Unique dynamic behaviors of ice divides: Siple Dome and the rheological properties of ice.

摘要

The constitutive relationship between applied stress and deformation rate of ice controls the dynamic behavior of the divide regions of ice sheets. I use finite-element modelling to explore three aspects of flow near a divide: (1) the increased relative activity of linear creep mechanisms at low stress, (2) the impact of sliding on stratigraphy, and (3) the role of crystal fabric in deformation.; Raymond (1983) showed that a special flow pattern emerges near an ice divide when ice is modelled using Glen's flow law. I show that the dominance of linear creep mechanisms at low stress tends to decrease the prominence of the special divide flow pattern. No Raymond bump forms in the isochrones, and younger ice appears deeper in the ice column, when compared to a more conventional Glen divide. When nonlinear rheological properties are coupled with a strongly anisotropic fabric, the special divide flow pattern is enhanced. Crystal fabric has little effect when the linear term dominates deformation rate. Finally, my model results show that basal sliding tends to redistribute the longitudinal stresses within the ice such that the special divide flow pattern is suppressed.; I use these results and available data to study Siple Dome, West Antarctica. The divide region of Siple Dome is presently in steady state, it has thinned at most 40 meters in the last 2000 years, and has been an elevated dome-like feature for much of Holocene. This contrasts with other sites around the Ross Sea Embayment with show modern thinning.; Using unique measurements of vertical strain throughout the depth of Siple Dome together with a finite-element flow model, I assess the relative importance of the linear term in the flow law compared to the effect crystal fabric. The linear term does contribute to flow at Siple Dome; the crossover stress is k = 0.22 bar. The band of strong crystal fabric around 750 m depth modifies the divide flow pattern, and, on the flanks, shear strain is concentrated within this layer, rather than in the deeper basal ice, creating a false-bed effect.