Distorted crystals carry useful information on processes involved in their formation, deformation and growth. The distortions are accommodated by geometrically necessary dislocations, and therefore characterising those dislocations is an informative task, to assist in, for example, deducing the slip systems that produced the dislocations. Electron Backscatter Diffraction (EBSD) allows detailed quantification of distorted crystal orientations and we summarise here a method for extracting information on dislocations from such data. The Weighted Burgers Vector (WBV) method calculates a vector at each point on an EBSD map, or an average over a region. The vector is a weighted average of the Burgers vectors of dislocation lines intersecting the map surface. It is weighted towards dislocation lines at a high angle to the map but that can be accounted for in interpretation. The method is fast and does not involve specific assumptions about dislocation types. It can be used, with care, to analyse subgrain walls (sharp orientation changes) as well as gradational orientation changes within individual grains. We describe new and published examples of the use of the technique to illustrate its potential; case studies to date mainly use the WBV direction not the magnitude. EBSD orientation data have angular errors, and so does the WBV. We present an analysis of these angular errors, showing there is a trade-off between directional accuracy and area sampled. In summary the technique is fast, free from assumptions, and errors can be taken into account to allow testing of hypotheses about dislocation types.

Slip on the active Mai’iu low-angle normal fault in Papua New Guinea that dips 15–24° at the surface has exhumed in its footwall a single, continuous fault surface across a >25 km-wide dome. Derived from a metabasaltic protolith, the fault zone consists of a ≤3 m-thick zone of gouges and cataclasites that overprint a structurally underlying carapace of extensional mylonites. Detailed microstructural and geochemical data, combined with chlorite-based geothermometry, reveal changing deformation processes and conditions in the Mai’iu fault rocks as they were exhumed. The microstructure of non-plastically deformed actinolite grains inherited from the fine-grained (6–35 µm) basaltic protolith indicate that shearing at depth was controlled by diffusion creep accompanied by grain-boundary sliding of these grains together with chlorite neo-crystallization at T≥270–370°C. In a foliated cataclasite unit at shallower crustal levels (T≈150–270°C), fluid-assisted mass transfer and metasomatic reactions accommodated aseismic, distributed shearing; pseudotachylites and ultracataclasites in the same unit indicate that such creep was punctuated by episodes of seismic slip—after which creep resumed. At the shallowest levels (T≤150°C), gouges contain abundant saponite, a frictionally weak mineral that promotes creep on the shallowest-dipping (≤24°), most poorly oriented part of the Mai’iu fault. Our field, microstructural and geochemical data of freshly exhumed fault rocks support geodetic, seismological, and geomorphic evidence for mixed seismic-to-aseismic slip on this active low-angle normal fault.

Strain weakening plays an important role in the continent-scale flow of rocks and minerals, including ice. In laboratory experiments, strain weakening coincides with microstructural changes including grain size reduction and the development of crystallographic preferred orientation (CPO). To interrogate the relative contributions of CPO development and grain size towards the strain weakening of viscously anisotropic minerals at very high homologous temperatures (Th ≥ 0.9), we deformed initially isotropic polycrystalline ice samples to progressively higher strains. We subsequently combined microstructural measurements from these samples with ice flow laws to separately model the mechanical response arising from CPO development and grain size reduction. We then compare the magnitudes of strain weakening measured in laboratory experiments with the magnitudes of strain weakening predicted by the constitutive flow laws. Strain weakening manifests as strain rate enhancement after minimum strain rate under constant load conditions, and as a stress drop after peak stress under constant displacement rate conditions. However, flow laws that only consider grain size evolution predict a nominally constant sample strength with increasing strain. On the other hand, flow law modelling that solely considers CPO effects can accurately reproduce the experimental strain weakening measurements. These observations suggest that at high homologous temperatures (Th ≥ 0.9), CPO development governs the strain weakening behaviour of viscously anisotropic materials like ice. Grain size, on the other hand, plays a negligible role in strain weakening under such conditions. Overall, we suggest that geodynamic and glaciological models should incorporate evolving CPOs to account for strain weakening, especially at high homologous temperatures.

Quantifying lithospheric strength is essential to better understand seismicity in continental regions. We estimate differential stresses and principal stress orientations driving rapid slip (~10 mm/yr) on the active Mai’iu low-angle normal fault (dipping 15–24° at the Earth’s surface) in Papua New Guinea. The fault’s mafic footwall hosts a well-preserved sequence of mylonite, foliated cataclasite, ultracataclasite and gouge. In these fault rocks, we combine stress inversion of fault-slip data and paleostress analysis of syntectonically emplaced calcite veins with microstructural and clumped-isotope geothermometry to constrain a syn-exhumational sequence of deformation stresses and temperatures, and to construct a stress profile through the exhumed footwall of the Mai’iu fault. This includes: 1) at ~12–20 km depth (T≈275–370°C), mylonites accommodated slip on the Mai’iu fault at low differential stresses (>25–135 MPa) before being overprinted by localized brittle deformation at shallower depths; 2) at ~6–12 km depth (T≈130–275°C) differential stresses in the foliated cataclasites and ultracataclasites were high enough (>150 MPa) to drive slip on a mid-crustal portion of the fault (dipping 30–40°), and to trigger brittle yielding of mafic footwall rocks in a zone of mixed-mode seismic/aseismic slip; and 3) at the shallowest crustal levels (T<150°C) on the most poorly oriented part of the Mai’iu fault (dipping ~20–24°), slip occurred on frictionally weak clay-rich gouges (μ≈0.15–0.38). Subvertical σ1 and subhorizontal σ3 parallel to the extension direction, with σ1≈σ2 (constriction), reflect vertical unloading and 3-D bending stresses during rolling-hinge style flexure of the footwall.

Strain weakening during plastic deformation can be achieved via strain energy reduction due to intragranular boundary development and grain boundary formation. To examine intragranular boundary formation at high temperatures (Th≈0.9), we analysed electron backscatter diffraction (EBSD) data of coarse-grained ice deformed at -30°C. Misorientation and weighted Burgers vector (WBV) statistics were calculated along planar intragranular boundaries. Neighbour-pair and random-pair misorientation distributions intersect at misorientation angles of 10–30°, indicating an upper limit to the misorientation threshold angle at which neighbouring grains begin to interact, e.g., rotate relative to each other. Misorientation angles change markedly along each analysed intragranular boundary, linking low- (<10°) and high-angle (10–38°) segments, with each segment exhibiting distinct misorientation axes and WBV directions. We suggest that these boundaries might be produced by the growth and intersection of individual boundary segments comprised of dislocations with distinct slip systems. This new kinematic model does not require a change in the boundary geometry after its formation, as required by the other models, to modify the crystallographic geometry of a planar boundary. Misorientation axis distributions are fundamentally different between intragranular boundaries (mostly confined to the ice basal plane) and grain boundaries (largely dispersed). This observation suggests a strong crystallographic control of intragranular boundary development via subgrain rotation. The apparent lack of crystallographic control for grain boundaries, on the other hand, suggests that misorientation axes become randomized upon grain boundary formation, likely due to the operation of other mechanisms/processes that can modify misorientation axes.