Formation of band ogives and associated structures at Bas Glacier d’Arolla, Valais, Switzerland

Abstract

ABSTRACT. Structural glaciological, sedimentological and geophysical techniques are used to provide new insight concerning the formation of band ogives and associated structures at Bas Glacier d' Arolla, Switzerland. Sedimentary stratification, crevasse traces and transverse foliation are identified as planar structures in the lower icefall and glacier tongue. Stratification and crevasse traces are progressively deformed into, and enhance, the transverse foliation found in the glacier tongue. Three-dimensional geometry has been defined using ground-penetrating radar, which portrays four main characteristics: (i) deep reflectors interpreted as the ice/bed interface, (ii) alternating reflection-rich and reflection-poor zones interpreted as ogives, (iii) up-glacier-dipping reflectors, interpreted as planar structures, and (iv) down-glacier-dipping reflectors of uncertain origin. At the glacier surface, each band ogive consists of a light and dark band. The dark bands contain more intense foliation which, on differential weathering, traps fine debris. Clasts and clear ice of basal character within dark ogive bands suggest that basal ice has been raised to the glacier surface.The most applicable model for the formation of band ogives at Bas Glacier d' Arolla is a refinement of Posamentier's (1978)``reverse faulting'' hypothesis. In this context, multiple shear zones are formed, through which basal ice is uplifted to the glacier surface to produce the dark, foliated ogive bands. This model fits observations reported from other glaciers with band ogives.

Recent structural evolution of Forni Glacier tongue (Ortles-Cevedale Group, Central Italian Alps)

“…The ogives observed on Forni Glacier can be classified as band ogives, that is, formed by repeated bands of light and dark ice (Figure 4(E) convex in flow direction (Goodsell et al, 2002). Ogives are found exclusively at the base of the eastern and central icefall, the latter in 2003 only, whereas no ogives can be seen below the western icefall.…”

“…On the eastern tongue, ogives are located about 350 m downvalley from the icefall and their wavelength (i.e. the distance between two dark ice bands) is approximately between 15 and 30 m. Although their mechanism of formation is not entirely clear, the most widely accepted explanation is that they reflect the passage of ice through an icefall in different seasons (Goodsell et al, 2002).…”

“…Although traces of foliation were also identified, on the orthophotos and during field surveys, few of these structures can be clearly distinguished on the orthophotos, and therefore, they were not considered in this study. Mapping of crevasses and crevasse traces on orthophotos follows the criteria laid out by Goodsell, Hambrey, Glasser, Nienow, and Mair (2005), while ogives were mapped and classified based on the description provided by Goodsell et al (2002). All identified structures were digitized using GIS software; digitized features were then classified as crevasses, crevasse traces, faults, normal faults and ring faults within a database.…”

“…A number of studies have conducted detailed structural surveys of polar and Alpine glaciers (e.g. Goodsell, Hambrey, & Glasser, 2002;Goodsell, Hambrey, Glasser, Nienow, & Mair, 2005;Hambrey & Lawson, 2000;Jennings, Hambrey, Glasser, James, & Hubbard, 2016), but a few have assessed the temporal evolution of glacier structures in response to evolving stress and strain regimes (Glasser & Scambos, 2008;Hambrey et al, 2005;Phillips et al, 2017). This study aims to map brittle and ductile structures originated from the glacier dynamics, by means of the analysis of high-resolution orthophotos, and to describe structural controls of glacier disintegration for a rapidly downwasting Alpine glacier by investigating structure evolution.…”

The Structural Glaciology of a Temperate Valley Glacier: Haut Glacier d'Arolla, Valais, Switzerland

“…There appears to be little difference between the structures and structural evolution described at temperate Haut Glacier d'Arolla, and those described for similar shaped polythermal glaciers. However, Haut Glacier d'Arolla differs from surge-type glaciers which demonstrate multiple phases of deformation during successive quiescent and surge phases (e.g., Lawson et al, 1994), or glaciers at the base of icefalls, which are dominated by transverse foliation (e.g., Allen et al, 1960;Ragan, 1969;Hambrey and Milnes, 1977;Goodsell et al, 2002).…”