TIMING OF LANDSLIDES IN THE UPPER REACHES OF THE MINJIANG RIVER, EASTERN TIBETAN PLATEAU: DEFINED BY OPTICAL DATING OF DAMMED LAKE SEDIMENTS

The chronology of landslides plays an important role in understanding their causes, frequency and hazards. Although the rapid development of Quaternary dating techniques makes multiple methods date landslides, landslide chronologies are still relatively scarce and limited due to the lack of datable materials associated with landslides. This is especially true in the eastern margin of the Tibetan Plateau, where landslides occur frequently and develop into catastrophic mass movements. For example, the 2008 Wenchuan earthquake triggered more than 30,000 landslides. In order to understand the mechanism further, it is necessary to establish the chronology of paleolandslides in the region. In this study, we tested the applicability of optically stimulated luminescence (OSL) dating techniques on a landslide located within the Minjiang River valley. The river flows along the Minjiang fault situated within the plateau margin close to the Sichuan Basin. Many lacustrine sediment exposures or sections on the banks of the river can be seen. It is obvious that the lakes were formed due to river blockage by landslides in the river bank cliffs. In the field, the largest landslide along the river was identified at the Diexi Town. An exposed lacustrine sediment section associated with the landslide-dammed lake was sampled for OSL measurements and particle size analyses. The similar particle-size distribution patterns of 17 samples from the ~20-m section demonstrate that the depositional environment is relatively stable from bottom to top. The luminescence properties of quartz grains extracted were rigorously tested in order to examine their applicability as dosimeter materials. The results show that the samples are ideal for luminescence dating, and their equivalent doses can be determined using the ‘Standardised growth curve’ method. The 17 samples yield a nearly identical OSL age of 20 ka, suggesting that the landslide-dammed lake burst within a short period of formation. It also implies that the chronology of the similar landslides in the region can be constructed using luminescence dating techniques.