Authors / CoAuthors
Zhao, S. | Lambeck, K.
Abstract
Modern geodetic techniques, especially the Global Positioning System (GPS) have allowed the accurate determination of the Earth's surface deformation of Glacial Isostatic Adjustment (GIA) associated with the ongoing stress release of the viscoelastic mantle after removal of the Late Pleistocene ice-sheets. We present an inversion analysis of the GPS derived deformation in North America to determine the effective lithosphere thickness and mantle viscosity, and examine whether the GPS observations can be fit with the ice-sheets and earth models, which were constructed and inferred mainly from geomorphologic/geological and relative sea level (RSL) data. The inversion computation is conducted for horizontal and vertical deformation, separately and jointly with two ice-sheet models (ANU-ICE and ICE-5G) developed independently by the Australian National University (ANU) and University of Toronto. The results from a simple three-layer earth model give a lithosphere thickness of 100~130km, an upper-mantle viscosity of 7~10 × 1020 Pa s, and a lower-mantle viscosity of 1.5~2.8 × 1021 Pa s. More sophisticated models such as introducing a transition zone of 400-670km failed to improve model fit, and the related parameters are mostly consistent with those of three-layer models. Further tests show that models of a thin-layer (30~40km) of large viscosity (~1022 Pa s) did not provide a better fit to the data. Ice scaling tests show that vertical deformation is more sensitive to local ice configuration. An increase of ice thickness by ~40% in Alberta and a reduction by ~50% between Saskatchewan and West Ontario are required to fit both horizontal and vertical deformation observed in Southwest Canada, whereas a reduction of ice thickness by ~25% for ANU-ICE produced an improved fit to both horizontal and vertical deformation in Quebec. Results from inversion analysis of two sub-datasets in Southwest and Southeast Canada revealed a 40% difference in the lower-mantle viscosity, which indicates that the lower-mantle in Southeast Canada could be relatively stronger. There is a discrepancy in the upper-mantle viscosity estimate between horizontal and vertical deformation: a low value (3~5 × 1020 Pa s) required by vertical deformation, and a high value (~9 × 1020 Pa s) favoured by horizontal deformation, which is due possibly to under-represented vertical deformation in the region as well as uncertainties in local ice topography. Overall, the earth model estimated from inversion analysis of GPS data in North America is consistent with the early inference from forward analyses of RSL data (e.g. Tushingham & Peltier, 1992): the lower-mantle viscosity is a factor of 1.5~2.0 larger than upper-mantle viscosity of ~1021 Pa s, reflecting that the main features of the earlier constructed North American ice-sheets (e.g. ICE-3G) are unchanged after two decade refinements.
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nonGeographicDataset
eCat Id
73724
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- External Publication
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2012-02-27T00:00:00
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