Magnetic Till Fabric: Applications of anisotropy of magnetic susceptibility (AMS) to subglacial deformation of till and ice. 1. Till kinematics and the origins of drumlins, New York State. 2. Till kinematics of the Baltic Ice Stream, Sweden. 3. Characterization of deformation within the Basal Stratified Ice of the Matanuska, Glacier, Alaska.

摘要

Subglacial processes are significant contributors to the dynamics and sediment transport of glaciers and ice sheets, drive much of the observed variability in modern systems, and are responsible for much of the landscape in glaciated terrains. Subglacial deformation (i.e., deformation of the glacier substrate) is now recognized as a fundamental process of motion and sediment transport for warm-based glaciers; however, the spatial and temporal variability of this deformation is poorly understood, owing in part to the inaccessibility of the modern subglacial environment. The Pleistocene sedimentary record provides a complementary view, and many processes are recorded within the sediments. Herein, the variability of subglacial deformation is assessed in three distinct glacial settings using anisotropy of magnetic susceptibility (AMS) fabric analysis. AMS is a robust, quantitative, volume-averaged, and objective measure of the orientations of the axes of magnetic susceptibility, which is predominantly controlled by grain shape such that the orientation of the maximum susceptibility axis parallels the long axis of the grain. Thus, AMS provides a useful proxy for the orientation of magnetic grains within sediments, from which direction of ice flow and the magnitude of deformation can be inferred.;First, I assess the spatial variability of ice flow and till transport around streamlined bedforms (drumlins) and demonstrate relationships between internal drumlin fabric and the drumlin surface morphology, indicating these landforms cannot be erosional remnants, but are likely formed as till is transported to the drumlin locality and syndepositionally streamlined. Next, I evaluate stratigraphic (i.e., temporal) variation as recorded in a late-Weichselian Baltic Ice Stream (BIS) till in southern Sweden. Our analysis records systematic and dramatic changes in ice flow direction and bed deformation, demonstrating the dynamic nature of the BIS and allowing for the discrimination of intra-till kinematic zones. Finally, I address the distribution of deformation within the debris-rich basal stratified ice of the Matanuska Glacier, a modern temperate glacier in southern Alaska and a type-locality of stratified basal ice. Basal ice that is debris-rich relative to englacial ice and interacts with the glacier bed is common to many glaciers and ice sheets, but its genesis and contributions to glacier motion are poorly understood. This analysis reveals basal stratified ice has experienced significant shear, and that shear appears concentrated in debris-rich layers in the form of simple shear along sub-horizontal shear planes. Debris-poor layers possess 'compaction' fabrics indicative of pure shear driven by the force of the overlying englacial ice. Thus, the debris-rich basal ice is characterized by rheological inhomogeneities resulting from the competing factors of debris-content and ice crystal size, among others. These results indicate that deformation is pervasive within the subglacial environment, extending from the unfrozen substrate into the debris-rich basal ice, but that this deformation is characterized by high spatial and temporal variability in both magnitude and direction.