Microstructures in a shear margin: Jarvis Glacier, Alaska

Abstract

Microstructures, including crystallographic fabric, within the margin of streaming ice can exert strong control on flow dynamics. To characterize a natural setting, we retrieved three cores, two of which reached bed, from the flank of Jarvis Glacier, eastern Alaska Range, Alaska. The core sites lie ~1 km downstream of the source, with abundant water present in the extracted cores and at the base of the glacier. All cores exhibit dipping layers, a combination of debris bands and bubble-free domains. Grain sizes coarsen on average approaching the lateral margin. Crystallographic orientations are more clustered and with c-axes closer to horizontal nearer the lateral margin. The measured fabric is sufficiently weak to induce little mechanical anisotropy, but the data suggest that despite the challenging conditions of warm ice, abundant water and a short flow distance, many aspects of the microstructure, including measurable crystallographic fabric, evolved in systematic ways.

Modeling Ice‐Crystal Fabric as a Proxy for Ice‐Stream Stability

“…There are few measurements to validate this result. Recent measurements of Jarvis Glacier in Alaska indicate that shear margins indeed exhibit stronger horizontal patterns than their surroundings, though the fabrics there were relatively weak (Gerbi et al, 2021). A core in the margin of Ice Stream B showed a double-maximum in the horizontal, consistent with our modeling but indicating that migration recrystallization may be active (Jackson & Kamb, 1997).…”

Microstructure and Crystallographic Preferred Orientations of an Azimuthally Oriented Ice Core from a Lateral Shear Margin: Priestley Glacier, Antarctica

“…CPOs from lateral shear zones in the margins of the Whillans ice stream have two horizontal maxima, one being more intense than the other (Jackson and Kamb, 1997;Jackson, 1999). Recent data from the margin of the Jarvis Glacier, Alaska show a preponderance of horizontal c-axes (Gerbi et al, 2021). The kinematics of the Priestley shear margin, as measured by stakes spaced 50-100 m, is dominated by simple shear (Still et al, this volume).…”

“…In reality, shear margin fabric development is affected by temperature, impurity content and the presence of structural heterogeneities, which can vary spatially and with depth (Lawson et al, 1994;Harrison et al, 1998;Barnes and Wolff, 2004;Pettit et al, 2014). To date, few studies have directly observed CPOs from temperate or polar glacier and ice stream margins (Hudleston, 1977;Jackson and Kamb, 1997;Samyn et al, 2008;Gerbi et al, 2021;Monz et al, 2021); Sample collection in these areas is generally hindered by safety or logistical problems. Of these studies, even fewer describe CPOs fully oriented in a kinematic reference frame (Hudleston, 1977;Monz et al, 2021;Hellman et al, 2021).…”

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

“…As result of high flow velocities, the margins of these streaming ice bodies undergo large strain as they are in contact with stationary ice or rock. Crystallographic preferred orientation (CPO) patterns inside glacier margins have been found to indicate a very high degree of crystal alignment in the horizontal direction (Jackson and Kamb, 1997;Monz et al, 2021;Gerbi et al, 2021; The presence of a CPO results in anisotropic mechanical properties and so influences the viscous behaviour of ice significantly (Azuma and Goto-Azuma, 1996;Budd et al, 2013;Faria et al, 2014;Hudleston, 2015). Shear margins of glaciers are therefore expected to affect the character of ice flow in ice streams due to their distinct mechanical properties (Minchew et al, 2018;Hruby et al, 2020;Drews et al, 2021).…”