Rates of periglacial processes in the Central Tianshan, China

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.