East Antarctic ice mass loss as an explanation for GPS observations of horizontal bedrock motion

The Antarctic Network (ANET) component of the Polar Earth Observing Network (POLENET) records GPS observations of the solid earth response to ice mass change in Antarctica. The densest sector of the network falls along the Transantarctic Mountains, spanning the boundary between the markedly different ice history and earth structure profiles of East and West Antarctica. Observed horizontal motions in the region are towards West Antarctic centers of major ice mass loss modelled in ice sheet reconstructions, opposite to the pattern of radial crustal motion expected in an unloading scenario. We investigate the viability of East Antarctic ice mass loss as an explanation for the observed pattern of deformation. We alter the W12 ice history model to incorporate unloading in the Wilkes Subglacial Basin after the last glacial maximum (LGM) by increasing past ice thickness in the region, with maximum loading at 20 ka decreasing linearly to 2 ka. A `realistic' ice model scenario is developed based on available glaciological records and invoking a wedge-shaped loading profile with the magnitude of thickening greatest at the grounding line and smallest/zero towards the interior. A less conservative case with an increased magnitude of thickening and a uniform load pattern across the entire Wilkes region was also tested to amplify the degree of unloading. Each W12-modified ice history scenario was coupled with 56 different earth model combinations, and resulting predicted horizontal motions compared with ANET GPS observations. Both the direction and magnitude of observed horizontal motions are well-matched in the northern sector of the study region where simulated Wilkes loading is strongest. Additionally, predicted and observed motions agree for a subset of stations exhibiting small magnitude motion located within the East Antarctic craton, where influences from West Antarctic and simulated Wilkes unloading intersect. Our results support the case for ice unloading in East Antarctica since the LGM as a viable explanation for observed present-day bedrock motion.