Application of borehole optical televiewing to investigating the 3-D structure of glaciers: implications for the formation of longitudinal debris ridges, midre Lovénbreen, Svalbard

Abstract

ABSTRACT. Digital optical televiewing (OPTV) of hot-water-drilled boreholes is evaluated as a technique for the investigation of englacial ice and debris structures on the basis of six boreholes drilled in the terminus region of midre Lovénbreen, Svalbard. The resulting OPTV logs successfully reveal several visually distinctive englacial ice properties and deformation structures (e.g. oblique englacial fractures imaged here for the first time). Combining these OPTV logs with surface mapping has resulted in the identification of eight separate structural elements, several of which can be interpolated onto 3-D grids at a node spacing of 1 m vertically and 10 m horizontally. Basally derived englacial sediment layers are also found to be intercalated with primary stratification, elevated into near-vertical planes around a central fold axis by large-scale lateral folding. The analysis also allows supraglacial longitudinal debris ridges to be subclassified into two types: a previously described (type-I) form, which are the exposed fold axes of large-scale lateral folds, and a new (type-II) form experiencing secondary deformation by small-scale horizontal folding in association with vertical displacements across arcuate shear planes in response to longitudinally compressive stresses near the glacier terminus. Although using boreholes to investigate glacier structure is limited (e.g. by parallelism with vertical planes), applying OPTV to multiple boreholes at midre Lovénbreen has successfully revealed a range of 3-D structural elements at high spatial resolution. As such, interpolating between multiple OPTV logs overcomes many of the problems associated with interpretations made solely on the basis of surface-based structural mapping, and combining the two techniques represents a powerful glaciological tool.

Former dynamic behaviour of a cold-based valley glacier on Svalbard revealed by basal ice and structural glaciology investigations

“…Hambrey and Dowdeswell, 1997;Hambrey and Glasser, 2003;Hambrey and others, 2005;Roberson and Hubbard, 2010), we suggest that these probably represent two end members of the same structure rather than distinct features. Primary stratification (S 0 ) describes the layers of bubble-rich and bubble-poor ice formed through firnification processes in the accumulation area (cf.…”

“…Planar glaciological structures were first identified within the ice caves and measured as strike and dip using a compass/clinometer. These field data were then integrated with the structures mapped on the glacier surface from aerial photographs and classified and coded from S 0 upwards based on order of formation, in accordance with structural geology conventions (Hambrey and others, 2005;Roberson and Hubbard, 2010). The dip direction and dip (linear structure) of sheared englacial debris laminae, or 'mineral stretching lineation ' (Fleming and others, 2013), were also recorded.…”

“…Where the hinge lines of the isoclinal folds intersect the glacier surface, they form linear stripes, mapped as longitudinal foliation or S 1 structures (e.g. Hambrey and Dowdeswell, 1997;Hambrey and Glasser, 2003;Roberson and Hubbard, 2010). As with any continuum, some structural elements must exist between the two end members; at Tellbreen, this is represented by the bubble stratification within the englacial ice facies (Fig.…”

“…Similar to Roberson (2008) and Roberson and Hubbard (2010), S 2 arcuate fracture traces are interpreted as shear planes based on the evidence that: (1) they cut across S 0 /S 1 layering at low angles and in places displace it vertically, and (2) the fine material within S 2 has a strong linear component (Fig. 9d), or stretching lineation (Fleming and others, 2013), indicative of shear in the direction of ice flow (Table 2; Fig.…”

Glacial geomorphology of the terrestrial margins of the tidewater glacier, Nordenskiöldbreen, Svalbard

“…Therefore, the geomorphology on the southern foreland is a result of interaction between five narrow ice flow units, which facilitated the delivery of supraglacial, englacial and subglacial debris from nunataks and valley sides and its transportation towards the ice margin along longitudinal flow stripes/foliations (cf. Casassa & Brecher, 1993;Ely & Clark, 2016;Glasser & Gudmundsson, 2012;Hambrey, 1975;Roberson & Hubbard, 2010). One further aspect of the southern foreland is the fact that the five southern ice flow units were squeezed by the two larger northern flow units during the advance of Nordenskiöldbreen, resulting in them advancing relatively further and being compressed and contorted over the site of the present foreland, where they are now melting out.…”