Internal structure and composition of a rock glacier in the Andes (upper Choapa valley, Chile) using borehole information and ground-penetrating radar

Abstract

ABSTRACT. This study uses boreholes, ground temperature monitoring and ground-penetrating radar (GPR) in order to understand the internal structure and composition of a rock glacier in the upper Choapa valley, northern Chile. The rock glacier is a small valley-side feature, 200 m long and ranging between 3710 and 3780 m a.s.l. Two boreholes were drilled down to depths of 20 and 25 m, respectively, using the diamond drillhole technique. An ice-rock mixture was encountered in the boreholes, with heterogeneous ice content averaging 15-30%. Data from common-midpoint (CMP) and constant-offset (CO) GPR surveys acquired, respectively, near the boreholes and across the whole rock glacier were processed to highlight the internal stratigraphy and variations in the radar-wave velocity. The GPR profiles depict a rock glacier constituted of stacked and generally concordant layers, with a thickness ranging from 10 m in its upper part to $30 m towards its terminus. The CMP analysis highlights radarwave velocities of 0.13-0.16 m ns -1 in the first 20 m of the structure. Larger vertical and lateral velocity variations are highlighted from CO data, reflecting the heterogeneous composition of the rock glacier and the likely presence of unfrozen water in the structure. Given the average air temperature registered at the site (+0.58C), the near-melting-point temperature registered in the boreholes over more than a year and the presence of locally high water content inferred from GPR data, it is thought that the permafrost in the rock glacier is currently degrading.

Resolution capacity of geophysical monitoring regarding permafrost degradation induced by hydrological processes

“…A reliable interpretation of small-scale geophysical anomalies (or their temporal changes) remains, therefore, difficult (Hilbich et al, 2009). Current challenges for the application of geophysical methods to rock glaciers include (a) the resolution potential for small-scale anomalies and processes (ideally down to the depth of the shear horizon at 15-20 m or more), and (b) the delineation of the complex internal structure and the quantification of the overall ice/water content in a rock glacier (Maurer and Hauck, 2007;Monnier et al, 2011;Monnier and Kinnard, 2013;Hausmann et al, 2007;Merz et al, 2015).…”

The origins of Antarctic rock glaciers: periglacial or glacial features?

“…Rock glaciers are widespread geomorphological features throughout the periglacial areas of both the Southern and Northern Hemispheres (Azócar and Brenning, 2010;Lilleøren and Etzelmüller, 2011;Kellerer-Pirklbauer et al, 2012;Seppi et al, 2012;Janke et al, 2015;Sattler et al, 2016), including Antarctica (Hassinger and Mayewski, 1983;Humlum 1998;Serrano and López-Martínez, 2000;Swanger et al, 2010;López-Martínez et al, 2012;Vieira et al, 2012;Bockheim, 2014). Only a few rock glaciers have been drilled, including the Galena Creek rock glacier (Clark et al, 1996) in North America, the Choapa Valley in the Chilean Andes (Monnier and Kinnard, 2013), the Foscagno rock glacier Guglielmin, 2004) and Lazaun rock glacier (Krainer et al, 2015) in the Italian Alps, and Gruben, Murtel, Muragl and Pontresina-Schafberg (Haeberli, 1985;Arenson et al, 2002;Haeberli et al, 2006) in the Swiss Alps. Only a few rock glaciers have been drilled, including the Galena Creek rock glacier (Clark et al, 1996) in North America, the Choapa Valley in the Chilean Andes (Monnier and Kinnard, 2013), the Foscagno rock glacier Guglielmin, 2004) and Lazaun rock glacier (Krainer et al, 2015) in the Italian Alps, and Gruben, Murtel, Muragl and Pontresina-Schafberg (Haeberli, 1985;Arenson et al, 2002;Haeberli et al, 2006) in the Swiss Alps.…”

Degradation of buried ice and permafrost in the Veleta cirque (Sierra Nevada, Spain) from 2006 to 2013 as a response to recent climate trends

“…In other ranges in the Mediterranean Alpine fringe, such as broad sections of the Alps, this positive thermal inertia recorded in recent decades has resulted in increased rates of movement of the rock glaciers, even leading to collapses and landslides in their detritus structure (Ikeda et al, 2003;Roer et al, 2005;Kääb et al, 2007;Delaloye et al, 2010). This process of degradation of permafrost has been seen for years in rock glaciers in various sectors of the Andes (Francou et al, 1999;Monnier and Kinnard, 2013) and in the final stretches of stabilised glacial tongues covered with stone blocks, as is occurring in several valleys in central Chile's Andes (Ferrando-Acuña, 2014).…”