Research ArticleGEOLOGY

Quantifying landslide frequency and sediment residence time in the Nepal Himalaya

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Science Advances  24 Apr 2019:
Vol. 5, no. 4, eaav3482
DOI: 10.1126/sciadv.aav3482
  • Fig. 1 Influence of bedrock landslides on a detrital age distribution, and predicted and observed age distributions from the Nyadi catchment in central Nepal.

    (A) A river sediment sample in a drainage basin may record denudation of the entire basin (light gray) or sediment generated dominantly by bedrock landslides (dark gray), producing (B) significantly different age distributions with the same age range [after Stock et al. (3)]. (C) Shaded relief digital elevation model of the Marsyandi drainage basin (heavy black border) in central Nepal with major tectonic structures and the location of the ~200-km2 Nyadi catchment (white fill). MBT, Main Boundary Thrust; MCT, Main Central Thrust; STD, South Tibetan Detachment. (D) Predicted detrital MAr age distributions (gray) based on uniform basin denudation show variable goodness of fit to the observed age distributions (black solid, dashed lines), with very similar ages to the 2002 sample and age distributions that do not match the 1997 sample. The 2002 data are from Ruhl and Hodges (17), and the 1997 data are from Brewer et al. (18).

  • Fig. 2 Overview of catchment bedrock age prediction.

    (A) Example 3D thermokinematic numerical model of the Marsyandi River region including the effects of active thrusting on the model equivalent of the Main Himalayan Thrust [see Whipp et al. (15) for model design details]. Thermochronometer ages are predicted across the entire model surface as particles travel toward the surface along their exhumation pathways (e.g., white arrow). (B) Example of a catchment subsample of predicted MAr bedrock ages. (C) Predicted SPDF (SPDFp) for all catchment bedrock ages. Age prevalence in the SPDFp is scaled by the instantaneous exhumation rate at the surface in the thermokinematic numerical model (A).

  • Fig. 3 Predicted MAr age distributions for varying sediment residence time tr and observed ages from the Nyadi catchment.

    One hundred of 10,000 predicted age distributions (gray) for (A) tr = 1 year, (B) tr = 10 years, (C) tr = 100 years, and (D) tr = 1000 years. The percent similarity refers to the percentage of predicted age distributions that pass the two-sample Kuiper’s test [see Press et al. (33) and the Supplementary Materials]. Decreasing variation in peak age is the result of an increase in the fraction of the drainage basin that is sampled by landslide-generated sediment as tr increases. Data are from Ruhl and Hodges (17). The predicted age SPDFs are smoother than the observed age distribution because they were created using the median percent uncertainty in the observed ages (σ = 10.9%), whereas the percent uncertainty for individual observed ages ranges from <1% to >1000%. Abbreviations as in Fig. 2 caption.

  • Fig. 4 The effects of bedrock landsliding on predicted detrital age distributions as a function of drainage basin area, sediment residence time, and fluvial sediment mixing.

    (A) Predicted landslide-generated age distributions from the whole Nyadi catchment (blue) and three subcatchments (green, red, and black) show increasing agreement with equivalent predicted age distributions for uniform basin denudation as the sediment residence time tr increases. (B) Mixing of landslide sediment with sediment generated by other hillslope processes shows that a lower fraction of landslide-derived sediment increases the probability that the predicted age distributions and the 2002 observed age distribution (17) are statistically equal, as does increasing the sediment residence time. Black filled circles indicate results from individual models.

  • Fig. 5 Factors affecting the sensitivity of detrital thermochronometers to stochastic erosion processes.

    (A) Factors that increase sensitivity of detrital thermochronometers to stochastic erosion include a small basin area, frequent erosion by landsliding or other stochastic mass wasting processes, high-relief and/or steep topography, rapid evacuation of catchment sediment from the basin (heavy blue lines), and a large range of ages in the catchment bedrock. (B) In contrast, detrital thermochronometers are less likely to be affected by stochastic erosion when the basin area is large, erosion by landslides is infrequent or landslide deposits remain perched on hillslopes, the topographic relief is low, fluvial evacuation is inefficient (thinner blue lines) and large volumes of sediment are stored in the catchment for decades or more (brown regions), or the range of ages in the catchment bedrock is small.

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Overview of landslide age prediction.
    • Fig. S2. Satellite imagery of landslides in the Nyadi catchment.
    • Reference (34)

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