Abstract
The Haupapa/Tasman Glacier is receding rapidly, like glaciers in mountainous regions globally. Ice exposed in the ablation area reveals a range of features that enable the study of structural and microstructural responses to strain in a temperate valley glacier. Large-scale observations and structural measurements provided a framework to select large oriented samples for quantitative microstructural analysis. Satellite imagery was used to map crevasse orientations. Transverse, longitudinal, and splaying crevasses are observed and present spatial variations as the glacier flows around corners and past confluences. The structure of the lower debris-free white ice of the Tasman Glacier was mapped, identifying three different zones consisting of differing fold and shear structures and microstructures. Layers are sub-vertical with traces sub-parallel to glacial flow, with newer generation layering cross cutting older. A very complex shear margin was documented on the true-left lateral margin. The shear margin comprises three zones: Zone 1 (~ 10 m wide) has complex fold bands, and runs parallel to the eastern glacier edge, Zone 2 (~ 100 m) has large scale folds and adjacent and true-right of Zone 1, Zone 3 is mostly featureless, and adjacent to Zone 2 extending across the centre of the glacier to where it is buried under the true-right lateral moraine.
Fold types observed include rounded hinge and chevron folds with sizes of 10s of cm (mostly Zone 1) to a few metres (Zone 2 and 3). Parasitic folding is present. Tight to isoclinal folding with shear structures were observed along the eastern margin, with larger, more open sinusoidal folds inboard of the margin. Ice towards the centre of the glacier contained mostly coarse grains with the occasional > metre scale fold and lineation. Dominance of ‘s’ fold vergence across the true-left side indicates sinistral shear and shortening. Fold axes of individual folds are curvilinear and measured fold axes are distributed in the plane of layering suggesting sheath folds. The fold axis maximum plunges in the up-flow direction, suggesting a vertical component to 3D shear, which is consistent with high ablation rates at the sample site.
The samples were cut into parallel 5 mm wide slabs of ~ 20 x 30 cm size. The 3D ice microstructure (grain shapes and sizes) was built from polarized light images of each slice. Selected slices were analysed further using a fabric analyser (measures c-axis orientation) and cryo-electron backscattered diffraction (cryo-EBSD, measures full crystallographic orientation) to measure CPOs (crystallographic preferred orientations) and to quantify the microstructure (e.g. grain size). Coarse grained ice in all three zones comprises ice grains (~ > 20 mm) with lobate irregular boundaries, internal low angle subgrain boundaries and lattice distortion. The coarse grains have c-axes strongly aligned sub-perpendicular to the dominant layering with single or double maxima, and small circle patterns. Aggregates of finer (mostly ~ 1 mm diameter, with some aggregates ≤ 0.1 mm) sub-polygonal ice grains form in localised blobs and planar layers < 10 mm thick, dominantly in Zones 1 and 2. The fine grains have a weaker CPO with a primary maximum perpendicular to the imposed shear plane. The layers of coarse and fine grains are folded, with fine grains not documented in a glacier before.
The microstructures of coarse grains are consistent with dynamic recrystallisation associated with dislocation creep during deformation. Comparison of CPOs with laboratory experiments suggest kinematics including simple shear on steep shear planes with a shallow shear direction (the velocity vector) and uniaxial compression slightly clockwise of the across flow direction. Similar sizes of subgrains and some finer grains suggests that subgrain rotation recrystallisation may explain fine grain nucleation. The weaker CPOs and grain shapes of fine grains are consistent with the operation of grain boundary sliding and the CPOs match closely those generated in laboratory shear experiments. CPO pattens indicate a simple shear dominated deformation regime.
A grainsize piezometer indicates stresses of 0.3 – 0.5 MPa (and perhaps up to ~ 1.5 MPa) for the fine grains and a composite flow law with a grain size sensitive component predicts strain rates of 1 x 10⁻⁶ s⁻¹ for 1 mm grains, and up to 1 x 10⁻³ s⁻¹ for 0.1 mm grains. Ice flow velocity gradient averages over a year or more yield much slower strain rates of ~10⁻⁸ s⁻¹ that would require stresses ~ < 0.2 MPa. Temporary glacial accelerations initiated by intense rainfall in the Tasman catchment could provide means to reach appropriate stress and strain rates for rapid localised shear at the margin and production of these super fine grains close to the glacier margin. Increased velocity from rainfall-induced pore pressure increase and basal lubrication creates strain localisation events. These events have increased stress and strain rate, promoting reduced grainsize and changes in the dominant deformation mechanism. Strain localisation can occur in an infinite number of different zone arrangements, influencing and is influenced by anisotropy in the ice. Anisotropy controls the flow regime and structural integrity, influencing the ice response to surrounding conditions imposed on it e.g. global warming. There is much difficulty in discerning specific events due to continued overprinting of structures, which adds complexity. This study has important implications for shear margin mechanics, with temperate glaciers providing information for inaccessible polar basal shear heating in Antarctic or Greenland. This improves ice sheet climate models.