Understanding the pattern of glaciations of Shaluli Shan in the southeastern Tibetan Plateau by using geomorphological mapping

Geomorphological mapping at a large scale is a powerful tool in understanding the paleoglaciology of northern hemispheric ice sheets, and has recently been used in reconstructing the paleoglaciation of the northeastern and southeastern Tibetan Plateau. The Quaternary glaciations of the Tibetan Plateau have long been debated and it is now generally accepted that glacier expansions were limited to valley glaciers and ice caps centered on high mountain ranges. Because of the importance of a detailed glacial chronology and development of new dating techniques, great efforts have been devoted to dating glacial landform on the plateau. The dating work has provided an extensive glacial chronological database and new insights on the glaciations of the plateau. However, dating alone, without proper delineation of glacial landforms, is insufficient for glaciation reconstruction. Given the vast area of the plateau, detailed and consistent mapping is required before a detailed picture can emerge of the extent and timing of Quaternary glaciations on the Tibetan Plateau.

Shaluli Shan is an extensive mountain region located on the margin of the southeastern Tibetan Plateau (Fig. 1). It includes both high relief mountains and relatively low relief upland landscapes. The plateau elevation primarily ranges between 3500 and 4800 m above sea level (asl), with the highest peak reaching 7556 m asl (Mt. Gongga). The summer monsoon system strongly influences this region and brings in more than 90% of the annual precipitation. Abundant glacial landforms indicate that this area was extensively glaciated during the Quaternary. However, only a few glaciers remain in the area today, primarily in Gongga Shan, Genie Shan, and Quer Shan.

Geomorphological mapping was performed using remotely sensed images and a digital elevation model (DEM). The mapped glacial landforms include glacial valleys, marginal moraines, hummocky terrain, scoured terrain, and glacial lineations (Fig. 2). A glacial valley is identified by its U-shaped cross section and smooth valley sides that result from the erosion of valley glaciers. Marginal moraines are arc-shaped ridges that run across the valley or on the plateau surface, or are slightly curved ridges located on the valley sides. They consist of glacial debris and locate along the margin of glacier or ice cap. Hummocky terrain is an irregularly shaped sedimentary accumulation exhibiting a typical surface expression of small hills and mounds. Glacial lineations are long, thin landforms that usually occur in clusters. They are normally visible on Landsat imagery and in Google Earth. Scoured terrain consists of low-relief, bedrock-dominated surfaces with rock basins and intervening rock knobs and is typically dotted with lakes.

Glacial landform patterns indicate that Quaternary glaciation of Shaluli Shan has been characterized by alpine glaciers and ice caps. The alpine glaciers were primarily concentrated in mountain massifs reaching above 5200 m asl. During the glaciations, the high mountains were occupied by valley glaciers. Around some of the highest peaks, glaciers were once connected to form small ice fields as indicated by the eroded walls between cirques (bowl-shaped depression, formed at the head of valley glacier by glacier erosion). At the maximum stage, large glaciers extended outside of valley mouth and neighboring valley glaciers merged to build large piedmont glaciers, forming long marginal moraines. Ice caps only occurred on low-relief plateaus primarily above 4000 m asl, the Haizishan Plateau and the Xinlong Plateau, of which the former has a more distinct zonal distribution of landforms formed by an ice cap. Three major moraine sequences were identified in the Haizishan Plateau. Delineated by the marginal moraines, the paleo-valley glaciers and ice caps were fairly extensive. However, a more extensive glaciation is indicated by glacial erratics (glacial boulder that differs from the size and type of rock native to the area in which it rests) and glacial landform remnants found in the field beyond the mapped landforms (Fig. 2).

A thorough examination of the landform pattern across the Shaluli Shan area uncovers an asymmetric pattern of glaciers controlled by climate and topography (Figs. 3, 4). The glacier extended lower in the east than west, as shown in the latitudinal transect in Fig. 4. Glaciers were generally longer and wider on the east slope of the mountains than on the west slope. This is because of the Southwest Asian Monsoon influence and the topographic control. The mountain ranges primarily have an S-N orientation, and the Monsoon ascends to the interior Tibetan Plateau along a pathway in southeast-northwest direction. Therefore it resulted in more rapid weakening of the Southwest Asian Monsoon from east to west, because of blocking by high mountains. In contrast, longitudinally extending river gorges enable the monsoon to penetrate S-N more effectively. The lee side (west) of the mountains gained less precipitation than the windward side (east) limiting glacier expansion.

The occurrence of paleo-ice caps on the two low-relief plateaus, Haizishan Plateau and Xinlong Plateau, in contrast to alpine glacier systems in high mountain range areas, can be explained by the topography (Figs. 2, 4). Both plateau surfaces have elevations primarily between 4200 and 4800 m asl, with low relief of 0 - 300 m. An equilibrium line (the location on glacier where winter accumulation of snow is equal to the summer loss) altitude drop to 4600 m asl during maximum glaciation allowed for buildup of ice across a broad area of the two plateaus and growth of ice caps, while areas with peaks above 4600 m asl, but lacking extensive surfaces at or above the threshold of 4600 m asl, were dominated by valley glaciers.

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