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The Pacific Coast of Central North America is a geodynamically complex region which has been subject to various geophysical processes operating on different timescales. Glacial isostatic adjustment (GIA), the ongoing deformational response of the solid Earth to past deglaciation, is an important geodynamic process in this region. In this study, we apply earth models with 3-D structure to determine if the inclusion of lateral structure can explain the poor performance of 1-D models in this region. Three different approaches are used to construct 3-D models of the Earth structure. For the first approach, we adopt an optimal 1-D viscosity structure from previous work and add 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 addition of lateral structure significantly impacts modelled RSL changes, but the data-model fits are not improved. The global seismic models are limited in spatial resolution and so two other approaches were considered to produce higher resolution models of 3-D structure: inserting a regional seismic model into two of the global seismic models and, explicitly incorporating regional structure of the Cascadia subduction zone and vicinity, that is 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 history. The different realizations of 3-D Earth structure gives useful insight to uncertainty associated with this aspect of the GIA model. Our results indicate that improving constraints on the deglacial history of the southwest sector of the Cordilleran ice sheet is an important step towards developing more accurate of GIA models for this region. [ABSTRACT FROM AUTHOR]

We determine the contribution of glacial isostatic adjustment ( GIA) to future relative sea-level change for the North American coastline between Newfoundland and Texas. We infer GIA model parameters using recently compiled and quality-assessed databases of past sea-level changes, including new databases for the United States Gulf Coast and Atlantic Canada. At 13 cities along this coastline, we estimate the GIA contribution to range from a few centimeters (e.g., 3 [−1 to 9] cm Miami) to a few decimeters (e.g., 18 [12-22] cm, Halifax) for the period 2085-2100 relative to 2006-2015 (1− σ ranges given). We provide estimates of uncertainty in the GIA component using two different methods; the more conservative approach produces total ranges (1− σ confidence) that vary from 3 to 16 cm for the cities considered. Contributions from ocean steric and dynamic changes as well as those from changes in land ice are also estimated to provide context for the GIA projections. When summing the contributions from all three processes at the 13 cities considered along this coastline, using median or best-estimate values, the GIA signal comprises 5-38% of the total depending on the adopted climate forcing and location. The contributions from ocean dynamic/steric changes and ice mass loss are similar in amplitude but with spatial variation that approximately cancels, resulting in GIA dominating the net spatial variability north of 35°N. [ABSTRACT FROM AUTHOR]

Highlights • Atlantic and Gulf coast relative sea level data require peripheral bulge suppression. • Bulge suppression requires high mantle viscosity in 1-D Earth models. • Bulge suppression also obtained by adding plausible lateral viscosity variations. • Lateral viscosity structure can thus reconcile discrepant 1-D viscosity inferences. Abstract The isostatic response of the Earth to past mass exchange between ice sheets and oceans, so-called glacial isostatic adjustment (GIA), is an important geodynamic process in North America. Modelling GIA observables provides one of the few direct methods for estimating Earth viscosity structure. We investigate here the requirement for anomalously high mantle viscosities to fit the relative sea level (RSL) data along the Atlantic and Gulf coasts of North America found in recent GIA studies. We demonstrate that this requirement is primarily related to the modelled geometry and evolution of the peripheral bulge of the Laurentide ice sheet. We show that a 3D Earth model with a global average viscosity in the upper mantle of 0.3 × 10 21 Pa s and 3 × 10 22 Pa s in the lower mantle, which is consistent with both the Richmond Gulf (Hudson Bay) relaxation time and several recent global analyses, is able to produce a much improved fit to the Gulf and Atlantic coast RSL data relative to the 1D Earth model results when no lateral structure is applied. Therefore, we conclude that realistic implementations of lateral structure can explain the markedly different viscosity inferences obtained using 1D GIA models and RSL data from different regions of North America. A necessary caveat is that there are significant differences among the computed RSL curves corresponding to the three different realisations of lateral structure considered here, demonstrating significant uncertainty associated with this model input. [ABSTRACT FROM AUTHOR]