Professor David Prior will showcase the lateral thinking of using geophysical and geological methods to study the physics of ice, which is relevant to our society as it informs ice sheet modelling and predictions for a warmer world.

Most significant polar ice loss is through fast ice streams and outlet glaciers. The fast-flowing ice deforms by shear where it drags against rock, sediments and relatively static ice at the basal and lateral margins of the flow. These shear zones are a significant control on the rate of flow. Fast ice flow feeds large floating ice shelves and fossil shear zones dissect ice shelves and may influence their break-up. For these reasons predicting future sea level rise related to polar ice loss requires a better quantitative understanding of the shear zones at the margins of fast ice flow.

This talk will outline new work on three lateral shear margins

• the Priestley Glacier that feeds into the Nansen Ice Shelf, Antarctica
• the Whillans Ice stream that feeds the Ross Ice Shelf, Antarctica
• the Haupapa/Tasman Glacier in New Zealand

At the Priestley site, satellite data and high precision tracking of marker stakes show the pattern of ice velocity and the complex interaction of shear with flexure related to tidal elevation of the floating ice shelf. Explosion source and hammer-plate seismology, together with polarimetric radar data constrain the large-scale structure. The geophysical data are informed by a classical structural geology analysis of ice layers, folds and foliations visible at the surface and in a 60m deep ice core. Vertical ice layers form a complex pattern of local shear zones and folds. Microstructural analysis of samples from the ice core shows that ice crystals have their planes of easy slip strongly aligned with the shear margin and slightly oblique to layers, as predicted by laboratory experiments. The seismic data confirm that these crystal alignments extend to the kilometre scale. The Priestley samples are characteristic of ice from the upper, cold (~-20°C) region of a lateral polar shear margin.

Samples from the Whillans shear margin are deeper and warmer and show that the deformation and recrystallisation mechanisms that allow the shear to occur change with depth and temperature.

The Haupapa/Tasman shear margin is more accessible and provides a much better data set to correlate the discrete shear zones and folds with ice microstructures and physical properties.

Our new data suggest that, rather than continuous homogenous shear, localised shear zones that are short lived dominate the shear margin deformation. Laboratory experiments show that ice viscosity has a strong non-linear dependence on strain rate, so that a shear margin with localization will be significantly weaker. Implementation of weaker shear margins is likely to affect the outcome of large-scale ice sheet models. The kinematic complications highlighted likely have importance for crustal and mantle shear zones. Ice shear zones may provide a superb analogue for the deeper Earth as we have the ability to make real-time measurements that are challenging deep in the Earth.

Geoscience Society of New Zealand - 2022 Hochstetter Lecturer

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Zoom link: https://bit.ly/otagogeology