Rates of periglacial processes in the Central Tianshan, China

Abstract

The periglacial landforms of the Central Tianshan are mainly distributed above the lower boundary (3200 m ASL) of permafrost. In the eastern part of the region, there are 130 days per year when the daily average air temperature fluctuates around 0°C. The annual precipitation is 430 mm. Thus, periglacial landform development reflects the high frost-thawing frequency. By contrast, in the western part, there are only 90 days per year when the daily average air temperature fluctuates around 0°C; thus, periglacial landforms are those typical of areas with low frost-thawing fequencies, even though the annual precipitation reaches 827 mm. Based on observations from 1985 to 1989, schist (bedrock) cracks have a maximal dilatational value of 19 mm/a in the region, and gneiss cracks have a maximal dilatational value of 4 mm/a. The frost-heaving of diorite reaches 8–40 mm/a. The moving rate of rockfall talus is 1 m/a. On sunny slopes (195°SW), the talus debris has amounted to as much as 31.5 cm in the past 25 years, but on semi-shady slopes (65°NE), the debris accumulation is only 5 cm. The alluvial talus (scree) has a downward movement rate of 146 cm/a on south-facing slopes, and 73 cm/a on north-facing slopes. Block slopes have a downward movement rate of 96 mm/a on sunny or semi-sunny slopes and 72 mm/a on the shady or semi-shady slopes; the average rate is 81 mm/a in the study region. East of 86°E, the lobate rock glacier is the main type and its mean moving rate is about 6 cm/a, as deduced by debris discharge which has covered the highway over the past 25 years. Measurements from 1985 to 1986 show that the movement of lobate rock glaciers is from 1 to 49 cm/a. Rates at the higher elevation (15.5–49 cm/a) are faster than at the lower elevation. At the same elevation, the rates on the northward slopes (about 49 cm/a) are greater than on southward slopes (45 cm/a). According to lichenometry data, lobate rock glaciers formed between 3949 a BP to 180 a BP. West of 86°E, the main rock glacier type is the tongue-shaped rock glacier. The mean movement rate is 0.4 m/a. Gelifluction lobes and terraces have an average annual movement of 25 cm in the east. In the west, the average annual movement rate of gelifluction is 49 cm/a.