Ultrasonic and seismic constraints on crystallographic preferred orientations of the Priestley Glacier shear margin, Antarctica

Abstract

Abstract. Crystallographic preferred orientations (CPOs) are particularly important in controlling the mechanical properties of glacial shear margins. Logistical and safety considerations often make direct sampling of shear margins difficult and geophysical measurements are commonly used to constrain the CPOs. We present here the first direct comparison of seismic and ultrasonic data with measured CPOs in a polar shear margin. The measured CPO from ice samples from a 58 m deep borehole in the left lateral shear margin of the Priestley Glacier, Antarctica, is dominated by horizontal c-axes aligned sub-perpendicular to flow. A vertical seismic profile experiment with hammer shots up to 50 m away from the borehole, in four different azimuthal directions, shows velocity anisotropy of both P-waves and S-waves. Matching P-wave data to the anisotropy corresponding to CPO models defined by horizontally aligned c-axes gives two possible solutions for c-axis azimuth, one of which matches the c-axis measurements. If both P-wave and S-wave data are used, there is one best fit for azimuth and intensity of c-axis alignment that matches well the measurements. Azimuthal P-wave and S-wave ultrasonic data recorded in the laboratory on the ice core show clear anisotropy that matches that predicted from the CPO of the samples. With good quality data, azimuthal increments of 30° or less will constrain well the orientation and intensity of c-axis alignment. Our experiments provide a good framework for planning seismic surveys aimed at constraining the anisotropy of shear margins.

Can changes in deformation regimes be inferred from crystallographic preferred orientations in polar ice?

“…In a shear margin of glaciers and ice streams, as well as in some ice shelves, ice experiences a gradient in velocity in the lateral direction perpendicular to that velocity, resulting in simple shear deformation with a vertical shear plane (zone III and IV in Fig. 1a-b), observed by a single or double maxima (Jackson and Kamb, 1997) oriented sub-perpendicular to the shear plane (Monz et al, 2021;Thomas et al, 2021) and consistent with geophysical data (Smith et al, 2017;Lutz et al, 2020Lutz et al, , 2022.…”

Tidal Modulation of a Lateral Shear Margin: Priestley Glacier, Antarctica

“…Very small strain measurements of synthetic isotropic ice, made at high frequency, give E = ~9 GPa (Gold, 1958;Gammon et al, 1983b;Vaughan et al, 2016). Values of E calculated from ultrasonic and seismic velocities of natural ice (Bentley, 1972;Kohnen and Gow, 1979;Hellmann et al, 2021) are also on the order of 9 GPa at these high frequencies, and seismic velocities at the Priestley Glacier site give a similar value (Lutz et al, 2022). McCarthy and Cooper (2016) show that measured E values reduce with frequency, down to values more than an order of magnitude lower at tidal frequencies.…”