Abstract Quantitative stress measurements related to the development of mesoscale structures in rock are difficult, if not impossible. A method for determining the stress distribution and history during the development of mesoscale boudinage structures in ice is introduced here. Boudinage structures in fracture trace ice have been observed in the outlets glaciers of the Framnes Mountains, east Antarctica. Fracture traces are preserved when crevasses fill with surface melt water, which freezes to form coarse-grained columnar ice. A 4.0 km flow-parallel traverse across the Central Ice Stream of the Framnes Mountains is presented to illustrate the boudinage of fracture traces with a mean width of 0.30 m. Field-based measurement of the geometric evolution of boudinage structures has been combined with surface flow rate measurements to quantitatively determine the strain rate at which the boudinage structures formed. The strain rate measurements provide boundary constraints on several two-dimensional finite difference models that have been used to analyse the stress distribution related to the formation of boudinage structures in a visco-elastic solid. The results reveal the development of pressure-shadows during the boudinage of layered rocks, and demonstrate the degree of refraction of stress across rheological boundaries, which have important ramifications for the analysis of planar and linear fabrics in rocks.

Abstract The break-up of Gondwana during the Early–Middle Jurassic was associated with flood basalt volcanism in southern Africa and Antarctica (Karoo–Ferrar provinces), and formed one of the most extensive episodes of continental magmatism of the Phanerozoic. Contemporaneous felsic magmatism along the proto-Pacific margin of Gondwana has been referred to as a silicic large igneous province, and is exposed extensively in Patagonian South America, the Antarctic Peninsula and elsewhere in West Antarctica. Jurassic-age silicic volcanism in Patagonia is defined as the Chon Aike province and forms one of the most voluminous silicic provinces globally. The Chon Aike province is predominantly pyroclastic in origin, and is characterized by crystal tuffs and ignimbrite units of rhyolite composition. Silicic volcanic rocks of the once contiguous Antarctic Peninsula form a southward extension of the Chon Aike province and are also dominated by silicic ignimbrite units, with a total thickness exceeding 1 km. The ignimbrites include high-grade rheomorphic ignimbrites, as well as unwelded, lithic-rich ignimbrites. Rhyolite lava flows, air-fall horizons, debris-flow deposits and epiclastic deposits are volumetrically minor, occurring as interbedded units within the ignimbrite succession.

Abstract Early structural glaciological research focused on analysis of particular structures or on mapping of structural features at particular glaciers. More recently, glacier structures have been interpreted in the context of deformation rates and histories measured or estimated using a range of techniques. These measurements indicate that glacier ice experiences complex, polyphase deformation histories that can include a wide range of types, rates and orientations of strain. Deformation styles in glacier ice resemble those in rocks, but occur at a much faster rate, allowing direct measurements to be undertaken, and providing potentially useful models of rock deformation. Structural analysis in the context of measured deformation shows that a wide range of structures (e.g. folds, foliations, boudins, shear zones, crevasses and faults) develop in response to complex strain environments, but strain does not necessarily result in the generation of structures. In the future, three-dimensional numerical modelling may be able to interpret and predict deformation histories and structural development.