Glacial Isostatic Adjustment Data Research Articles

Asthenospheric rheology beneath mainland China inferred from mantle flow simulation and shear-wave splitting measurements

Mantle rheology is an essential property to understand the dynamics and evolution of the Earth. Estimates of mantle viscosity date from Haskell's canonical estimate of 1021 Pa s. For the asthenosphere, as a mechanically weak layer beneath the lithosphere, estimates have varied over a large viscosity range from 4 × 1017 to 1 × 1022 Pa s. In this paper, we use global and regional tomography-based geodynamic models with lateral viscosity variations in both the lithosphere and the mantle to predict the asthenospheric flow field, then use D-Rex to calculate transverse isotropy axis which is taken as the proxy of the fast polarization direction of shear-wave splitting, and finally determine the asthensopheric viscosity beneath Mainland China under the constraints of seismic anisotropy examined by a shear-wave splitting technique. Our inference, 1.27–5.58 × 1020 Pa s, lies in the range of previous estimates, and is comparable to most global estimates inferred from global convection-related observables, glacial isostatic adjustment data, global long wavelength geoid anomalies or their combination, but slightly larger than asthenospheric viscosity in Africa (3.5 × 1019 Pa s) and global estimates (3–10 × 1019 Pa s) inferred from the shear-wave splitting measurements. These findings show asthenospheric viscosity inferred from shear-wave splitting data has a small spatial change (< one order of magnitude).

Postseismic Deformation due to the 2012 Mw 7.8 Haida Gwaii and 2013 Mw 7.5 Craig Earthquakes and its Implications for Regional Rheological Structure

AbstractObservations of ∼7 years of postseismic deformation following the 2012 M w 7.8 Haida Gwaii and 2013 M w 7.5 Craig earthquakes in western British Columbia and southeastern Alaska provides insight into the regional rheologic structure and seismological hazards. A stress‐driven afterslip‐only model cannot alone explain rates or spatial patterns of the far field and later stage postseismic deformation measured by Global Positioning System (GPS). We thus develop an integrated model that fits the observations by combining afterslip on the deeper extension of the faults and distributed viscous flow in the asthenosphere. The best‐fit integrated model has a 60‐km thick lithosphere overlying a Burgers body asthenosphere with a Maxwell viscosity of 2 × 1019 Pa s (0.8–3 × 1019 Pa s at 95% confidence), with a Kelvin viscosity assumed equal to 10% of that value. These values are in good agreement with rheological models derived from glacial isostatic adjustment data. The early, near‐field postseismic deformation from both events is dominated by their corresponding afterslip, however, throughout the Craig postseismic period, the dominant mechanism in the far field is viscoelastic relaxation. For the intermediate region between the two earthquakes, both events contribute to the observed postseismic displacements. Because there was no obvious afterslip following the Haida Gwaii event or known large historical earthquakes on the southernmost part of the QCF, this region continues to have great potential to suffer a large earthquake in the future.

Inferences of Mantle Viscosity Based on Ice Age Data Sets: The Bias in Radial Viscosity Profiles Due to the Neglect of Laterally Heterogeneous Viscosity Structure

AbstractInferences of mantle viscosity using glacial isostatic adjustment (GIA) data are hampered by data sensitivity to the space‐time geometry of ice cover. A subset of GIA data is relatively insensitive to this ice history: the Fennoscandian relaxation spectrum (FRS), postglacial decay times in Canada and Scandinavia, and the rate of change of the degree‐2 zonal harmonic of the geopotential ( ). These geographically limited data have been inverted to constrain the radial (one‐dimensional [1D]) mantle viscosity profile. We explore potential biases in these 1D inversions introduced by neglecting a three‐dimensional (3D) viscosity structure. We perform 1D Bayesian inversions of synthetic GIA data generated from Earth models with realistic 3D variations in mantle viscosity and lithospheric thickness and compare results to the 1D viscosity profile associated with the 3D model used to generate the synthetics. Differences between these two 1D profiles reflect GIA data resolution and biasing introduced by neglecting, in the inversions, a 3D viscosity structure. We focus on the second issue, demonstrating that the largest bias occurs within the upper mantle (in particular, the transition zone). This remains consistent when varying inversion parameters (e.g., prior/starting models) and the 1D/3D viscosity fields adopted in generating the synthetics. Inversions of individual data sets show 3D biasing increases for data exhibiting shallower (thus more localized) sensitivity to viscosity. Of the data considered herein, inversions of the FRS are subject to the largest bias followed by decay time data. The bias is minimal for , as its deeper sensitivity is accompanied by broader averaging of structure in radial and lateral directions.

Inferences of mantle viscosity based on ice age data sets: Radial structure

AbstractWe perform joint nonlinear inversions of glacial isostatic adjustment (GIA) data, including the following: postglacial decay times in Canada and Scandinavia, the Fennoscandian relaxation spectrum (FRS), late‐Holocene differential sea level (DSL) highstands (based on recent compilations of Australian sea level histories), and the rate of change of the degree 2 zonal harmonic of the geopotential, J2. Resolving power analyses demonstrate the following: (1) the FRS constrains mean upper mantle viscosity to be ∼3 × 1020 Pa s, (2) postglacial decay time data require the average viscosity in the top ∼1500 km of the mantle to be 1021 Pa s, and (3) the J2 datum constrains mean lower mantle viscosity to be ∼5 × 1021 Pa s. To reconcile (2) and (3), viscosity must increase to 1022–1023 Pa s in the deep mantle. Our analysis highlights the importance of accurately correcting the J2 observation for modern glacier melting in order to robustly infer deep mantle viscosity. We also perform a large series of forward calculations to investigate the compatibility of the GIA data sets with a viscosity jump within the lower mantle, as suggested by geodynamic and seismic studies, and conclude that the GIA data may accommodate a sharp jump of 1–2 orders of magnitude in viscosity across a boundary placed in a depth range of 1000–1700 km but does not require such a feature. Finally, we find that no 1‐D viscosity profile appears capable of simultaneously reconciling the DSL highstand data and suggest that this discord is likely due to laterally heterogeneous mantle viscosity, an issue we explore in a companion study.

The sensitivity of GNSS measurements in Fennoscandia to distinct three-dimensional upper-mantle structures

Abstract. The sensitivity of global navigation satellite system (GNSS) measurements in Fennoscandia to nearby viscosity variations in the upper mantle is investigated using a 3-D finite element model of glacial isostatic adjustment (GIA). Based on the lateral viscosity structure inferred from seismic tomography and the location of the ice margin at the last glacial maximum (LGM), the GIA earth model is subdivided into four layers, where each of them contains an amalgamation of about 20 blocks of different shapes in the central area. The sensitivity kernels of the three velocity components at 10 selected GNSS stations are then computed for all the blocks. We find that GNSS stations within the formerly glaciated area are most sensitive to mantle viscosities below and in its near proximity, i.e., within about 250 km in general. However, this can be as large as 1000 km if the stations lie near the center of uplift. The sensitivity of all stations to regions outside the ice margin during the LGM is generally negligible. In addition, it is shown that prominent structures in the second (250–450 km depth) and third layers (450–550 km depth) of the upper mantle may be readily detected by GNSS measurements, while the viscosity in the first mantle layer below the lithosphere (70–250 km depth) along the Norwegian coast, which is related to lateral lithospheric thickness variation there, can also be detected but with limited sensitivity. For future investigations on the lateral viscosity structure, preference should be on GNSS stations within the LGM ice margin. But these stations can be grouped into clusters to improve the inference of viscosity in a specific area. However, the GNSS measurements used in such inversion should be weighted according to their sensitivity. Such weighting should also be applied when they are used in combination with other GIA data (e.g., relative sea-level and gravity data) for the inference of mantle viscosity.

A new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data

We present new profiles of mantle viscosity derived on the basis of non-linear, Occam-style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment (GIA). The convection related observables include satellite-derived free-air gravity harmonics, the geodetically inferred excess ellipticity of the CMB and tectonic plate motions. The GIA constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late Pleistocene ice history: the so-called Fennoscandian relaxation spectrum (FRS) and a set of site-specific decay times determined from the postglacial sea-level history in Hudson Bay and Sweden. The inverted viscosity profiles show a significant, three orders of magnitude, increase from the upper mantle (mean value of ∼4×10 20 Pa s) to a high-viscosity (>10 23 Pa s) peak at 2000 km depth, followed by a reduction toward the core-mantle boundary.

Glacial isostatic adjustment in Fennoscandia with a three-dimensional viscosity structure as an inverse problem

Glacial isostatic adjustment data are commonly used to invert for the radial viscosity structure of the mantle. However, the effects of lateral variations in mantle viscosity in such inversions are not yet accounted for. Here we analysed synthetic sea-level data for the Fennoscandian region, which are derived from a three-dimensional (3D) earth model with realistic lateral and vertical viscosity variations deduced from seismological and geological information. The inversion of the 3D synthetic data for a best-fitting 1D viscosity profile reveals that (i) lateral lithospheric thickness variations can be detected with 1D model predictions, if the data are grouped into regional subsets, (ii) combined lateral variations in lithospheric thickness and asthenospheric viscosity are not properly resolved with 1D model predictions, and (iii) the spatial and temporal distribution of observational data strongly affects the resulting 1D viscosity profile.

The inverse problem for mantle viscosity

The site-specific relaxation-time parametrization of relative sea level histories, for sites that were ice covered at the last glacial maximum, is herein applied in the context of formal inversions of the highest quality fraction of such data in order to infer the depth dependence of mantle viscosity. These data are jointly inverted together with additional relevant information that includes the relaxation spectrum for Fennoscandian rebound, originally derived by McConnell and two data derived from observed anomalies in the Earth's present rotational state. The latter include both the so-called `nontidal' component of the observed axial acceleration of rotation and the ongoing motion of the north pole of rotation relative to the surface geography at a rate near per million years that is directed southwards along the west meridian of longitude. The formal inversion of the totality of these data is based upon the Bayesian methodology of Tarantola and Valette, Jackson and Matsu'ura, and Backus. Both the preparation of the data that is required for the formal analysis and the derivation of the Fréchet kernels for the individual data types are discussed in some detail. The results obtained in these analyses, preliminary versions of which were recently described in abbreviated form elsewhere, appear to be rather significant. The variation of mantle viscosity with depth deduced from the glacial isostatic adjustment data is found to agree, within a constant scale factor and then rather precisely, with a model previously inferred on the basis of nonhydrostatic geoid anomalies related to the mantle convection process. Since the latter data are insensitive to a constant rescaling, this implies that the rheology of the bulk mantle may exhibit no significant transient behaviour and thus may well be Newtonian.

A space geodetic target for mantle viscosity discrimination: Horizontal motions induced by glacial isostatic adjustment

A model of mantle viscosity recently obtained on the basis of the inversion of glacial isostatic adjustment data is herein shown to predict a highly characteristic signature in tangential motion due to this cause.

Viscoelastic perturbations of the earth: significance of the incremental gravitational force in models of glacial isostasy

SUMMARY The incremental gravitational force (IGF) arises from perturbations of the earth's gravitational potential. In glacial isostasy, its sources are the surjicial and internal mass redistributions associated with the growth and decay of the continental ice sheets. We examine the errors caused by the neglect of the IGF using closed-form solutions of the equations governing surface-load induced perturbations of two types of incompressible, spherical-earth models: (1) Maxwellian-viscoelastic mantle enclosed by elastic lithosphere, and (2) Maxwellian-viscoelustic mantle enclosing inviscid core. Calculations in the Legendre domain of the radial surface displacement for these models show that neglecting the IGF causes enhancement of the elastic response and acceleration of the viscous relaxation. In the space domain, these changes entail corresponding modifications of the calculated land adjustment. The magnitude of the error caused by the neglect of the IGF strongly depends on the deglaciation history and load radius adopted. Assuming a typical deglaciation history, the error reaches a maximum of less than 20 m at the end of the deglaciation phase for loads comparable in size to the Canadian or the Fennoscandian ice sheets. We also compare sphericul-earth models with IGF and plane-earth models without IGF. Calculations of the radial surface displacement show that the errors due to the neglect of sphericity and the IGF partially compensate each other. Taking the uncertainties of the observational data into account, we conclude that the majority of the Canadian and Fennoscandian glacial-isostatic adjustment data can be modelled with sufficient accuracy using a plane-earth model without IGF.