Flow at ice-divide triple junctions: 2. Three-dimensional views of isochrone architecture from ice-penetrating radar surveys

Ground-based surveys of ice-divide triple junctions in two Antarctic ice rises, the Fletcher Promontory and Berkner Island, have been carried out using low-frequency ice-penetrating radars. These surveys have focused on understanding the internal layer architecture at and around the triple junctions, with a particular emphasis on understanding how the Raymond effect operates. A variety of features have been observed, including Raymond cupolas under the junctions, bump stacks of varying amplitude with distance from the divide, double bumps, crooked stacks implying divide motion, and the presence of ridges containing weak and strong bump stacks. It was generally not possible to elucidate the three-dimensional structure by correlation of pick crossovers, so instead a least squares cross-correlation technique was used which computes nominal age surfaces using radial basis function interpolation. Double bumps are absent from the central cupola of Fletcher Promontory, permitting us to infer that the ice here has thinned by around 500 m in the past 5000 years. Estimates of the age scale suggest that there will be recoverable Eemian ice. A technique for inferring the velocity profile at divides using layer traces and assumed velocities in the flanks is presented.