Modern Horizontal Crustal Motions in Victoria Land, Antarctica: Influence of Heterogeneous Earth Structure on Solid Earth Deformation

Measurements of crustal motion in regions of ice mass change provide a direct observation of glacial isostatic adjustment (GIA). In Antarctica, vertical surface motions derived from GPS have been used to assess GIA model accuracy, whereas horizontal motions have been underutilized. Modeling studies have shown that the pattern and magnitude of horizontal crustal motions are strongly influenced by lateral variations in lithospheric thickness and, particularly, in mantle viscosity. We present a set of GPS horizontal crustal motion data for Victoria Land, Antarctica, a region that spans the boundary in earth properties between cratonic East Antarctica and rifted West Antarctica. For more than a decade, GPS has been used to measure bedrock motion as part of the TransAntarctic Mountain DEFormation (TAMDEF) and the Antarctic Network (ANET)/Polar Earth Observing Network (POLENET) projects, providing a robust set of crustal deformation data. Observed horizontal surface motions in Victoria Land, after plate motion is removed, are to the southeast, which is toward, rather than away from the site of major ice mass loss in West Antarctica modeled in ice sheet reconstructions. GPS results do not document divergent displacements across the East-West Antarctic rift margin or offshore, indicating the measured motions are not tectonic. The uniform pattern of crustal motions are not compatible with rift-related extension and must be attributed to GIA, yet the motion directions are opposite to crustal strains predicted by GIA models. Southeast-directed horizontal crustal motions could be driven by ice mass loss within the East Antarctic Ice Sheet to the northwest, but there is little glaciological data in support of this model. Alternatively, the horizontal crustal motions could result from the dominating influence of the rheological boundary in this region, with surface motions driven by mantle flow from a more rigid East Antarctica to a weaker West Antarctica. This interpretation is supported by a deflection in the orientation of horizontal crustal motions across the marked change in both crustal thickness and mantle properties beneath Victoria Land, as indicated by seismology. Specifically, the change in orientation of motions is spatially coincident with the East-West Antarctic earth properties boundary, and motions on the weaker, West Antarctic side are perpendicular to the boundary. These observations suggest that horizontal crustal motions differ from current models of GIA for Antarctica because of the strong influence of heterogeneity in earth structure on solid earth deformation. As models of GIA for Antarctica move towards incorporating lateral variations in earth structure, observations spanning the rheological boundary between East and West Antarctica, such as those that we present here, are of particular importance.