Glacial Isostatic Adjustment of the Pacific Coast of North America: The Influence of Lateral Earth Structure

Abstract:

The Pacific Coast of Central North America is a geodynamically complex region which has been subject to various geophysical processes operating on different time scales. Glacial isostatic adjustment (GIA), the ongoing deformational response of the solid Earth to past deglaciation, is an important geodynamic process in this region. Due to the complexity in Earth structure, likely associated with the presence of the Cascadia subduction zone, conventional 1D (spherically-symmetric) GIA models are not capable of providing a quality fit to paleo observations of relative sea level (RSL) at the regional scale (Yousefi et al., Quaternary Science Reviews, 2018). In this study we apply Earth models with 3D structure to determine if the inclusion of lateral structure can explain the poor performance of the 1D models. Three different approaches are used to construct the 3-D realizations of the Earth structure. For the first approach, we adopt the optimal 1D viscosity structure from previous work and add the lateral variations based on four global seismic shear wave velocity anomalies and two global lithosphere thickness models. The results based on these models indicate that the spatial resolution is not sufficient to resolve the data-model discrepancies associated with the 1D model. The two other approaches resulted in higher resolution models of 3D structure by, in one case, inserting a regional seismic model (Hawley and Allen, Geophysical Research Letters, 2019) into two of the global seismic models and, in the other case, explicitly incorporating regional structure of the Cascadia subduction zone, i.e., the subducting slab, the overlying mantle wedge, and the plate boundary interface. The results associated with these higher resolution models do not reveal any clear improvement in satisfying the RSL observations, suggesting that our estimates of lateral structure are inaccurate and/or the data-model misfits are primarily due to limitations in the adopted ice-loading histories. The application of different realizations of the 3D Earth structure in this study gives insight into the spread of modelled RSL values and is a necessary step towards estimating model uncertainty due to lateral variability in Earth viscosity structure.