Constraints on Crustal Structures, Residual Topography, and Isostasy in the Western US from Virtual Deep Seismic Sounding (VDSS)

Abstract:

How surface topography is supported at depth is a long-standing question in geodynamics. Overall, the western United States (US) stands high compared to the North American craton, but it has remained a challenge to distinguish crustal and mantle support of this topography due to the complex tectonic history of the western US and inadequate knowledge of crustal structure.

We provide new seismological constraints on crustal structure using virtual deep seismic sounding (VDSS). VDSS uses SV-to-P wave conversion at the free surface near each seismograph as a virtual source, which, in turn, generates a strong, post-critical reflection off the Moho. This signal remains robust even if the Moho is complex or transitional in nature. Compared to traditional receiver functions, VDSS is less prone to contamination by scattering from other crustal structures, such as thick sediments or intra-crustal discontinuities. More important, VDSS can provide simultaneous constraints on both the total thickness and the overall P-wave speed of the crust – two key parameters for estimating the crustal contribution to isostasy.

Based on data from USArray (EarthScope), we estimate the residual topography (that is, the difference between observed elevation and that predicted from the Airy model given the inferred crustal structure) in the western US (Figure 1). Positive values, indicative of mantle-supported topography, are wide spread in the Great Basin, the southern Rocky Mountains, the Snake River Plain-Yellowstone system, and the High Lava Plains. The periphery of the Colorado Plateau, the Central Sierra Nevada batholith, and the Idaho batholith also show positive residual topography. In contrast, our analysis suggests that thick crust in the interior of the Colorado Plateau and the Wyoming craton provides more than enough support for the topography, consistent with a thick, cold lithospheric root below.