Internal structure of a Himalayan debris-covered glacier revealed by borehole optical televiewing

Abstract

Characterising the structures within glaciers can give unique insight into ice motion processes. On debris-covered glaciers, traditional structural glaciological mapping is challenging because the lower glacier is hidden by the supraglacial debris layer. Here, we use high-resolution optical televiewer (OPTV) image logs from four boreholes drilled into Khumbu Glacier, Nepal, to overcome this limitation and investigate englacial structural features within a Himalayan debris-covered glacier. The OPTV logs show structural features that are up to an order of magnitude thinner than those observed at the glacier surface and reveal five structural units: (I) primary stratification of ice; (II) debris-rich planes that conform with the primary stratification; (III) water-healed crevasse traces; (IV) healed crevasse traces; and (V) steeply dipping planes of basally derived fine sediment near the glacier terminus. The OPTV logs also reveal that the primary stratification both decreases in dip with depth (by up to 56° over 20 m) and rotates with depth (by up to 100° over 20 m) towards parallelism with the proximal lateral moraine. This transformation and the presence of relict layers of basally derived sediment raised into an englacial position – possibly involving thrusting – near the glacier's now stagnant terminus reveal a previously more dynamic glacier regime.

Englacial Drainage Drives Positive Feedback Depression Growth on the Debris‐Covered Ngozumpa Glacier, Nepal

“…Human‐traversable conduits exist within debris‐covered glaciers and develop when meltwater drains through preexisting lines of permeability (Benn et al., 2017; Gulley & Benn, 2007; Gulley, Benn, Screaton, & Martin, 2009). Surface observations (Benn et al., 2001; Benn et al., 2012; E. S. Miles et al., 2017; K. E. Miles et al., 2020), borehole investigations (K. E. Miles et al., 2022), and englacial cave surveys documented (Benn et al., 2017; Gulley & Benn, 2007; Gulley, Benn, Müller, & Luckman, 2009, Gulley, Benn, Screaton, & Martin, 2009) small scale (∼10 −3 –10 −1 m thick) permeable structures within debris‐covered glaciers. We infer that these structures are widespread and act as sink points for supraglacial meltwater and debris.…”

“…S. Miles et al, 2017;K. E. Miles et al, 2020), borehole investigations (K. E. Miles et al, 2022), and englacial cave surveys documented (Benn et al, 2017;Gulley & Benn, 2007;Gulley, Benn, Müller, & Luckman, 2009, Gulley, Benn, Screaton, & Martin, 2009 small scale (∼10 −3 -10 −1 m thick) permeable structures within debris-covered glaciers. We infer that these structures are widespread and act as sink points for supraglacial meltwater and debris.…”