Origin of a bouldery diamiction, Kunlun pass, Qinghai-Xizang Plateau, People's Republic of China

Abstract

On the west side of the military road to Tibet in the Kunlun Shan, a major body of diamiction is moving slowly downslope from the ridge crest at 4800 m in a northerly and easterly direction. The material is derived from Middle Pleistocene till deposits and the underlying Pliocene alluvial gravels. More than 10% of the material is composed of boulders longer than 2m, 45% has long axes between 0.5 and 2 m, while the matrix is a poorly sorted sandy loam. The mean annual air temperature is -7°C to -5°C and the mean annual precipitation is under 3000 mm a-1. The diamiction lacks a vegetation cover, in contrast to meadow tundra on the surrounding slopes. The diamiction mantles the north slope of the ridge, but splits into at least 16 separate tongues which are moving down fluvially graded valleys. The average slope of the landform is about 19°, while the mean slope of the fronts of the tungues is 21°C. With one exception, the slope of the landform is about 19°, while the mean slope of the fronts of the tongues is 21°. With one exception, the slope of the fronts does not exceed 25°, unlike true rock glaciers. The diamicton is up to 40 m thick in valley 4. The active layer was 12 to 30 cm deep in July at 4780 m, increasing to 1.5 to 2 m at about 4650 m. Ice contents in the permafrost may reach 57% but 30% is more usual. The larger boulders act as braking blocks on the upper slopes of the landform and are frozen into the permafrost. The lower parts of the landform move at under 3 cm a-1 whereas the fine-grained material in the active layer moves past the braking blocks on the upper slopes at up to 30 cm a-1. There is no direct evidence for flowage of the icy diamiction forming the deposit. It is therefore best referred to as a gelifluction slope deposit, and is the longest and most spectacular of such deposits described so far in the world. Show more