Structural glaciology of a temperate maritime glacier: lower fox glacier, new zealand

Abstract

This paper describes the structural glaciology of the lower Fox Glacier, a 12.7 km-long valley glacier draining the western side of the Southern Alps, New Zealand. Field data are combined with analysis of aerial photographs to present a structural interpretation of a 5 km-long segment covering the lower trunk of the glacier, from the upper icefall down-glacier to the terminus. The glacier typifies the structural patterns observed in many other alpine glaciers, including: primary stratification visible within crevasse walls in the lower icefall; foliation visible in crevasses below the lower icefall; a complex set of intersecting crevasse traces; splaying and chevron crevasses at the glacier margins; transverse crevasses forming due to longitudinal extension; longitudinal crevasses due to lateral extension near the snout; and, arcuate up-glacier dipping structures between the foot of the lower icefall and the terminus. The latter are interpreted as crevasse traces that have been reactivated as thrust faults, accommodating longitudinal compression at the glacier snout. Weak band-ogives are visible below the upper icefall, and these could be formed by multiple shearing zones uplifting basal ice to the glacier surface to produce the darker bands, rather than by discrete fault planes. Many structures such as crevasses traces do not show a clear relationship with measured surface strain-rates, in which case they may be 'close to crevassing', or are undergoing passive transport down-glacier.

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“…They cited two cases where low-angle refl ectors dipped up glacier (i.e., opposite to the glacier fl ow) and intersected an undulating ice surface in a manner that could be interpreted as thrust faults bringing deeper ice nearer the surface in the hanging wall of the fault (i.e., the fault verges down glacier [see also Goodsell et al, 2002]). Crevasse traces may also provide zones of preexisting weakness (e.g., as shear planes) that can be reactivated as thrust faults by compressive stresses (Nye, 1951;Goodsell et al, 2002Goodsell et al, , 2005Appleby et al, 2010). Bamber (1987) explained such refl ectors to be water, which locally would imply temperate ice.…”

“…For example, Appleby et al (2010) interpreted them as thrusts formed by longitudinal compression below an icefall. They cited two cases where low-angle refl ectors dipped up glacier (i.e., opposite to the glacier fl ow) and intersected an undulating ice surface in a manner that could be interpreted as thrust faults bringing deeper ice nearer the surface in the hanging wall of the fault (i.e., the fault verges down glacier [see also Goodsell et al, 2002]).…”

“…Goodsell et al (2002) invoked this shearing model to explain the ogiverelated radar scattering pattern observed on the Bas Glacier d'Arolla. Similarly, Appleby et al (2010) interpreted band ogives on the Fox glacier to be formed by multiple thrust zones that brought deeper, banded ice to the surface.…”

“…Two mechanisms are widely cited for ogive formation (mainly focused on band ogives), the shearing hypothesis (Posamentier, 1978) and the "summer versus winter passage" hypothesis (Nye, 1958; summarized by Appleby et al, 2010; for a review of the differing mechanisms, see Goodsell et al, 2002). The "summer versus winter passage" hypothesis states that as ice accelerates across the icefall zone, it is stretched.…”

Longitudinal surface structures (flowstripes) on Antarctic glaciers

“…Field investigations on valley glaciers in Norway, Svalbard and the European and New Zealand Alps have demonstrated that these longitudinal surface structures are typically three-dimensional in nature (Hambrey, 1975(Hambrey, , 1977Hambrey and Glasser, 2003;Goodsell et al, 2005;Appleby et al, 2010). Where field relationships have been used to establish a three-dimensional nature, these features have been termed longitudinal foliation.…”