Viscous creep of ice-rich permafrost debris in a recently uncovered proglacial area in the Tianshan Mountains, China

Abstract

Since the Little Ice Age and as a consequence of climate warming, many recently deglaciated forefields have become and will continue to evolve into large ice-debris complexes exposed to periglacial processes and environment. Such transitional processes have significant implications for geomorphologic shaping and water supply for the downstream communities, especially in arid regions, but our understanding of their evolutionary processes and their potential geomorphic and hydrological impacts is still limited. A landform transition from partly debris-covered glaciers to ice-rich permafrost debris undergoing slow viscous creep was revealed in the Aerzailaikunai Valley in the eastern Tianshan Mountains in China based on the results of in-situ observations and measurements (boreholes, ground temperature monitoring, electrical resistivity tomography surveys, and continuous global positioning system measurements, among others). The internal structure of ice-till mixture contains pure ice layers, supersaturated frozen sands with ice lenses, and ice-bearing blocks with maximum volumetric contents of heterogeneous ice at 35%–60%. Beneath an 1.5-m-thick active layer, permafrost reached far into the underlying bedrock with the mean annual ground temperature of −2.1 °C at the depth of 20 m. The higher surface velocities (with an accumulative displacement of 65 mm from October 2019 to May 2020) and extremely high electrical resistivity (several million Ω m) of the debris-covered glacier margin were in sharp contrast to those of the progressively stabilizing ground surface (up to 16 mm) and the lower zones with relatively smaller electrical resistivity (several thousand Ω m). Combined with the borehole stratigraphy (higher rock content), monitored ground temperatures (permafrost environment), lower electrical resistance (ice-rich moraine), and continuous global positioning system results (viscous creeping), this study documents a transition from glacial to periglacial conditions, materials and processes characteristic of cold-dry ice-clad mountains, and reinforces the theory of the transition from debris-covered glaciers into morainically originated rock glaciers.

Keywords

• Alpine permafrost
• Viscous creep
• Debris-covered glacier margins
• Climate warming
• Landform transition
• Evolution of rock glacier