crustal deformation pattern and the origin of the ISB remain debated. Previously proposed mechanisms include lateral gradients of gravitational potential energy (GPE) and dynamic topography due to mantle convection, but a model that could simultaneously explain the formation of intraplate earthquakes, crustal motion inferred by GPS, and seismic anisotropy is still lacking.

Here, we utilize a data-oriented geodynamic modeling approach to simultaneously investigate the WUS lithospheric and mantle dynamics. In this model, the structure of convecting mantle is based on a data-assimilation approach that satisfies multiple geophysical and geological observations in the WUS. We then combine this mantle structure with the seismically inferred crust and mantle lithosphere to reproduce the observed surface topography, crustal stress state, GPS velocity, and asthenospheric flow that satisfies observed anisotropy. Our results show that the main driving force for crustal deformation varies across different tectonic regions, with plate boundary force controlling the coastal region, the gradient of GPE dominant along the ISB, and the mantle flow affecting the entire WUS, especially the pattern of crustal motion. We find that the lithospheric thickness variation plays a key role in defining this dynamic system. The thick lithosphere along the east boundary of B&R blocks the eastward mantle flow, as explains the sudden change in the observed seismic anisotropy, modulates crustal stress, as well as forms the rotational WUS crustal motion. Furthermore, the overlapping regions of predicted strong lithospheric deformation and mantle upwelling highly correlate with the recent WUS volcanism, better than with only the widespread slow seismic anomalies beneath the lithosphere.