Geophysical constraints on mantle viscosity and its influence on Antarctic glacial isostatic adjustment




Darlington, Andrea

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Glacial isostatic adjustment (GIA) is the process by which the solid Earth responds to past and present-day changes in glaciers, ice caps, and ice sheets. This thesis focuses on vertical crustal motion of the Earth caused by GIA, which is influenced by several factors including lithosphere thickness, mantle viscosity profile, and changes to the thickness and extent of surface ice. The viscosity of the mantle beneath Antarctica is a poorly constrained quantity due to the rarity of relative sea-level and heat flow observations. Other methods for obtaining a better-constrained mantle viscosity model must be investigated to obtain more accurate GIA model predictions. The first section of this study uses seismic wave tomography to determine mantle viscosity. By calculating the deviation of the P- and S-wave velocities relative to a reference Earth model (PREM), the viscosity can be determined. For Antarctica mantle viscosities obtained from S20A (Ekstrom and Dziewonski, 1998) seismic tomography in the asthenosphere range from 1016 Pa∙s to 1023 Pa∙s, with smaller viscosities beneath West Antarctica and higher viscosities beneath East Antarctica. This agrees with viscosity expectations based on findings from the Basin and Range area of North America, which is an analogue to the West Antarctic Rift System. Section two compares bedrock elevations in Antarctica to crustal thicknesses, to infer mantle temperatures and draw conclusions about mantle viscosity. Data from CRUST 2.0 (Bassin et al., 2000), BEDMAP (Lythe and Vaughan, 2001) and specific studies of crustal thickness in Antarctica were examined. It was found that the regions of Antarctica that are expected to have low viscosities agree with the hot mantle trend found by Hyndman (2010) while the regions expected to have high viscosity are in better agreement with the trend for cold mantle. Bevis et al. (2009) described new GPS observations of crustal uplift in Antarctica and compared the results to GIA model predictions, including IJ05 (Ivins and James, 2005). Here, we have generated IJ05 predictions for a three layered mantle (viscosities ranging over more than four orders of magnitude) and compared them to the GPS observations using a χ2 measure of goodness-of-fit. The IJ05 predictions that agree best with the Bevis et al. observations have a χ2 of 16, less than the null hypothesis value of 42. These large values for the best-fit model indicate the need for model revisions and/or that uncertainties are too optimistic. Equally important, the mantle viscosities of the best-fit models are much higher than expected for West Antarctica. The smallest χ2 values are found for an asthenosphere viscosity of 1021 Pa•s, transition zone viscosity of 1023 Pa∙s and lower mantle viscosity of 2 x 1023 Pa∙s, whereas the expected viscosity of the asthenosphere beneath West Antarctica is probably less than 1020 Pa∙s. This suggests that revisions to the IJ05 ice sheet history are required. Simulated annealing was performed on the ice sheet history and it was found that changes to the recent ice load history have the strongest effect on GIA predictions.



glacial isostatic adjustment, Global Positioning System, crustal thickness, elastic lithosphere thickness, mantle viscosity, Antarctica, post glacial rebound, seismic tomography, ice sheet thickness, West Antarctic Rift System, Transantarctic Mountains, Marie Byrd Land, Ellsworth Land, Antarctic Peninsula, East Antarctica, simulated annealing