Folding of stratigraphic layers in ice domes
Jacobson, Herbert Paul
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This dissertation presents a two part model of the formation of recumbent folds in stratigraphic layers of ice sheets such as those in Greenland and Antarctica. The initial disturbances in the stratigraphy have their roots in transient dynamic processes and local rheological inhomogeneities, but the kinematics of even a simple ice flow model can deform these disturbances enough to alter paleoclimatic interpretation of an ice core. This study focuses on this deformation, treating the disturbed strati-graphic layers as passive markers in the large-scale flow.The two major tools that I use to study overturning are core-relative isochrones (precores) and the deformation gradient tensor along a particle path. This tensor is used to calculate the rotation of stratigraphic segments of various orientations. It is also used in a stochastic model to derive the probability distribution of observing overturned segments given a probability distribution of the initial disturbances.Overturn probabilities increase downstream along a path. They also increase with depth, unless the initial disturbance processes are concentrated at a particular depth. Because of rapid rate of rotation of near-vertical segments, the probability of seeing obvious overturning in a small ice-core cross-section is low even if recumbent folds are present. Gentle disturbances are more likely to be overturned if they occur deep in the ice where the shear is strongest.Overlaying precores on divide arch isochrones shows that ice divide movement can produce recumbent folds. This movement must have a stop-start quality, with enough stability to produce mature arching, but also enough movement to leave an orphaned disturbance on the flank where it can be overturned. Such folding may be most likely downstream from ice ridges bounded by variable ice streams.The variation in the vertical thickness of a disturbed layer is also examined. Even if a disturbance is not overturned, some portions will be thinned more than undisturbed layers while other portions will be thickened. Passive deformation may reduce or exaggerate certain aspects of dynamically produced stratigraphic disturbances, but it does not obliterate them.
- Geophysics