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Glacial Isostatic Adjustment: Inferences on properties and processes in the upper mantle from 3D dynamical modeling
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Observations of glacial isostatic adjustment (GIA) offers a powerful window into the properties of the Earth's interior. Combined with dynamical modeling of the GIA process we can use the observations to infer properties such as the elastic structure of the lithosphere, the rheology of the mantle and changes in the stress conditions in the Earth. This information aids our understanding of the long term evolution of the Earth, e.g. mantle convection, but also illuminates short term processes such as magma generation, earthquakes and shoreline migration. As present day warming trends causes glacier retreat world wide, GIA offers the opportunity to gain local insight into the Earth.

In this thesis I develop an implementation of the pre-stress advection term in finite element modeling. I apply this to current GIA in Iceland, and conclude that local variations in the elastic thickness of the lithosphere can potentially be detected close to the largest ice cap. I study the magnitude of dehydration stiffening in the uppermost Icelandic mantle. The results indicate that the increase in viscosity over the dry solidus is of small magnitude, implying a non-linear rheology in the uppermost mantle beneath Iceland. The present deglaciation in Iceland causes additional melting of the mantle. I find an increased melt production rate of 100-140% at present, although the melt supply rate at the base of the lithosphere is found to be delayed, with estimated present day perturbations ranging from neglible up to 120%.

In the last section of the thesis I focus on the role of ice sheet reconstructions in GIA modeling. I compare three reconstruction of the Weichselian ice sheet and discuss similarities and difference as well as the fit to present day uplift rates in Fennoscandia. The results provide input to improvements in the ice sheet models.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 83 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 906
National Category
URN: urn:nbn:se:uu:diva-169790ISBN: 978-91-554-8294-7 (print)OAI: diva2:507702
Public defence
2012-04-20, Hamberg, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Available from: 2012-03-29 Created: 2012-03-06 Last updated: 2012-04-19Bibliographically approved
List of papers
1. Implementation of the glacial rebound pre-stress advection correction in general-purpose finite element analysis software: Springs versus foundations
Open this publication in new window or tab >>Implementation of the glacial rebound pre-stress advection correction in general-purpose finite element analysis software: Springs versus foundations
2012 (English)In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 40, 97-106 p.Article in journal (Refereed) Published
Abstract [en]

When general-purpose finite element analysis software is used to model glacial isostatic adjustment (GIA), the first-order effect of prestress advection has to be accounted for by the user. We show here that the common use of elastic foundations at boundaries between materials of different densities will produce incorrect displacements, unless the boundary is perpendicular to the direction of gravity. This is due to the foundations always acting perpendicular to the surface to which they are attached, while the body force they represent always acts in the direction of gravity. If prestress advection is instead accounted for by the use of elastic spring elements in the direction of gravity, the representation will be correct. The use of springs adds a computation of the spring constants to the analysis. The spring constant for a particular node is defined by the product of the density contrast at the boundary, the gravitational acceleration, and the area supported by the node. To be consistent with the finite element formulation, the area is evaluated by integration of the nodal shape functions. We outline an algorithm for the calculation and include a Python script that integrates the shape functions over a bilinear quadrilateral element. For linear rectangular and triangular elements, the area supported by each node is equal to the element area divided the number of defining nodes, thereby simplifying the computation. This is, however, not true in the general nonrectangular case, and we demonstrate this with a simple 1-element model. The spring constant calculation is simple and performed in the preprocessing stage of the analysis. The time spent on the calculation is more than compensated for by a shorter analysis time, compared to that for a model with foundations. We illustrate the effects of using springs versus foundations with a simple two-dimensional GIA model of glacial loading, where the Earth model has an inclined boundary between the overlying elastic layer and the lower viscoelastic layer. Our example shows that the error introduced by the use of foundations is large enough to affect an analysis based on high-accuracy geodetic data.

National Category
Research subject
Geophysics with specialization in Solid Earth Physics
urn:nbn:se:uu:diva-159553 (URN)10.1016/j.cageo.2011.07.017 (DOI)000301624600009 ()
Available from: 2011-10-04 Created: 2011-10-04 Last updated: 2017-12-08Bibliographically approved
2. Glacial isostatic adjustment constrains dehydration stiffening beneath Iceland
Open this publication in new window or tab >>Glacial isostatic adjustment constrains dehydration stiffening beneath Iceland
2012 (English)In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 359-360, 152-161 p.Article in journal (Refereed) Published
Abstract [en]

During melting in the upper mantle the preferred partitioning of water into the melt will effectively dehydrate the solid residue. Linear extrapolation of laboratory experiments suggests that dehydration can produce a sharp viscosity contrast (increase) of a factor 500 across the dry solidus. In this study we show that the suggested magnitude of dehydration stiffening in a plume–ridge setting is incompatible with the present glacial isostatic adjustment (GIA) in Iceland. Using GPS observations of current GIA in Iceland, we find that the data are best fit by a viscosity contrast over the dry solidus in the range 0.5–3. A viscosity contrast higher than 10 requires a mantle viscosity below the dry solidus lower than , depending on the thickness of the dehydrated layer. A viscosity contrast of 100 or more demands a mantle viscosity of or less. However, we show here that a non-linear extrapolation of the laboratory data predicts a viscosity contrast as low as a factor 3–29, assuming conditions of constant strain rate to constant viscous dissipation rate. This is compatible with our GIA results and suggests that the plume–ridge interaction beneath Iceland is governed by a non-linear rheology and controlled by a combination of kinematic and dynamic boundary conditions rather than buoyant forces alone.

glacial isostatic adjustment, Iceland, dehydration stiffening, rheology, viscosity
National Category
urn:nbn:se:uu:diva-169787 (URN)10.1016/j.epsl.2012.10.015 (DOI)000312924200016 ()
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2017-12-07Bibliographically approved
3. Effects of present day deglaciation on melt production rates beneath Iceland
Open this publication in new window or tab >>Effects of present day deglaciation on melt production rates beneath Iceland
Show others...
2013 (English)In: Journal of Geophysical Research-Solid Earth, ISSN 2169-9313, Vol. 118, no 7, 3366-3379 p.Article in journal (Other academic) Published
Abstract [en]

Ongoing deglaciation in Iceland not only causes uplift at the surface but also increases magma production at depth due to decompression of the mantle. Here we study glacially induced decompression melting using 3-D models of glacial isostatic adjustment in Iceland since 1890. We find that the mean glacially induced pressure rate of change in the mantle increases melt production rates by 100–135%, or an additional 0.21–0.23 km3 of magma per year beneath Iceland. Approximately 50% of this melt is produced underneath central Iceland. The greatest volumetric increase is found directly beneath Iceland's largest ice cap, Vatnajökull, colocated with the most productive volcanoes. Our models of the effect of deglaciation on mantle melting predict a significantly larger volumetric response than previous models which only considered the effect of deglaciation of Vatnajökull, and only mantle melting directly below Vatnajökull. Although the ongoing deglaciation significantly increases the melt production rate, the increase in melt supply rate at the base of the lithosphere is delayed and depends on the melt ascent velocity through the mantle. Assuming that 25% of the melt reaches the surface, the upper limit on our deglaciation-induced melt estimates for central Iceland would be equivalent to an eruption the size of the 2010 Eyjafjallajökull summit eruption every seventh year.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2013
decompression melting, GIA, Iceland, mantle melting, volcanism, deglaciation
National Category
urn:nbn:se:uu:diva-169788 (URN)10.1002/jgrb.50273 (DOI)000324952300008 ()
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2013-11-04Bibliographically approved
4. Glacial isostatic adjustment in Fennoscandia, a comparativestudy of three ice sheet reconstructions
Open this publication in new window or tab >>Glacial isostatic adjustment in Fennoscandia, a comparativestudy of three ice sheet reconstructions
(English)Manuscript (preprint) (Other academic)
National Category
urn:nbn:se:uu:diva-169786 (URN)
Available from: 2012-03-06 Created: 2012-03-06 Last updated: 2012-04-19

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