TIMING AND EXTENT OF LATE PLEISTOCENE GLACIATION IN THE CENTRAL MONGOLIAN ALTAI

Many of the subranges of the Mongolian Altai were extensively glaciated during the late Pleistocene; however, relatively few numerical dates from glacial deposits exist for this vast region of Central Asia. Improved understanding of glacial chronologies in the Altai has important implications for linking the processes of regional climate change, denudation, and active tectonic deformation. The extent and timing of glacial advances around the Delüün Valley (47.87°N, 90.75°E) were determined via field mapping and 20 new cosmogenic ¹⁰Be surface exposure ages from moraine crests in five valleys draining westward from the Höh Serh Range (HSR) and the east-draining Chuluut valley. Five ¹⁰Be samples from the outermost moraines along the HSR front at Ice Lake (48.01°N, 90.825°E) and Yamaat (47.9°N, 90.87°E) valleys produce a weighted mean age of 35.8 ± 5.4 ka. Corresponding ELAs calculated with both the AAR and TSAM methods in all five HSR valleys vary between 3050 to 3330 m, with a mean of 3200 ± 125 m, approximately 800 m below the modern ELA. Field evidence supports two younger Pleistocene advances of the Höh Serh and Chuluut valley glaciers. The youngest of these readvances, dated by eight ¹⁰Be samples from four valleys, occurred at 12.9 ± 0.8 ka, and may record one of the first recognitions of the glacial response to the Younger Dryas stadial in western Mongolia. In the southernmost glaciated valley of the HSR (47.74°N, 90.94°E), catastrophic failure of the Younger Dryas moraine (¹⁰Be age of 12.5 ±1.1) and its impounded lake resulted in the deposition of a 2.5 km² debris flow on the piedmont.

To the west of the Delüün Valley, in Chuluut valley (47.85°N, 90.62°E), an older terminal moraine with a minimum ¹⁰Be exposure age of 115 ± 10.5 ka is preserved 8 km downvalley of the LGM moraines. The lack of glacial deposits with similar ages along the HSR may be the result of localized ongoing tectonic uplift of the range by the dextral-oblique Höh Serh fault, resulting in increased land area above the ELA that enabled LGM glaciers to extend further than might be expected.