Quantifying the forebulge of the last glaciation

A glacial forebulge is a load-driven bending-related upheaval of the lithosphere outside a glaciated area. As a typical feature of the glacial isostatic adjustment process the forebulge forms contemporaneously to the depression of the lithosphere below the ice sheet. Forebulge development and collapse related to the last glaciation has led to significant topographic changes in the order of several tens of meters in North America and Europe. Furthermore, forebulge behaviour has a significant effect on the evolution of lithospheric stresses, which can induce intraplate earthquakes, even in areas that were not covered by an ice sheet. Therefore, quantifying the present-day position, amplitude and subsidence of the forebulge is crucial for the estimation of future sea-level changes, the evolution of fluvial networks and understanding the distribution of deglaciation seismicity. Though the forebulge of the last glaciation attracted attention over more than one century, quantitative descriptions on the geometry and position of the forebulge are still rare. Key controlling factors for the position, amplitude and dynamic behaviour of the forebulge are the flexural rigidity of the lithosphere, asthenospheric flow processes, as well as ice-sheet geometry and history. Numerical simulations indicate that a higher flexural rigidity of the lithosphere leads to a lower amplitude of the forebulge and a greater distance to the load. Forebulge formation is also supported by the flow of asthenospheric material, which can occur as channel-flow or deep flow. In case of channel-flow, the forebulge shows an outward migration during collapse, whereas deep-flow leads to an inward migration. A non-linear mantle rheology is seen as a reason for stationary forebulge collapse. The height of the glacial forebulge of the last glaciation was in a range of several tens of meters, with a greater height in North America than in Europe due to the larger Laurentide ice sheet.