Inference of the viscosity structure and mantle conditions beneath the Central Nevada Seismic Belt from combined postseismic and lake unloading studies

Abstract

We test whether a single depth‐dependent Newtonian viscosity structure can be found to explain measured surface deformation in Western Nevada from two separate loading events: tectonic loading from a series of seven historic earthquakes in the Central Nevada Seismic Belt and nontectonic loading from the formation and evaporation of co‐located Pleistocene‐aged Lake Lahontan. Rheologic studies are generally plagued with nonuniqueness issues due to the limitations of observational constraints. Here, we reduce nonuniqueness by solving for a single rheologic structure that can simultaneously satisfy all observational constraints associated with all events. Model results suggest that Western Nevada is underlain by a strong lower crust (order 1020 Pa s), a relatively weak mantle (order 5 × 1018 Pa s) from 40 to 80 km, and a much weaker mantle (order 1018 Pa s) below 80 km. We would thus place the mechanical lithosphere/asthenosphere boundary (LAB) at 40 km depth. Thermal modeling of conductive geothermal gradients, combined with melting curves calculated for enriched and depleted mantle compositions suggest that the viscosity decrease at 40 km depth (the LAB) is associated with the onset of wet melting of mantle lithosphere hydrated by past subduction and is about 10 km shallower than the inferred transition from conduction to convection.

Constraints on mantle viscosity and Laurentide ice sheet evolution from pluvial paleolake shorelines in the western United States

“…This minimum viscosity is 90 comparable to the values obtained for the Lake Bonneville region but might extend over a larger 91 depth range (Bills et al, 2007). Post-seismic studies from Lake Lahontan find slightly lower 92 viscosities over the same depth range but overlap within uncertainty with the lake rebound 93 obtained viscosities (Dickinson et al, 2016). 94 95 These Basin and Range sub-lithospheric viscosity estimates are significantly lower than global 96 average estimates at this depth of ~5 x 10 20 Pa s, obtained from observations derived from 97 postglacial rebound (Peltier et al, 2015).…”

GPS Imaging of vertical land motion in California and Nevada: Implications for Sierra Nevada uplift

“…While estimates of the viscosities of the lower crust and upper mantle differ somewhat among the various studies [ Thatcher and Pollitz , ], the interpretation of the uplift anomaly as a long‐lived viscoelastic response seems to be agreed upon. More detailed modeling with realistic rheological properties that integrates coseismic, early, and later postseismic deformation suggests that a symmetric uplift response in the later postseismic period is expected, that the deformation source is mantle flow immediately beneath the crust, and that the mechanical lithosphere is ~40 km deep [ Thompson and Parsons , ; Dickenson et al , ].…”

Glacial Isostatic Adjustment modelling: historical perspectives, recent advances, and future directions

“…There is the additional complication that afterslip must be accounted for in post-seismic studies (Ingleby and Wright, 2017), but in general, the changing deformation rates observed during an earthquake cycle suggest that the Earth either follows a power-law rheology (Freed and Burgmann, 2004;Freed et al, 2006), or a rheology comprising several different 820 relaxation times (Pollitz, 2005;Hetland and Hager, 2006). Is there a single rheological law that can explain GIA, post-seismic deformation, intra-plate deformation, and deformation in response to sediment or lake loading (Gilbert, 1890;Dickinson et al, 2016)? It is clear that the Earth behaves differently over different timescales (Burgmann and Dresen, 2008;Watts et al, 2013), and it will be interesting to see how parallel fields of research progress towards quantifying the rheological structure of the solid Earth, and the degree to which the rheology reflects the forcing that is applied.…”