Kinking

Motivation

Kinking in minerals preserves the memory of earthquakes—evidence of intense, short-lived stress recorded as sharp lattice bending. These distortions mark the moments when crystals accommodate seismic shock through rapid plastic deformation. In ice, similar bursts of strain occur during glacier surges, ice-shelf flexure, and brittle–ductile transitions near calving fronts, where stress builds and releases in sudden pulses. Ice can form and relax kinks close to its melting point, offering a unique window into transient high-stress flow at high homologous temperatures. Studying these processes connects the deep-Earth mechanics of rocks with the dynamic, ever-changing behaviour of glaciers and icy moons.

Experimental Procedure

To explore how ice responds to high stress, coarse-grained polycrystalline samples (∼1.3 mm grain size) were fabricated using the flood-freeze method and deformed under uniaxial compression at −30 °C within a gas-medium apparatus. The experiments were conducted at confining pressures of ~40 MPa and controlled strain rates ranging from 1 × 10⁻⁵ to 6 × 10⁻⁵ s⁻¹. Post-deformation, the samples were preserved at cryogenic temperatures and examined using cryogenic electron backscatter diffraction (cryo-EBSD), providing high-resolution maps of grain orientation and intragranular distortion.

Observation: Kink-band Formation

Cryo-EBSD analyses reveal that intragranular boundaries are pervasive throughout the deformed ice samples, occurring in both low-angle (4–10°, blue) and high-angle (>10°, red) forms. These boundaries appear mostly straight or gently curved, often extending across large portions of individual grains before terminating within the grain interior rather than forming complete grain boundaries. Many of these intragranular boundaries correspond to kink bands, characterised by sharp lattice bending with misorientation axes lying predominantly within the basal plane.

Observation: Coupling Kinking & CPO

Kinking activity varies systematically with crystallographic orientation. Kink bands are most abundant in grains whose basal planes are sub-parallel to the compression axis and become rare as orientations approach perpendicular (left), showing that kinking is favoured where shear stress on the basal plane is greatest. CPO data reveal contrasting fabrics among grain populations: remnant grains retain a weak cone-shaped c-axis pattern, recrystallised grains show near-random orientations, and kinked domains form a strong girdle of c-axes normal to compression (right).

Our model

Our model links kinking, recrystallisation, and crystallographic orientation during high-stress ice deformation. Grains with basal planes oblique to compression (G3) deform easily by basal slip and grain boundary migration (GBM), relaxing strain without kinking. Those with basal planes parallel to compression (G2) are hard-oriented and accumulate strain, releasing it through kink-band formation. Grains with basal planes perpendicular to compression (G1) experience little shear stress and remain stable. With continued strain, kinking in hard-oriented grains promotes lattice bending and segmentation, while GBM and subgrain rotation (SGR) in neighbouring grains drive recrystallisation. These combined processes generate new grain boundaries and progressively reshape the crystallographic fabric.