Flow at ice-divide triple junctions: 1. Three-dimensional full-Stokes modeling

Ice domes are either axisymmetric, high points along ridges, or ridge triple junctions. We model time-dependent isothermal flow near triple junctions, solving the full set of mechanical equations with a nonlinear power law rheology. Forcing is applied through the boundary conditions, which affect flow patterns at outlets. Where such forcing is purely axisymmetric, an axisymmetric dome is formed. If a threefold symmetry in the forcing is applied, the axisymmetric dome breaks up into three ridges subtending angles of 120 degrees. Sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state. Here, in steady state, the ridges join the triple junction at nearly 120 degrees, but one ridge curves to satisfy the boundary forcing. The slope pattern in the immediate dome vicinity depends only on a dimensionless parameter, which is a function of the ice consistency, the accumulation, and the rheological power law index. Attempts to replicate the topography around Summit, Greenland, obtained a good fit with n = 3. At a triple junction the dome is really distinct from the surrounding ridges, contrary to the highest point of a single ridge divide. As a consequence, the Raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center. Along the ridges leaving the dome, the Raymond effect is still present and decreases with the ratio of the flow across and along the ridge. In the vicinity of the dome, horizontal strain rates vary strongly from uniaxial to biaxial. Large-scale effects, represented in our model as fluxes at boundaries, seem to be the primary controls on dome position and shape.

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